intel/fs: Don't delete coalesced MOVs if they have a cmod

Shader-db results on ICL:

    total instructions in shared programs: 17133088 -> 17133287 (<.01%)
    instructions in affected programs: 61300 -> 61499 (0.32%)
    helped: 0
    HURT: 199

This means it's likely fixing 199 bugs. :-)  All the changed shaders are
in Mad Max.  It's surprisingly difficult to get the back-end compiler to
generate a pattern that hits this we don't tend to emit a lot coalescable
MOVs.  The pattern in Mad Max that's able to hit is fsign(fsat(x)) under
the right conditions.

Closes: #2820
Cc: [email protected]
Tested-by: Ian Romanick <[email protected]>
Reviewed-by: Ian Romanick <[email protected]>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4773>

1 files changed, 15 insertions, 2 deletions

diff --git a/src/intel/compiler/brw_fs_register_coalesce.cpp b/src/intel/compiler/brw_fs_register_coalesce.cpp
index 5d24240c393..671ced5bc95 100644
--- a/src/intel/compiler/brw_fs_register_coalesce.cpp
+++ b/src/intel/compiler/brw_fs_register_coalesce.cpp
@@ -277,13 +277,26 @@ fs_visitor::register_coalesce()
       progress = true;
 
       for (int i = 0; i < src_size; i++) {
-         if (mov[i]) {
+         if (!mov[i])
+            continue;
+
+         if (mov[i]->conditional_mod == BRW_CONDITIONAL_NONE) {
             mov[i]->opcode = BRW_OPCODE_NOP;
-            mov[i]->conditional_mod = BRW_CONDITIONAL_NONE;
             mov[i]->dst = reg_undef;
             for (int j = 0; j < mov[i]->sources; j++) {
                mov[i]->src[j] = reg_undef;
             }
+         } else {
+            /* If we have a conditional modifier, rewrite the MOV to be a
+             * MOV.cmod from the coalesced register.  Hopefully, cmod
+             * propagation will clean this up and move it to the instruction
+             * that writes the register.  If not, this keeps things correct
+             * while still letting us coalesce.
+             */
+            assert(mov[i]->opcode == BRW_OPCODE_MOV);
+            assert(mov[i]->sources == 1);
+            mov[i]->src[0] = mov[i]->dst;
+            mov[i]->dst = retype(brw_null_reg(), mov[i]->dst.type);
          }
       }