Patent Publication Number: US-6338160-B1

Title: Constant pool reference resolution method

Description:
RELATED U.S. APPLICATIONS 
     This application is a continuation-in-part of the application “Java Virtual Machine Hardware for RISC and CISC Processors”, Ser. No. 09/208,741, filed Dec. 8, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     Java™ is an object oriented programming language developed by Sun Microsystems. The Java language is small, simple and portable across platforms and operating systems, both at the source and at the binary level. This makes the Java programming language very popular on the Internet. 
     Platform independence and code compaction are the most significant advantages of Java over conventional programming languages. In conventional programming languages, the source code of a program is sent to a compiler which interprets the program into machine code or processor instructions. The processor instructions are native to the system&#39;s processor. If the code is compiled on an Intel-based system, the resulting program will only run on other Intel-based systems. If it is desired to run the program on another system, the user must go back to the original source code, obtain a compiler for the new processor, and recompile the program into the machine code specific to that other processor. 
     Java operates differently. The Java compiler takes a Java program and, instead of generating machine code for a particular processor, generates bytecodes. Bytecodes are instructions that look like machine code, but aren&#39;t specific to any processor. To execute a Java program, a bytecode interpreter takes the Java bytecode converts them to equivalent native processor instructions and executes the Java program. The Java byte code interpreter is one component of the Java Virtual Machine. 
     Having the Java programs in bytecode form means that instead of being specific to any one system, the programs can run on any platform and any operating system as long a Java Virtual Machine is available. This allows a binary bytecode file to be executable across platforms. 
     Most computer languages, such as C, are compiled languages. All references to objects are resolved before running of the program. Because Java is run from the Virtual Machine, it is possible to operate without having all of the references resolved. The advantage of this arrangement is that it allows for operation of the program or the program is completely downloaded from another location, such as off the Internet. 
     For example, a Java instruction that invokes ‘ABC’ may be run when the Class ‘ABC’ is not loaded into memory. The Invoke instruction must cause the reference to be resolved. This can take a considerable amount of time to resolve the reference each time that the instruction is run. Instructions that reference the constant pool, such as invoke instructions, often can have this problem. 
     One version of a Java Virtual Machine can reduce this problem with the use of quick variants of bytecodes. The quick variants of bytecodes are not officially part of the Java Virtual Machine specification and are invisible outside specific Java Virtual Machine implementations. When the quick optimization is turned on, each non-quick bytecode resolves the specified item in the constant pool, signals if an item in the constant pool could not be resolved for some reason, turns itself into the quick variant of itself, and then performs its intended operation. Thus, the bytecode is written over by the quick bytecode variant. The next time the code is run, the system assumes that the item in the constant pool has already been resolved, and that this resolution did not produce any errors. The system can then simply performs the intended operation on the resolved item. A discussion of such a system is described in Gosling, U.S. Pat. No. 5,367,685 incorporated herein by reference. 
     This optimization cannot be run from read-only memory (ROM) because the optimization requires writing over the normal bytecode with its quick variant. Running a program from read-only memory could be quite useful in some circumstances particularly for embedded systems. It is desired to have an improved method of resolving the constant pool references to avoid some of the problems of the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention, a Resolution Data Field, is used in the constant pool entries. When a reference to the constant pool is done by a bytecode, the data in the resolution data field acts as an index to a jump table to determine the native code to be run next. The first time a bytecode, which references the constant pool, is run, the data in the resolution data field causes the system to jump to code to resolve the reference. This resolution code could search for the object reference bytecode within the memory. If the object is not within the memory, the system can load the object into memory and then operate in the normal bytecode operation. This system also sets the resolution data field, so as to indicate that the reference has been resolved. An indication of the location of the resolved object is also stored within the constant pool entry. The next time the bytecode is operated on, the data in the resolution data field acts as an index to the jump table which causes a jump to operation code that assumes that the resolution has been resolved. In this way, after the first operation of the bytecode, searching for the object in memory need not be done. 
     The advantage of the present invention is that it is not required to write the bytecode over with another value, such as a quick bytecode. Thus, the bytecode could be run from read-only memory. The advantage of using a jump table in the preferred embodiment is that the data in the resolution data field need only be added to a base value rather than compared to a value. In most current processors, the add step can be done quicker than a compare step. In an alternate embodiment, a comparison of the data in the resolution data field could be done. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be further understood from the following description in conjunction with the drawings. 
     FIG. 1 is a diagram of the system of the parent case including the hardware Java accelerator. 
     FIG. 2 is a diagram illustrating the use of the hardware Java accelerator of the parent case. 
     FIG. 3 is a diagram illustrating some the details of a Java hardware accelerator of one embodiment of the parent case. 
     FIG. 4 is a diagram illustrating the details of one embodiment of a Java accelerator instruction translation in the system of the parent case. 
     FIG. 5 is a diagram illustration the instruction translation operation of one embodiment of the parent case. 
     FIG. 6 is a diagram illustrating the instruction translation system of one embodiment of the parent case using instruction level parallelism. 
     FIG. 7 is a table showing one possible list of bytecodes which can cause exceptions in a preferred embodiment of the parent case. 
     FIG. 8 is a flow chart illustrating the operation of the present invention. 
     FIG. 9A and 9B are diagrams illustrating the operation of one embodiment of the present invention 
     FIG. 10 is a diagram illustrating one embodiment of the constant pool entry for use with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1-7 illustrate the operation of one embodiment of the application of the parent case. FIGS. 8-10 illustrate the operation of the present invention. 
     FIG. 1 is a diagram of the system  20  showing the use of a hardware Java accelerator  22  in conjunction with a central processing unit  26 . The Java hardware accelerator  22  allows part of the Java Virtual Machine to be implemented in hardware. This hardware implementation speeds up the processing of the Java byte codes. In particular, in a preferred embodiment, the translation of the Java bytecodes into native processor instructions is at least partially done in the hardware Java accelerator  22 . This translation has been part of a bottleneck in the Java Virtual Machine when implemented in software. In FIG. 1, instructions from the instruction cache  24  or other memory is supplied to the hardware Java accelerator  22 . If these instruction are Java bytecode, the hardware Java accelerator  22  can convert these bytecodes into native processor instruction which are supplied through the multiplexer  28  to the CPU. If a non-Java code is used, the hardware accelerator can be by-passed using the multiplexer  26 . 
     The Java hardware accelerator can do, some or all of the following tasks: 
     1. Java bytecode decode; 
     2. identifying and encoding instruction level parallelism (ILP), wherever possible; 
     3. interpreting bytecodes to native instructions; 
     4. managing the Java stack on a register file associated with the CPU or as a separate stack; 
     5. generating exceptions on instructions on predetermined Java byte codes; 
     6. switching to native CPU operation when native CPU code is provided; 
     7. performing bounds checking on array instructions; and 
     8. managing the variables on the register file associated with the CPU. 
     In a preferred embodiment, the Java Virtual Machine functions of bytecode interpreter, Java register, and Java stack are implemented in the hardware Java accelerator. The garbage collection heap and constant pool area can be maintained in normal memory and accessed through normal memory referencing. 
     The major advantages of the Java hardware accelerator is to increase the speed in which the Java Virtual Machine operates, and allow existing native language legacy applications, software base, and development tools to be used. A dedicated microprocessor in which the Java bytecodes were the native instructions would not have access to those legacy applications. 
     Although the Java hardware accelerator is shown in FIG. 1 as separate from the central processing unit, the Java hardware accelerator can be incorporated into a central processing unit. In that case, the central processing unit has a Java hardware accelerator subunit to interpret Java bytecode into the native instructions operated on by the main portion of the CPU. 
     FIG. 2 is a state machine diagram that shows the operation of one embodiment of the parent case. Block  32  is the power-on state. During power-on, the multiplexer  28  is set to bypass the Java hardware accelerator. In block  34 , the native instruction boot-up sequence is run. Block  36  shows the system in the native mode executing native instructions and by-passing the Java hardware accelerator. 
     In block  38 , the system switches to the Java hardware accelerator mode. In the Java hardware accelerator mode, Java bytecode is transferred to the Java hardware accelerator  22 , converted into native instructions then sent to the CPU for operation. 
     The Java accelerator mode can produce exceptions at certain Java bytecodes. These bytecodes are not processed by the hardware accelerator  22  but are processed in the CPU  26 . As shown in block  40 , the system operates in the native mode but the Java Virtual Machine is implemented in the CPU which does the bytecode translation and handles the exception created in the Java accelerator mode. 
     The longer and more complicated bytecodes that are difficult to handle in hardware can be selected to produce the exceptions. FIG. 7 is a table showing one possible list of bytecodes which can cause exceptions in a preferred embodiment. An implementation of bytecode instruction operations that reference the constant pool is discussed below with respect to FIGS. 8-10. 
     FIG. 3 is a diagram illustrating details of one embodiment of the Java hardware accelerator of the parent case. The Java hardware accelerator includes Java accelerator instruction translation hardware  42 . One embodiment of the Java accelerator instruction translation hardware  42  is described in more detail below with respect to FIG.  4 . This instruction translation hardware  42  uses data stored in hardware Java registers  44 . The hardware Java Registers store the Java Registers defined in the Java Virtual Machine. The Java Registers contain the state of the Java Virtual Machine, affect its operation, and are updated after each bytecode is executed. The Java registers in the Java virtual machine include the PC, the program counter indicating what bytecode is being executed; Optop, a pointer to the top of the operand stack; Frame, a pointer to the execution environment of the current method; and Vars, a pointer to the first local variable available of the currently executing method. The virtual machine defines these registers to be a single 32-bit word wide. The Java registers are also stored in the Java stack which can be implemented as the hardware Java stack  50  or the Java stack can be stored into the CPU associated register file. 
     In a preferred embodiment, the hardware Java registers  44  can include additional registers for the use of the instruction translation hardware  42 . These registers can include a register indicating a switch to native instructions and a register indicating the version number of the system. 
     The Java PC can be used to obtain bytecode instructions from the instruction cache  24 . In one embodiment the Java PC is multiplexed with the normal program counter  54  of the central processing unit  26  in multiplexer  52 . The normal PC  54  is not used during the operation of the Java hardware bytecode translation. In another embodiment, the normal program counter  54  is used as the Java program counter. 
     The Java registers are a part of the Java Virtual Machine and should not be confused with the general registers  46  or  48  which are operated upon by the central processing unit  26 . In one embodiment, the system uses the traditional CPU register file  46  as well as a Java CPU register file  48 . When native code is being operated upon the multiplexer  56  connects the conventional register file  46  to the execution logic  26   c  of the CPU  26 . When the Java hardware accelerator is active, the Java CPU register file  48  substitutes for the conventional CPU register file  46 . In another embodiment, the conventional CPU register file  46  is used. 
     As described below with respect to FIGS. 3 and 4, the Java CPU register file  48 , or in an alternate embodiment the conventional CPU register file  46 , can be used to store portions of the operand stack and some of the variables. In this way, the native register-based instructions from the Java accelerator instruction translator  42  can operate upon the operand stack and variable values stored in the Java CPU register file  48 , or the values stored in the conventional CPU register file  46 . Data can be written in and out of the Java CPU register file  48  from the data cache or other memory  58  through the overflow/underflow line  60  connected to the memory arbiter  62 . The overflow/underflow transfer of data to and from the memory to can done concurrently with the CPU operation. Alternately, the overflow/underflow transfer can be done explicitly while the CPU is not operating. The overflow/underflow bus  60  can be implemented as a tri-state bus or as two separate buses to read data in and write data out of the register file when the Java stack overflows or underflows. 
     The register files for the CPU could alternately be implemented as a single register file with native instructions used to manipulate the loading of operand stack and variable values to and from memory. Alternately, multiple Java CPU register files could be used: one register file for variable values, another register file for the operand stack values, and another register file for the Java frame stack holding the method environment information. 
     The Java accelerator controller (co-processing unit)  64  can be used to control the hardware Java accelerator, read in and out from the hardware Java registers  44  and Java stack  50 , and flush the Java accelerator instruction translation pipeline upon a “branch taken” signal from the CPU execute logic  26   c.    
     The CPU  26  is divided into pipeline stages including the instruction fetch  26   a,  instruction decode  26   b,  execute logic  26   c,  memory access logic  26   d,  and writeback logic  26   e.  The execute logic  26   c  executes the native instructions and thus can determine whether a branch instruction is taken and issue the “branch taken” signal. 
     FIG. 4 illustrates an embodiment of a Java accelerator instruction translator which can be used with the parent case. The instruction buffer  70  stores the bytecode instructions from the instruction cache. The bytecodes are sent to a parallel decode unit  72  which decodes multiple bytecodes at the same time. Multiple bytecodes are processed concurrently in order to allow for instruction level parallelism. That is, multiple bytecodes may be converted into a lesser number of native instructions. 
     The decoded bytecodes are sent to a state machine unit  74  and Arithmetic Logic Unit (ALU)  76 . The ALU  76  is provided to rearrange the bytecode instructions to make them easier to be operated on by the state machine  74 . The state machine  74  converts the bytecodes into native instructions using the look-up table  78 . Thus, the state machine  74  provides an address which indicates the location of the desired native instruction in the look-up table  78  . Counters are maintained to keep a count of how many entries have been placed on the operand stack, as well as to keep track of the top of the operand stack. In a preferred embodiment, the output of the look-up table  78  is augmented with indications of the registers to be operated on at line  80 . The register indications are from the counters and interpreted from bytecodes. Alternately, these register indications can be sent directly to the Java CPU register file  48  shown in FIG.  3 . 
     The state machine  74  has access to the Java registers in  44  as well as an indication of the arrangement of the stack and variables in the Java CPU register file  48  or in the conventional CPU register file  46 . The buffer  82  supplies the interpreted native instructions to the CPU. 
     The operation of the Java hardware accelerator of one embodiment of the parent case is illustrated in FIGS. 5 and 6. FIG. 5, section I shows the instruction translation of the Java bytecode. The Java bytecode corresponding to the mnemonic iadd is interpreted by the Java virtual machine as an integer operation taking the top two values of the operand stack, adding them together and pushing the result on top of the operand stack. The Java translating machine interprets the Java bytecode into a native instruction such as the instruction ADD R 1 , R 2 . This is an instruction native to the CPU indicating the adding of value in register R 1  to the value in register R 2  and the storing of this result in register R 2 . R 1  and R 2  are the top two entries in the operand stack. 
     As shown in FIG. 5, section II, the Java register includes a PC value of “Value A” that is incremented to “Value A+1”. The Optop value changes from “Value B” to “Value B−1” to indicate that the top of the operand stack is at a new location. The Vars value which points to the top of the variable list is not modified. In FIG. 5, section III, the contents of a Java CPU register file, such as the Java CPU register file  48  in FIG. 3, is shown. The Java CPU register file starts off with registers R 0 -R 5  containing operand stack values and registers R 6 -R 7  containing variable values. Before the operation of the native instruction, register R 1  contains the top value of the operand stack. Register R 6  contains the first variable. After the execution of the native instruction, register R 2  now contains the top value of the operand stack. Register R 1  no longer contains a valid operand stack value and is available to be overwritten by a operand stack value from the memory sent across the overflow/underflow line  60  or from the bytecode stream. 
     FIG. 5, section IV, shows the memory locations of the operand stack and variables which can be stored in the data cache  58  or in main memory. For convenience, the memory is illustrated without illustrating any virtual memory scheme. Before the native instruction executes, the address of the top of the operand stack, Optop, is “Value B”. After the native instruction executes, the address of the top of the operand stack is “Value B−1” containing the result of the native instruction. Note that the operand stack value “4427” can be written into register R 1  across the overflow/underflow line  60 . Upon a switch back to the native mode, the data in the Java CPU register file  48  should be written to the data memory. 
     Consistency must be maintained between the Hardware Java Registers  44 , the Java CPU register file  48  and the data memory. The CPU  26  and Java Accelerator Instruction Translation Unit  42  are pipelined and any changes to the hardware java registers  44  and changes to the control information for the Java CPU register file  48  must be able to be undone upon a “branch taken” signal. The system preferably uses buffers (not shown) to ensure this consistency. Additionally, the Java instruction translation must be done so as to avoid pipeline hazards in the instruction translation unit and CPU. 
     FIG. 6 is a diagram illustrating the operation of instruction level parallelism with the parent case. In FIG. 6 the Java bytecodes iload_n and iadd are converted by the Java bytecode translator to the single native instruction ADD R 6 , R 1 . In the Java Virtual Machine, iload_n pushes the top local variable indicated by the by the Java register VAR onto the operand stack. 
     In the parent case the Java hardware translator can combine the iload_n and iadd bytecode into a single native instruction. As shown in FIG. 6, section II, the Java Register, PC, is updated from “Value A” to “Value A+2”. The Optop value remains “value B”. The value Var remains at “value C”. 
     As shown in FIG. 6, section III, after the native instruction ADD R 6 , R 1  executes the value of the first local variable stored in register R 6 , “1221”, is added to the value of the top of the operand stack contained in register R 1  and the result stored in register R 1 . In FIG. 6, section IV, the Optop value does not change but the value in the top of the register contains the result of the ADD instruction, 1371. 
     The Java hardware accelerator of the parent case is particularly well suited to a embedded solution in which the hardware accelerator is positioned on the same chip as the existing CPU design. This allows the prior existing software base and development tools for legacy applications to be used. In addition, the architecture of the present embodiment is scalable to fit a variety of applications ranging from smart cards to desktop solutions. This scalability is implemented in the Java accelerator instruction translation unit of FIG.  4 . For example, the lookup table  78  and state machine  74  can be modified for a variety of different CPU architectures. These CPU architectures include reduced instruction set computer (RISC) architectures as well as complex instruction set computer (CISC) architectures. The parent case can also be used with superscalar CPUs or very long instruction word (VLIW) computers. 
     The term Java in the specification or claims should be construed to cover successor programming languages or other programming languages using basic Java concepts (the use of generic instructions, such as bytecodes, to indicate the operation of a virtual machine). 
     FIG. 8 is a flow chart that illustrates the apparatus of one embodiment of the present invention. In step  120 , there is a check whether a bytecode references specific data structure, for example the constant pool. Every currently loaded class has a constant pool attached to it. The constant pool is allocated when a class is first loaded, the constants in this pool encode all the names (of variables, methods, and so forth) used by any method in the class. The class contains a count of how many constants there are and the start of the constant pool is available to the Java bytecodes. The bytecodes that reference the constant pool and include load constant instructions (ldc, ldc_w, ldc 2 _w ), anewarray allocation, multinewarray allocation, getfield operations, putfield operations, getstatic operations, putstatic operations, the invoke operations (invoke interface operations, invoke special operations, invoke static operations and invoke virtual operations), new operation, checkcast operations, and instanceof operations. 
     Step  120  is preferably done in hardware as described with respect to FIGS. 1-7 above. The references to the constant pool are software exceptions for that hardware implementation. Alternately, the present invention of FIGS. 8-10 can be completely implemented in software. 
     If the bytecode does not reference a constant pool, other steps  122  are done. If the bytecode does reference a constant pool, in step  124 , data is obtained from the specific data structure, i.e. the constant pool, including data from a resolution data field. The data from the resolution data field is used as an index to a jump table. In steps  126  and  128 , the operation of the jump table checks whether the reference is resolved. If the reference is resolved, the system jumps to do the normal operation on the resolved reference in block  130 . If the reference is not resolved, the system jumps to the resolution operation to check whether the object referred to is in memory. If the object referred to is not in memory, the object is loaded into memory in step  132 . The data in the data structure is modified to indicate in step  134  that the reference is resolved. Thus, the constant pool entry will indicate the location of the found or loaded reference. The resolution data field is also set, so as to indicate that the reference is resolved. In step  136 , the bytecode is done upon the resolved reference. Step  138  indicates getting the next bytecode. Note that the system moves to steps  126  and  128  since the resolution data field is set. The resolution data field will indicate that the reference is resolved. 
     Details of one embodiment of the present invention is shown with respect to FIG. 9A and 9B. Looking at FIG. 9A, different elements stored in the memory are shown. In the bytecode region  150 , includes a bytecode  150   a  that references the constant pool  152 . The operation of the invoke instruction has as one of its arguments, the index which points to entry  152   a  in the constant pool  152 . The constant pool entries include a resolution data field  160  and indication field  162 . In this case, the indication ‘5002’ points to address ‘5002’ that contains the data ‘ABC’. The resolution data field  160  indicates that the reference to the object has not been resolved. The first time that the instruction invoke  150   a  is run, the system obtains a string ‘ABC’ and uses the data in the data resolution field  160  as an index to the jump table  170 . The ‘0’ data in the data resolution field  160  causes the system to jump to location ‘9000’ in the native instruction region  172 . At address ‘9000’, the resolve instructions include code for the object search, the loading of the object if not in memory, the modification of the constant pool, and the normal invoke operations. FIG. 9B illustrates the operation of the native instruction starting at address ‘9000’. The object ‘ABC’ was not found in memory so is loaded starting at location ‘7500’. The address of the loaded object  174  is placed into the in the field  162 ′ of constant pool entry  152   a  . The data in the data resolution field  160 ′ is modified to ‘1’. The invoke bytecode  150   a  is not modified. 
     The next time the invoke bytecode  150   a  is run, the data in the data resolution field  160 ′ will cause a jump in the jump table  170  to location ‘9500’. This will be the native instruction for the invoke instruction. The contents of the field  162 ′ indicates the location of the loaded object  174 . Note that there can be different jump tables and instruction regions for each specific bytecode instruction referencing the constant pool. 
     FIG. 10 illustrates one embodiment of a constant pool entry  180 . The constant pool entry  180  includes a field  180   b  for the Java object address or nonJava address, also shown as the reference resolution field  180   a,  The reference resolution field can also indicate whether the reference is to a Java object. In one embodiment, “00” indicates an unresolved Java object, “01” indicates a resolved Java object, “10” indicates a numeric constant, and “11” indicates a text string. Field  180   c  contains garbage collection bits. 
     While the present invention has been described with reference to the above embodiments, this description of the preferred embodiments and methods is not meant to be construed in a limiting sense. It should also be understood that all aspects of the present invention are not to be limited to the specific descriptions, or to configurations set forth herein. Variations in the present invention will be apparent to a person skilled in the art upon reference to the present disclosure. It is therefore contemplated that the following claims will cover any such modifications or variations of the described embodiment as falling within the true spirit and scope of the present invention.