[Mesa-dev] [PATCH 1/2] i965: Do not apply CSE opt to MOV immediate

[Juan A. Suarez Romero]

If the shader asm code is something like:

mov vgrf2767.0:F, [13F, 14F, 15F, 16F]
mov vgrf2768.0:F, [9F, 10F, 11F, 12F]
mov m8:F, [13F, 14F, 15F, 16F]
mov m7:F, [9F, 10F, 11F, 12F]

And we apply Common Subexpresion Elimination optimization, we get the
following one:

mov vgrf2771.0:F, [13F, 14F, 15F, 16F]
mov vgrf2767.0:F, vgrf2771.xyzw:F
mov vgrf2772.0:F, [9F, 10F, 11F, 12F]
mov vgrf2768.0:F, vgrf2772.xyzw:F
mov m8:F, vgrf2771.xyzw:F
mov m7:F, vgrf2772.xyzw:F

The problem is that later we apply Copy Propagation optimization, which
reverts the change to the original one. If we run the optimizations in
a loop, there is always a progress, but we are in neverending loop.

Usually, when we have a sentence of the form:

X = exp

We apply CSE if "exp" is actually an expression. But if it is constant
we do not apply it, as the point of CSE is saving running the same
computation more than once, that doesn't happen when we have a constant.

So this commit ensures CSE is not applied to MOV immediate (as it
provides no gain, and it is reverted later by copy-propagation
optimization).

Signed-off-by: Juan A. Suarez Romero <jasuarez at igalia.com>
---
 src/mesa/drivers/dri/i965/brw_vec4_cse.cpp | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/src/mesa/drivers/dri/i965/brw_vec4_cse.cpp b/src/mesa/drivers/dri/i965/brw_vec4_cse.cpp
index 85cbf24..7ed7654 100644
--- a/src/mesa/drivers/dri/i965/brw_vec4_cse.cpp
+++ b/src/mesa/drivers/dri/i965/brw_vec4_cse.cpp
@@ -93,6 +93,17 @@ is_expression(const vec4_instruction *const inst)
 }
 
 static bool
+is_mov_constant(const vec4_instruction *const inst)
+{
+   if (inst->opcode == BRW_OPCODE_MOV &&
+       inst->src[0].file == BRW_IMMEDIATE_VALUE) {
+      return true;
+   } else {
+      return false;
+   }
+}
+
+static bool
 operands_match(const vec4_instruction *a, const vec4_instruction *b)
 {
    const src_reg *xs = a->src;
@@ -142,7 +153,7 @@ vec4_visitor::opt_cse_local(bblock_t *block)
    int ip = block->start_ip;
    foreach_inst_in_block (vec4_instruction, inst, block) {
       /* Skip some cases. */
-      if (is_expression(inst) && !inst->predicate && inst->mlen == 0 &&
+      if (is_expression(inst) && !inst->predicate && inst->mlen == 0 && !is_mov_constant(inst) &&
           ((inst->dst.file != ARF && inst->dst.file != FIXED_GRF) ||
            inst->dst.is_null()))
       {
-- 
2.5.0