Patent Publication Number: US-10761854-B2

Title: Preventing hazard flushes in an instruction sequencing unit of a multi-slice processor

Description:
BACKGROUND 
     Field of the Invention 
     The field of the invention is data processing, or, more specifically, methods, apparatus, and products for preventing hazard flushes in an instruction sequencing unit of a multi-slice processor. 
     Description of Related Art 
     The development of the EDVAC computer system of 1948 is often cited as the beginning of the computer era. Since that time, computer systems have evolved into extremely complicated devices. Today&#39;s computers are much more sophisticated than early systems such as the EDVAC. Computer systems typically include a combination of hardware and software components, application programs, operating systems, processors, buses, memory, input/output devices, and so on. As advances in semiconductor processing and computer architecture push the performance of the computer higher and higher, more sophisticated computer software has evolved to take advantage of the higher performance of the hardware, resulting in computer systems today that are much more powerful than just a few years ago. 
     One area of computer system technology that has advanced is computer processors. As the number of computer systems in data centers and the number of mobile computing devices has increased, the need for more efficient computer processors has also increased. Speed of operation and power consumption are just two areas of computer processor technology that affect efficiency of computer processors. 
     SUMMARY 
     Methods and apparatus for preventing hazard flushes in an instruction sequencing unit of a multi-slice processor are disclosed in this specification. Preventing hazard flushes in an instruction sequencing unit of a multi-slice processor includes receiving a load instruction in a load reorder queue, wherein the load instruction is an instruction to load data from a memory location; subsequent to receiving the load instruction, receiving a store instruction in a store reorder queue, wherein the store instruction is an instruction to store data in the memory location; determining that the store instruction causes a hazard against the load instruction; preventing a flush of the load reorder queue based on a state of the load instruction; and re-executing the load instruction. 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular descriptions of exemplary embodiments of the invention as illustrated in the accompanying drawings wherein like reference numbers generally represent like parts of exemplary embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  sets forth a block diagram of an example system configured for preventing hazard flushes in an instruction sequencing unit of a multi-slice processor according to embodiments of the present invention. 
         FIG. 2  sets forth a block diagram of a portion of a multi-slice processor according to embodiments of the present invention. 
         FIG. 3  sets forth a block diagram of a portion of a load/store slice of a multi-slice processor according to embodiments of the present invention. 
         FIG. 4  sets forth a flow chart illustrating an exemplary method for preventing hazard flushes in an instruction sequencing unit of a multi-slice processor according to embodiments of the present invention. 
         FIG. 5  sets forth a flow chart illustrating an exemplary method for preventing hazard flushes in an instruction sequencing unit of a multi-slice processor according to embodiments of the present invention. 
         FIG. 6  sets forth a flow chart illustrating an exemplary method for preventing hazard flushes in an instruction sequencing unit of a multi-slice processor according to embodiments of the present invention. 
         FIG. 7  sets forth a flow chart illustrating an exemplary method for preventing hazard flushes in an instruction sequencing unit of a multi-slice processor according to embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary methods and apparatus for preventing hazard flushes in an instruction sequencing unit of a multi-slice processor in accordance with the present invention are described with reference to the accompanying drawings, beginning with  FIG. 1 .  FIG. 1  sets forth a block diagram of an example system configured for preventing hazard flushes in an instruction sequencing unit of a multi-slice processor according to embodiments of the present invention. The system of  FIG. 1  includes an example of automated computing machinery in the form of a computer ( 152 ). 
     The computer ( 152 ) of  FIG. 1  includes at least one computer processor ( 156 ) or ‘CPU’ as well as random access memory ( 168 ) (‘RAM’) which is connected through a high speed memory bus ( 166 ) and bus adapter ( 158 ) to processor ( 156 ) and to other components of the computer ( 152 ). 
     The example computer processor ( 156 ) of  FIG. 1  may be implemented as a multi-slice processor. The term ‘multi-slice’ as used in this specification refers to a processor having a plurality of similar or identical sets of components, where each set may operate independently of all the other sets or in concert with the one or more of the other sets. The multi-slice processor ( 156 ) of  FIG. 1 , for example, includes several execution slices (‘ES’) and several load/store slices (‘LSS’)—where load/store slices may generally be referred to as load/store units. Each execution slice may be configured to provide components that support execution of instructions: an issue queue, general purpose registers, a history buffer, an arithmetic logic unit (including a vector scalar unit, a floating point unit, and others), and the like. Each of the load/store slices may be configured with components that support data movement operations such as loading of data from cache or memory or storing data in cache or memory. In some embodiments, each of the load/store slices includes a data cache. The load/store slices are coupled to the execution slices through a results bus. In some embodiments, each execution slice may be associated with a single load/store slice to form a single processor slice. In some embodiments, multiple processor slices may be configured to operate together. 
     The example multi-slice processor ( 156 ) of  FIG. 1  may also include, in addition to the execution and load/store slices, other processor components. In the system of  FIG. 1 , the multi-slice processor ( 156 ) includes fetch logic, dispatch logic, and branch prediction logic. Further, although in some embodiments each load/store slice includes cache memory, the multi-slice processor ( 156 ) may also include cache accessible by any or all of the processor slices. 
     Although the multi-slice processor ( 156 ) in the example of  FIG. 1  is shown to be coupled to RAM ( 168 ) through a front side bus ( 162 ), a bus adapter ( 158 ) and a high speed memory bus ( 166 ), readers of skill in the art will recognize that such configuration is only an example implementation. In fact, the multi-slice processor ( 156 ) may be coupled to other components of a computer system in a variety of configurations. For example, the multi-slice processor ( 156 ) in some embodiments may include a memory controller configured for direct coupling to a memory bus ( 166 ). In some embodiments, the multi-slice processor ( 156 ) may support direct peripheral connections, such as PCIe connections and the like. 
     Stored in RAM ( 168 ) in the example computer ( 152 ) is a data processing application ( 102 ), a module of computer program instructions that when executed by the multi-slice processor ( 156 ) may provide any number of data processing tasks. Examples of such data processing applications may include a word processing application, a spreadsheet application, a database management application, a media library application, a web server application, and so on as will occur to readers of skill in the art. Also stored in RAM ( 168 ) is an operating system ( 154 ). Operating systems useful in computers configured for operation of a multi-slice processor according to embodiments of the present invention include UNIX™, Linux™, Microsoft Windows™, AIX™, IBM&#39;s z/OS™, and others as will occur to those of skill in the art. The operating system ( 154 ) and data processing application ( 102 ) in the example of  FIG. 1  are shown in RAM ( 168 ), but many components of such software typically are stored in non-volatile memory also, such as, for example, on a disk drive ( 170 ). 
     The computer ( 152 ) of  FIG. 1  includes disk drive adapter ( 172 ) coupled through expansion bus ( 160 ) and bus adapter ( 158 ) to processor ( 156 ) and other components of the computer ( 152 ). Disk drive adapter ( 172 ) connects non-volatile data storage to the computer ( 152 ) in the form of disk drive ( 170 ). Disk drive adapters useful in computers configured for operation of a multi-slice processor according to embodiments of the present invention include Integrated Drive Electronics (‘IDE’) adapters, Small Computer System Interface (‘SCSI’) adapters, and others as will occur to those of skill in the art. Non-volatile computer memory also may be implemented for as an optical disk drive, electrically erasable programmable read-only memory (so-called ‘EEPROM’ or ‘Flash’ memory), RAM drives, and so on, as will occur to those of skill in the art. 
     The example computer ( 152 ) of  FIG. 1  includes one or more input/output (‘I/O’) adapters ( 178 ). I/O adapters implement user-oriented input/output through, for example, software drivers and computer hardware for controlling output to display devices such as computer display screens, as well as user input from user input devices ( 181 ) such as keyboards and mice. The example computer ( 152 ) of  FIG. 1  includes a video adapter ( 209 ), which is an example of an I/O adapter specially designed for graphic output to a display device ( 180 ) such as a display screen or computer monitor. Video adapter ( 209 ) is connected to processor ( 156 ) through a high speed video bus ( 164 ), bus adapter ( 158 ), and the front side bus ( 162 ), which is also a high speed bus. 
     The exemplary computer ( 152 ) of  FIG. 1  includes a communications adapter ( 167 ) for data communications with other computers ( 182 ) and for data communications with a data communications network ( 100 ). Such data communications may be carried out serially through RS-232 connections, through external buses such as a Universal Serial Bus (‘USB’), through data communications networks such as IP data communications networks, and in other ways as will occur to those of skill in the art. Communications adapters implement the hardware level of data communications through which one computer sends data communications to another computer, directly or through a data communications network. Examples of communications adapters useful in computers configured for operation of a multi-slice processor according to embodiments of the present invention include modems for wired dial-up communications, Ethernet (IEEE 802.3) adapters for wired data communications, and 802.11 adapters for wireless data communications. 
     The arrangement of computers and other devices making up the exemplary system illustrated in  FIG. 1  are for explanation, not for limitation. Data processing systems useful according to various embodiments of the present invention may include additional servers, routers, other devices, and peer-to-peer architectures, not shown in  FIG. 1 , as will occur to those of skill in the art. Networks in such data processing systems may support many data communications protocols, including for example TCP (Transmission Control Protocol), IP (Internet Protocol), HTTP (HyperText Transfer Protocol), WAP (Wireless Access Protocol), HDTP (Handheld Device Transport Protocol), and others as will occur to those of skill in the art. Various embodiments of the present invention may be implemented on a variety of hardware platforms in addition to those illustrated in  FIG. 1 . 
     For further explanation,  FIG. 2  sets forth a block diagram of a portion of a multi-slice processor according to embodiments of the present invention. The multi-slice processor in the example of  FIG. 2  includes a dispatch network ( 202 ). The dispatch network ( 202 ) includes logic configured to dispatch instructions for execution among execution slices. 
     The multi-slice processor in the example of  FIG. 2  also includes a number of execution slices ( 204   a ,  204   b - 204   n ). Each execution slice includes general purpose registers ( 206 ) and a history buffer ( 208 ). The general purpose registers and history buffer may sometimes be referred to as the mapping facility, as the registers are utilized for register renaming and support logical registers. 
     The general purpose registers ( 206 ) are configured to store the youngest instruction targeting a particular logical register and the result of the execution of the instruction. A logical register is an abstraction of a physical register that enables out-of-order execution of instructions that target the same physical register. 
     When a younger instruction targeting the same particular logical register is received, the entry in the general purpose register is moved to the history buffer, and the entry in the general purpose register is replaced by the younger instruction. The history buffer ( 208 ) may be configured to store many instructions targeting the same logical register. That is, the general purpose register is generally configured to store a single, youngest instruction for each logical register while the history buffer may store many, non-youngest instructions for each logical register. 
     Each execution slice ( 204 ) of the multi-slice processor of  FIG. 2  also includes an execution reservation station ( 210 ). The execution reservation station ( 210 ) may be configured to issue instructions for execution. The execution reservation station ( 210 ) may include an issue queue. The issue queue may include an entry for each operand of an instruction. The execution reservation station may issue the operands for execution by an arithmetic logic unit or to a load/store slice ( 222   a ,  222   b ,  222   c ) via the results bus ( 220 ). 
     The arithmetic logic unit ( 212 ) depicted in the example of  FIG. 2  may be composed of many components, such as add logic, multiply logic, floating point units, vector/scalar units, and so on. Once an arithmetic logic unit executes an operand, the result of the execution may be stored in the result buffer ( 214 ) or provided on the results bus ( 220 ) through a multiplexer ( 216 ). 
     The results bus ( 220 ) may be configured in a variety of manners and be of composed in a variety of sizes. In some instances, each execution slice may be configured to provide results on a single bus line of the results bus ( 220 ). In a similar manner, each load/store slice may be configured to provide results on a single bus line of the results bus ( 220 ). In such a configuration, a multi-slice processor with four processor slices may have a results bus with eight bus lines—four bus lines assigned to each of the four load/store slices and four bus lines assigned to each of the four execution slices. Each of the execution slices may be configured to snoop results on any of the bus lines of the results bus. In some embodiments, any instruction may be dispatched to a particular execution unit and then by issued to any other slice for performance. As such, any of the execution slices may be coupled to all of the bus lines to receive results from any other slice. Further, each load/store slice may be coupled to each bus line in order to receive an issue load/store instruction from any of the execution slices. Readers of skill in the art will recognize that many different configurations of the results bus may be implemented. 
     The multi-slice processor in the example of  FIG. 2  also includes a number of load/store slices ( 222   a ,  222   b - 222   n ). Each load/store slice includes a queue ( 224 ), a multiplexer ( 228 ), a data cache ( 232 ), and formatting logic ( 226 ), among other components described below with regard to  FIG. 3 . The queue receives load and store operations to be carried out by the load/store slice ( 222 ). The formatting logic ( 226 ) formats data into a form that may be returned on the results bus ( 220 ) to an execution slice as a result of a load or store instruction. 
     The example multi-slice processor of  FIG. 2  may be configured for flush and recovery operations. A flush and recovery operation is an operation in which the registers (general purpose register and history buffer) of the multi-slice processor are effectively ‘rolled back’ to a previous state. The term ‘restore’ and ‘recover’ may be used, as context requires in this specification, as synonyms. Flush and recovery operations may be carried out for many reasons, including missed branch predictions, exceptions, and the like. Consider, as an example of a typical flush and recovery operation, that a dispatcher of the multi-slice processor dispatches over time and in the following order: an instruction A targeting logical register  5 , an instruction B targeting logical register  5 , and an instruction C targeting logical register  5 . At the time instruction A is dispatched, the instruction parameters are stored in the general purpose register entry for logical register  5 . Then, when instruction B is dispatched, instruction A is evicted to the history buffer (all instruction parameters are copied to the history buffer, including the logical register and the identification of instruction B as the evictor of instruction A), and the parameters of instruction B are stored in the general purpose register entry for logical register  5 . When instruction C is dispatched, instruction B is evicted to the history buffer and the parameters of instruction C are stored in the general purpose register entry for logical register  5 . Consider, now, that a flush and recovery operation of the registers is issued in which the dispatch issues a flush identifier matching the identifier of instruction C. In such an example, flush and recovery includes discarding the parameters of instruction C in the general purpose register entry for logical register  5  and moving the parameters of instruction B from the history buffer for instruction B back into the entry of general purpose register for logical register  5 . 
     During the flush and recovery operation, in prior art processors, the dispatcher was configured to halt dispatch of new instructions to an execution slice. Such instructions may be considered either target or source instructions. A target instruction is an instruction that targets a logical register for storage of result data. A source instruction by contrast has, as its source, a logical register. A target instruction, when executed, will result in data stored in an entry of a register file while a source instruction utilizes such data as a source for executing the instruction. A source instruction, while utilizing one logical register as its source, may also target another logical register for storage of the results of instruction. That is, with respect to one logical register, an instruction may be considered a source instruction and with respect to another logical register, the same instruction may be considered a target instruction. 
     The multi-slice processor in the example of  FIG. 2  also includes an instruction sequencing unit ( 240 ). While depicted as a single unit, each of the plurality of execution slices may include a respective instruction sequencing unit similar to instruction sequencing unit ( 240 ). Instruction sequencing unit ( 240 ) may take dispatched instructions and check dependencies of the instructions to determine whether all older instructions with respect to a current instruction have delivered, or may predictably soon deliver, results of these older instructions from which the current instruction is dependent so that the current instruction may execute correctly. If all dependencies to a current instruction are satisfied, then a current instruction may be determined to be ready to issue, and may consequently be issued—regardless of a program order of instructions as determined by an ITAG. Such issuance of instructions may be referred to as an “out-of-order” execution, and the multi-slice processor may be considered an out-of-order machine. 
     In some cases, a load/store unit receiving an issued instruction, such as a load/store slice, may not yet be able to handle the instruction, and the instruction sequencing unit ( 240 ) may keep the instruction queued until such time as the load/store slice may handle the instruction. After the instruction is issued, the instruction sequencing unit ( 240 ) may track progress of the instruction based at least in part on signals received from a load/store slice. 
     For further explanation,  FIG. 3  sets forth a block diagram depicting an expanded view of a load/store slice ( 222   a ) implementing architectural components that include a load/store access queue (LSAQ) ( 224 ), a load reorder queue (LRQ) ( 304 ), a load miss queue (LMQ) ( 308 ), a store reorder queue (SRQ) ( 306 ), a data cache ( 232 ), among other components. 
     In previous systems, if a load/store unit received an instruction from an instruction sequencing unit, and the load/store unit was unable to handle the instruction for some reason, then the load/store unit would notify the instruction sequencing unit that the instruction was being rejected and the load/store unit would discard information related to the rejected instruction. In which case, the instruction sequencing unit would continue maintaining information to track and maintain the rejected instruction until the instruction is resent to the load/store unit. 
     The load/store slice ( 222   a ), by contrast to the above previous system, is configured to determine a rejection condition for an instruction received from an instruction sequencing unit, however, the load/store slice ( 222   a ), instead of sending a reject signal to the instruction sequencing unit, maintains tracking and handling of the instruction—including information usable to relaunch or reissue the instruction—until the rejection condition is resolved. Further, an entry in the load reorder queue ( 304 ) or the store reorder queue ( 306 ) may be configured to maintain information for tracking an instruction that would otherwise have been rejected and removed from the load reorder queue ( 304 ). For example, if the load/store slice ( 222   a ) determines that a rejection condition exists for a given load or store instruction, then logic within the load/store slice may notify the load reorder queue ( 304 ) or the store reorder queue ( 306 ) to place the instruction in a sleep state for a given number of cycles, or to place the instruction in a sleep state until notified to awaken, or to immediately reissue the instruction, among other notifications to perform other operations. 
     In this way, the load/store slice ( 222   a ) may save cycles that would otherwise be lost if the instruction were rejected to the instruction sequencing unit because the load/store slice ( 222   a ) may more efficiently and quickly reissue the instruction when the rejection condition is resolved in addition to more quickly detecting resolution of a rejection condition than an instruction sequencing unit. For example, if the load/store slice ( 222   a ) determines that an instruction that is in a sleep state may be reissued in response to determining that one or more rejection conditions have been resolved preventing the instruction from completing, then the load/store store slice may notify the load reorder queue ( 304 ) or the store reorder queue ( 306 ) to relaunch or reissue the instruction immediately or after some number of cycles. The number of cycles may depend upon a type of rejection condition or upon other factors affecting reissue or relaunch of the instruction. In this example, the load reorder queue ( 304 ) or the store reorder queue ( 306 ) may reissue or relaunch an instruction by providing the load/store access queue ( 224 ) with information to reissue the instruction, where the load reorder queue ( 304 ) or the store reorder queue ( 306 ) may communicate with the load/store access queue ( 224 ) along line ( 314 ). 
     Another improvement that results from the load/store slice ( 222   a ) maintaining an instruction if a rejection condition is determined is that the load/store slice ( 222   a ) uses fewer resources, such as logic and circuitry for latches and other components, to maintain the instruction than an instruction sequencing unit. In other words, given that the instruction sequencing unit may rely on the load/store slice ( 222   a ) in handling the instruction to completion, the instruction sequencing unit may free resources once the instruction is provided to the load/store slice ( 222   a ). 
     Further, the instruction sequencing unit ( 240 ), based at least in part on communications with the load/store slice ( 222   a ), may determine when and whether to wake instructions that may be dependent on a current instruction being handled by the load/store slice ( 222   a ). Therefore, if the load/store slice ( 222   a ) determines that a rejection condition exists, the load/store slice ( 222   a ) delays a notification to the instruction sequencing unit ( 240 ) to awaken dependent instructions to prevent the instruction sequencing unit ( 240 ) from issuing dependent instructions that are subsequently unable to finish due to lack of availability of results from a current instruction. In this way, the instruction sequencing unit ( 240 ) may avoid wasting execution cycles reissuing dependent instructions that are unable to finish. 
     For example, the load/store slice ( 222   a ) may communicate with the instruction sequencing unit ( 240 ) through the generation of signals indicating, at different points in handling a load instruction, that a load instruction is to be reissued or that data for a load instruction is valid. In some cases, in response to the instruction sequencing unit ( 240 ) receiving a signal from the load/store slice ( 222   a ) that a given instruction is to be reissued, the instruction sequencing unit ( 240 ) may awaken instructions dependent upon the given instruction with the expectation that the given instruction, after being reissued, is going to finish and provide valid data. 
     The load/store slice ( 222   a ) may also retrieve data from any tier of a memory hierarchy, beginning with a local data cache ( 232 ), and extending as far down in the hierarchy as needed to find requested data. The requested data, when received, may be provided to general purpose registers, virtual registers, or to some other destination. The received data may also be stored in a data cache ( 232 ) for subsequent access. The load/store slice ( 222   a ) may also manage translations of effective addresses to real addresses to communicate with different levels of memory hierarchy. 
     A store reorder queue ( 306 ) may include entries for tracking the cache operations for sequential consistency and may reissue operations into the load/store pipeline for execution independent of an execution slice. 
     A load miss queue ( 308 ) may issue requests for data to one or more data storage devices of a multi-tiered memory hierarchy, where a request for data may correspond to a load instruction for the data. 
     Responsive to the data being returned along the line ( 302 ) to the load/store slice ( 222   a ), the data may be delivered to a destination such as the results bus ( 220  of  FIG. 2 ) to be loaded into, for example, a general purpose register—where the delivery of the data may be from the data cache ( 232 ) or over the line ( 310 ). The line ( 310 ) bypasses the data cache ( 232 ) and allows implementation of a critical data forwarding path. The load reorder queue ( 304 ) may also use line ( 312 ) to notify an instruction sequencing unit, or some other logical component, that the data is available. 
     A load reorder queue ( 304 ) may track execution of cache operations issued to the load/store slice ( 222   a ) and includes entries for tracking cache operations for sequential consistency, among other attributes. The load reorder queue ( 304 ) may also reissue operations into the load/store pipeline for execution, which provides operation that is independent of the execution slices. 
     For further explanation,  FIG. 4  sets forth a flow chart illustrating an exemplary method for preventing hazard flushes in an instruction sequencing unit of a multi-slice processor. The method of  FIG. 4  may be carried out by a multi-slice processor similar to that in the examples of  FIGS. 1-3 . Such a multi-slice processor may include an instruction sequencing unit ( 240 ), and a plurality of load/store slices ( 220   a - 220   n ), where each of the load/store slices may implement a load/store access queue ( 224 ), a load reorder queue ( 304 ), and a store reorder queue ( 306 ), as described above with regard to  FIG. 3 . 
     The method of  FIG. 4  includes receiving ( 402 ) a load instruction in a load reorder queue ( 304 ), wherein the load instruction is an instruction to load data from a memory location. Receiving ( 402 ) a load instruction in a load reorder queue ( 304 ), wherein the load instruction is an instruction to load data from a memory location may be carried out by issuing, from the load/store unit, the load instruction and placing the load instruction in the load reorder queue ( 304 ). A load instruction is an instruction to read data from a memory location, and copy that data to a register, such as the general purpose register. 
     The method of  FIG. 4  also includes, subsequent to receiving the load instruction, receiving ( 404 ) a store instruction in a store reorder queue ( 306 ), wherein the store instruction is an instruction to store data in the memory location. Receiving a store instruction in a store reorder queue ( 306 ), wherein the store instruction is an instruction to store data in the memory location may be carried out by issuing, from the load/store unit, the store instruction and placing the store instruction in the store reorder queue ( 306 ). The store instruction is an instruction to read data from a register, such as a general purpose register, and copy that data to a memory location. 
     The method of  FIG. 4  also includes determining ( 406 ) that the store instruction causes a hazard against the load instruction. Determining that the store instruction causes a hazard against the load instruction may be carried out by detecting a store-hit-load hazard based on receiving an older store instruction targeting a memory location and determining that a younger load instruction resides in the load reorder queue ( 304 ) that targets the same memory location. 
     The store instruction may target data that the load instruction is attempting to read from the memory location. However, if the store instruction has not stored the targeted data into the memory location, the load instruction may read stale data (i.e., data stored by a previous instruction) from the memory location. This condition will cause the instruction sequencing unit ( 240 ) to detect a store-hit-load hazard. Further, load and store instructions may be dependent on multiple other load and store instructions, and any one of the dependencies may trigger a hazard. 
     The method of  FIG. 4  also includes preventing ( 408 ) a flush of the load reorder queue ( 304 ) based on a state of the load instruction. Preventing a flush of the load reorder queue ( 304 ) based on a state of the load instruction may be carried out by obtaining the state of the load instruction. The state of the load instruction refers to a level of completeness of the load instruction. Upon first issuing or waking up, the load instruction may attempt to retrieve the target data from a memory location. This may include attempting to read the data from the L1 cache. If the cacheline is not currently in the L1 cache, then a fetching mechanism (e.g., the load miss queue ( 308 )) may be employed to fetch the data from other memory. While waiting for the fetching mechanism to return the data, the load instruction is in a waiting state. 
     Preventing the flush of a load instruction may be carried out by avoiding the processes that cause a flush. A flush of a load instruction may be carried out by incrementing an age indicator of the corresponding store instruction. This may cause the instruction sequencing unit to flush load instructions in the load reorder queue ( 304 ) that are younger than the store instruction. A flush of a load instruction may also be carried out by obtaining the age of the all load instructions causing the hazard and flushing the oldest load instruction. Flushing the load replaces the incorrect load result in the general purpose register and re-executes instructions that were dependent on the result of the load instruction that may have previously executed. Consequently, flushing the load instruction may delay the execution of the load instruction and any other instruction dependent upon that load. 
     The method of  FIG. 4  also includes re-executing ( 410 ) the load instruction. Re-executing the load instruction may be carried out by re-executing the load instruction without having the load instruction re-issued to the load reorder queue ( 304 ). Specifically, re-executing ( 410 ) the load instruction may be carried out by waking up the load instruction previously placed in the load reorder queue ( 304 ) and executing the load instruction from the current state of the load instruction. 
     For further explanation,  FIG. 5  sets forth a flow chart illustrating an exemplary method for preventing hazard flushes in an instruction sequencing unit of a multi-slice processor according to embodiments of the present invention that includes receiving ( 402 ) a load instruction in a load reorder queue ( 304 ), wherein the load instruction is an instruction to load data from a memory location; subsequent to receiving the load instruction, receiving ( 404 ) a store instruction in a store reorder queue ( 306 ), wherein the store instruction is an instruction to store data in the memory location; determining ( 406 ) that the store instruction causes a hazard against the load instruction; preventing ( 408 ) a flush of the load reorder queue ( 304 ) based on a state of the load instruction; and re-executing ( 410 ) the load instruction. 
     The method of  FIG. 5  differs from the method of  FIG. 4 , however, in that the method of  FIG. 5  further includes forwarding ( 502 ) the data targeted by the store instruction using a store-forwarding mechanism. Forwarding ( 502 ) the data targeted by the store instruction using a store-forwarding mechanism may be carried out by sending the data targeted by the store instruction directly to the register entry targeted by the load instruction. Such a store-forwarding mechanism may bypass storing the target data in the memory location targeted by either the store instruction or load instruction. 
     For further explanation,  FIG. 6  sets forth a flow chart illustrating a further exemplary method for preventing hazard flushes in an instruction sequencing unit of a multi-slice processor according to embodiments of the present invention that includes receiving ( 402 ) a load instruction in a load reorder queue ( 304 ), wherein the load instruction is an instruction to load data from a memory location; subsequent to receiving the load instruction, receiving ( 404 ) a store instruction in a store reorder queue ( 306 ), wherein the store instruction is an instruction to store data in the memory location; determining ( 406 ) that the store instruction causes a hazard against the load instruction; preventing ( 408 ) a flush of the load reorder queue ( 304 ) based on a state of the load instruction; and re-executing ( 610 ) the load instruction. 
     The method of  FIG. 6  differs from the method of  FIG. 4 , however, in that preventing ( 408 ) a flush of the load reorder queue ( 304 ) based on a state of the load instruction includes determining ( 602 ) that the load instruction has not finished and has not sent results to a general purpose register; and setting ( 604 ) a prevent critical data forwarding flag in the load reorder queue ( 304 ). Determining ( 602 ) that the load instruction has not finished and has not sent results to a general purpose register may be carried out by querying the load miss queue ( 306 ) to determine whether the access request for the target memory location is pending. Determining ( 602 ) that the load instruction has not finished and has not sent results to a general purpose register may also be carried out by inspecting the target register entry to determine whether the load instruction has placed data in the register entry. 
     Setting ( 604 ) a prevent critical data forwarding flag in the load reorder queue ( 304 ) may be carried out by altering a bit stored in the load reorder queue ( 304 ) to indicate that no critical data forwarding is to be performed for the associated load instruction. Setting the prevent critical data forwarding flag prevents the load reorder queue ( 304 ) entry for the load instructions from waking up for a period of time, such as a single cycle. During the following period of time, the prevent critical data forwarding flag will be reset and the load instruction will be woken up. The prevent critical data forwarding flag may be stored in the load reorder queue ( 304 ). 
     The method of  FIG. 6  also differs from the method of  FIG. 4  in that re-executing ( 610 ) the load instruction includes performing ( 606 ) a load-hit-store check on the store reorder queue ( 306 ). Performing ( 606 ) a load-hit-store check on the store reorder queue ( 306 ) may be carried out by detecting that the store instruction is in the store reorder queue ( 306 ) and that load instruction has re-executed. This may cause a load-hit-store hazard. In response to the load-hit-store hazard, the instruction sequencing unit may forward the data targeted by the store instruction to the load instruction using a store-forwarding mechanism. Alternatively, the instruction sequencing unit may reject the load instruction and re-execute the load instruction if the data is unavailable for store-forwarding. 
     For further explanation,  FIG. 7  sets forth a flow chart illustrating a further exemplary method for preventing hazard flushes in an instruction sequencing unit of a multi-slice processor according to embodiments of the present invention that includes receiving ( 402 ) a load instruction in a load reorder queue ( 304 ), wherein the load instruction is an instruction to load data from a memory location; subsequent to receiving the load instruction, receiving ( 404 ) a store instruction in a store reorder queue ( 306 ), wherein the store instruction is an instruction to store data in the memory location; determining ( 406 ) that the store instruction causes a hazard against the load instruction; preventing ( 408 ) a flush of the load reorder queue ( 304 ) based on a state of the load instruction; and re-executing ( 410 ) the load instruction. 
     The method of  FIG. 7  differs from the method of  FIG. 4 , however, in that the method of  FIG. 7  further includes receiving ( 702 ) a subsequent store instruction in the store reorder queue ( 306 ), wherein the subsequent store instruction is an instruction to store the data in the memory location. Receiving ( 702 ) a subsequent store instruction in the store reorder queue ( 306 ), wherein the subsequent store instruction is an instruction to store the data in the memory location may be carried out by issuing, from the load/store unit, the subsequent store instruction and placing the subsequent store instruction in the store reorder queue ( 306 ). 
     The method of  FIG. 7  further includes determining ( 704 ) that the subsequent store instruction causes a subsequent hazard against the load instruction. Determining ( 704 ) that the subsequent store instruction causes a subsequent hazard against the load instruction may be carried out by determining that a load targeting the same memory location has previously been issued, as discussed above in step  406 . 
     More than one store instruction targeting the same memory location may be received after the load instruction has issued. In such a case, upon receiving each store instruction, the hazard detection mechanism is employed to determine the existence of a store-hit-load hazard. If a hazard is detected, flushing the load instruction may be avoided based on the state of the load instruction. 
     The method of  FIG. 7  further includes flushing ( 706 ) a portion of the load reorder queue ( 304 ) based on the subsequent hazard. Flushing ( 706 ) a portion of the load reorder queue ( 304 ) based on the subsequent hazard may be carried out by incrementing an age indicator of the corresponding store instruction. This may cause the instruction sequencing unit to flush load instructions in the load reorder queue ( 304 ) that are younger than the store instruction. A flush of a load instruction may also be carried out by obtaining the age of the all load instructions causing the hazard and flushing the oldest load instruction. Flushing the load replaces the incorrect load result in the general purpose register and re-executes instructions that were dependent on the result of the load instruction that may have previously executed. 
     Flushing ( 706 ) a portion of the load reorder queue ( 304 ) based on the subsequent hazard may further be in response to determining that the state of the load instruction does not support preventing the flush of the load instruction. For example, the load instruction may have, at the time the hazard is detected, completed fetching the target data from the L1 cache. As a result, the load instruction will have fetched stale data because the store has not placed the target data into the memory location. Therefore, the instruction sequencing unit will perform a flush of the load instruction. 
     Exemplary embodiments of the present invention are described largely in the context of a fully functional computer system for preventing hazard flushes in an instruction sequencing unit of a multi-slice processor. Readers of skill in the art will recognize, however, that the present invention also may be embodied in a computer program product disposed upon computer readable storage media for use with any suitable data processing system. Such computer readable storage media may be any storage medium for machine-readable information, including magnetic media, optical media, or other suitable media. Examples of such media include magnetic disks in hard drives or diskettes, compact disks for optical drives, magnetic tape, and others as will occur to those of skill in the art. Persons skilled in the art will immediately recognize that any computer system having suitable programming means will be capable of executing the steps of the method of the invention as embodied in a computer program product. Persons skilled in the art will recognize also that, although some of the exemplary embodiments described in this specification are oriented to software installed and executing on computer hardware, nevertheless, alternative embodiments implemented as firmware or as hardware are well within the scope of the present invention. 
     The present invention may be a system, a method, and/or a computer program product. The computer program product may include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the present invention. 
     The computer readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanically encoded device such as punch-cards or raised structures in a groove having instructions recorded thereon, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiber-optic cable), or electrical signals transmitted through a wire. 
     Computer readable program instructions described herein can be downloaded to respective computing/processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and/or a wireless network. The network may comprise copper transmission cables, optical transmission fibers, wireless transmission, routers, firewalls, switches, gateway computers and/or edge servers. A network adapter card or network interface in each computing/processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing/processing device. 
     Computer readable program instructions for carrying out operations of the present invention may be assembler instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Smalltalk, C++ or the like, and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The computer readable program instructions may execute entirely on the user&#39;s computer, partly on the user&#39;s computer, as a stand-alone software package, partly on the user&#39;s computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user&#39;s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present invention. 
     Aspects of the present invention are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer readable program instructions. 
     These computer readable program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer readable program instructions may also be stored in a computer readable storage medium that can direct a computer, a programmable data processing apparatus, and/or other devices to function in a particular manner, such that the computer readable storage medium having instructions stored therein comprises an article of manufacture including instructions which implement aspects of the function/act specified in the flowchart and/or block diagram block or blocks. 
     The computer readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable apparatus or other device to produce a computer implemented process, such that the instructions which execute on the computer, other programmable apparatus, or other device implement the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of instructions, which comprises one or more executable instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instructions. 
     It will be understood from the foregoing description that modifications and changes may be made in various embodiments of the present invention without departing from its true spirit. The descriptions in this specification are for purposes of illustration only and are not to be construed in a limiting sense. The scope of the present invention is limited only by the language of the following claims.