i965: Document conventions for counting layers in 2D multisample buffers.

The "layer" parameters used in blorp, and the
intel_renderbuffer::mt_layer field, represent a physical layer rather
than a logical layer.  This is important for 2D multisample arrays on
Gen7+ because the UMS and CMS multisample layouts use N physical
layers to represent each logical layer, where N is the number of
samples.

Also add an assertion to blorp to help catch bugs if we fail to follow
these conventions.
i965: Don't try to use HW blitter for glCopyPixels() when multisampled.

The hardware blitter doesn't understand multisampled layouts, so
there's no way this could possibly succeed.
i965/gen7+: Disentangle MSAA layout from fast clear state.

This patch renames the enum that's used to keep track of fast clear
state from "mcs_state" to "fast_clear_state", and it removes the enum
value INTEL_MCS_STATE_MSAA (which previously meant, "this is an MSAA
buffer, so we're not keeping track of fast clear state").  The only
real purpose that enum value was serving was to prevent us from trying
to do fast clear resolves on MSAA buffers, and it's just as easy to
prevent that by checking the buffer's msaa_layout.

This paves the way for implementing fast clears of MSAA buffers.
i965/blorp: Get rid of redundant num_samples blorp param.

Previously, brw_blorp_params contained two fields for determining
sample count: num_samples (which determined the multisample
configuration of the rendering pipeline) and dst.num_samples (which
determined the multisample configuration of the render target
surface).  This was redundant, since both fields had to be set to the
same value to avoid rendering errors.

This patch eliminates num_samples to avoid future confusion.
i965/blorp: allow multisample blorp clears

Previously, we didn't do multisample blorp clears because we couldn't
figure out how to get them to work.  The reason for this was because
we weren't setting the brw_blorp_params num_samples field consistently
with dst.num_samples.  Now that those two fields have been collapsed
down into one, we can do multisample blorp clears.

However, we need to do a few other pieces of bookkeeping to make them
work correctly in all circumstances:

- Since blorp clears may now operate on multisampled window system
  framebuffers, they need to call
  intel_renderbuffer_set_needs_downsample() to ensure that a
  downsample happens before buffer swap (or glReadPixels()).

- When clearing a layered multisample buffer attachment using UMS or
  CMS layout, we need to advance layer by multiples of num_samples
  (since each logical layer is associated with num_samples physical
  layers).

Note: we still don't do multisample fast color clears; more work needs
to be done to enable those.
i965/blorp: Refactor code for computing fast clear align/scaledown factors.

This will make it easier to add fast color clear support to MSAA
buffers, since they have different alignment and scaling requirements.
i965/gen7+: Implement fast color clears for MSAA buffers.

Fast color clears of MSAA buffers work just like fast color clears
with non-MSAA buffers, except that the alignment and scaledown
requirements are different.
glsl: Extract functions from loop_analysis::visit(ir_dereference_variable *).

This function is about to get more complex.
glsl: Fix loop analysis of nested loops.

Previously, when visiting a variable dereference, loop analysis would
only consider its effect on the innermost enclosing loop.  As a
result, when encountering a loop like this:

    for (int i = 0; i < 3; i++) {
      for (int j = 0; j < 3; j++) {
        ...
        i = 2;
      }
    }

it would incorrectly conclude that the outer loop ran three times.

Fixes piglit test "vs-inner-loop-modifies-outer-loop-var.shader_test".
glsl: Fix handling of function calls inside nested loops.

Previously, when visiting an ir_call, loop analysis would only mark
the innermost enclosing loop as containing a call.  As a result, when
encountering a loop like this:

    for (i = 0; i < 3; i++) {
      for (int j = 0; j < 3; j++) {
        foo();
      }
    }

it would incorrectly conclude that the outer loop ran three times.
(This is not certain; if foo() modifies i, then the outer loop might
run more or fewer times).

Fixes piglit test "vs-call-in-nested-loop.shader_test".
glsl: In loop analysis, handle unconditional second assignment.

Previously, loop analysis would set
this->conditional_or_nested_assignment based on the most recently
visited assignment to the variable.  As a result, if a vaiable was
assigned to more than once in a loop, the flag might be set
incorrectly.  For example, in a loop like this:

    int x;
    for (int i = 0; i < 3; i++) {
      if (i == 0)
        x = 10;
      ...
      x = 20;
      ...
    }

loop analysis would have incorrectly concluded that all assignments to
x were unconditional.

In practice this was a benign bug, because
conditional_or_nested_assignment is only used to disqualify variables
from being considered as loop induction variables or loop constant
variables, and having multiple assignments also disqualifies a
variable from being considered as either of those things.

Still, we should get the analysis correct to avoid future confusion.
glsl/loops: consolidate bounded loop handling into a lowering pass.

Previously, all of the back-ends (ir_to_mesa, st_glsl_to_tgsi, and the
i965 fs and vec4 visitors) had nearly identical logic for handling
bounded loops.  This replaces the duplicate logic with an equivalent
lowering pass that is used by all the back-ends.

Note: on i965, there is a slight increase in instruction count.  For
example, a loop like this:

    for (int i = 0; i < 100; i++) {
      total += i;
    }

would previously compile down to this (vec4) native code:

          mov(8)       g4<1>.xD 0D
          mov(8)       g8<1>.xD 0D
    loop:
          cmp.ge.f0(8) null     g8<4;4,1>.xD 100D
    (+f0) break(8)
          add(8)       g5<1>.xD g5<4;4,1>.xD g4<4;4,1>.xD
          add(8)       g8<1>.xD g8<4;4,1>.xD 1D
          add(8)       g4<1>.xD g4<4;4,1>.xD 1D
          while(8) loop

After this patch, the "(+f0) break(8)" turns into:

    (+f0) if(8)
          break(8)
          endif(8)

because the back-end isn't smart enough to recognize that "if
(condition) break;" can be done using a conditional break instruction.
However, it should be relatively easy for a future peephole
optimization to properly optimize this.
glsl/loops: replace loop controls with a normative bound.

This patch replaces the ir_loop fields "from", "to", "increment",
"counter", and "cmp" with a single integer ("normative_bound") that
serves the same purpose.

I've used the name "normative_bound" to emphasize the fact that the
back-end is required to emit code to prevent the loop from running
more than normative_bound times.  (By contrast, an "informative" bound
would be a bound that is informational only).
glsl/loops: Remove unused fields iv_scale and biv from loop_variable class.

These fields were part of some planned optimizations that never
materialized.  Remove them for now to simplify things; if we ever get
round to adding the optimizations that would require them, we can
always re-introduce them.
glsl/loops: Remove unnecessary list walk from loop_control_visitor.

When loop_control_visitor::visit_leave(ir_loop *) is analyzing a loop
terminator that acts on a certain ir_variable, it doesn't need to walk
the list of induction variables to find the loop_variable entry
corresponding to the variable.  It can just look it up in the
loop_variable_state hashtable and verify that the loop_variable entry
represents an induction variable.
glsl/loops: Allocate loop_terminator using new(mem_ctx) syntax.

Patches to follow will introduce code into the loop_terminator
constructor.  Allocating loop_terminator using new(mem_ctx) syntax
will ensure that the constructor runs.
glsl/loops: Move some analysis from loop_controls to loop_analysis.

Previously, the sole responsibility of loop_analysis was to find all
the variables referenced in the loop that are either loop constant or
induction variables, and find all of the simple if statements that
might terminate the loop.  The remainder of the analysis necessary to
determine how many times a loop executed was performed by
loop_controls.

This patch makes loop_analysis also responsible for determining the
number of iterations after which each loop terminator will terminate
the loop, and for figuring out which terminator will terminate the
loop first (I'm calling this the "limiting terminator").

This will allow loop unrolling to make use of information that was
previously only visible from loop_controls, namely the identity of the
limiting terminator.
glsl/loops: Simplify loop unrolling logic by breaking into functions.

The old logic of loop_unroll_visitor::visit_leave(ir_loop *) was:

    heuristics to skip unrolling in various circumstances;
    if (loop contains more than one jump)
      return;
    else if (loop contains one jump) {
      if (the jump is an unconditional "break" at the end of the loop) {
        remove the break and set iteration count to 1;
        fall through to simple loop unrolling code;
      } else {
        for (each "if" statement in the loop body)
          see if the jump is a "break" at the end of one of its forks;
        if (the "break" wasn't found)
          return;
        splice the remainder of the loop into the other fork of the "if";
        remove the "break";
        complex loop unrolling code;
        return;
      }
    }
    simple loop unrolling code;
    return;

These tasks have been moved to their own functions:
- splice the remainder of the loop into the other fork of the "if"
- simple loop unrolling code
- complex loop unrolling code

And the logic has been flattened to:

    heuristics to skip unrolling in various circumstances;
    if (loop contains more than one jump)
      return;
    if (loop contains no jumps) {
      simple loop unroll;
      return;
    }
    if (the jump is an unconditional "break" at the end of the loop) {
      remove the break;
      simple loop unroll with iteration count of 1;
      return;
    }
    for (each "if" statement in the loop body) {
      if (the jump is a "break" at the end of one of its forks) {
        splice the remainder of the loop into the other fork of the "if";
        remove the "break";
        complex loop unroll;
        return;
      }
    }

This will make it easier to modify the loop unrolling algorithm in a
future patch.
glsl/loops: Get rid of loop_variable_state::max_iterations.

This value is now redundant with
loop_variable_state::limiting_terminator->iterations and
ir_loop::normative_bound.
glsl/loops: Stop creating normatively bound loops in loop_controls.

Previously, when loop_controls analyzed a loop and found that it had a
fixed bound (known at compile time), it would remove all of the loop
terminators and instead set the loop's normative_bound field to force
the loop to execute the correct number of times.

This made loop unrolling easy, but it had a serious disadvantage.
Since most GPU's don't have a native mechanism for executing a loop a
fixed number of times, in order to implement the normative bound, the
back-ends would have to synthesize a new loop induction variable.  As
a result, many loops wound up having two induction variables instead
of one.  This caused extra register pressure and unnecessary
instructions.

This patch modifies loop_controls so that it doesn't set the loop's
normative_bound anymore.  Instead it leaves one of the terminators in
the loop (the limiting terminator), so the back-end doesn't have to go
to any extra work to ensure the loop terminates at the right time.

This complicates loop unrolling slightly: when deciding whether a loop
can be unrolled, we have to account for the presence of the limiting
terminator.  And when we do unroll the loop, we have to remove the
limiting terminator first.

For an example of how this results in more efficient back end code,
consider the loop:

    for (int i = 0; i < 100; i++) {
      total += i;
    }

Previous to this patch, on i965, this loop would compile down to this
(vec4) native code:

          mov(8)       g4<1>.xD 0D
          mov(8)       g8<1>.xD 0D
    loop:
          cmp.ge.f0(8) null     g8<4;4,1>.xD 100D
    (+f0) if(8)
          break(8)
          endif(8)
          add(8)       g5<1>.xD g5<4;4,1>.xD g4<4;4,1>.xD
          add(8)       g8<1>.xD g8<4;4,1>.xD 1D
          add(8)       g4<1>.xD g4<4;4,1>.xD 1D
          while(8) loop

(notice that both g8 and g4 are loop induction variables; one is used
to terminate the loop, and the other is used to accumulate the total).

After this patch, the same loop compiles to:

          mov(8)       g4<1>.xD 0D
    loop:
          cmp.ge.f0(8) null     g4<4;4,1>.xD 100D
    (+f0) if(8)
          break(8)
          endif(8)
          add(8)       g5<1>.xD g5<4;4,1>.xD g4<4;4,1>.xD
          add(8)       g4<1>.xD g4<4;4,1>.xD 1D
          while(8) loop