[Mesa-dev] [PATCH] nir/copy_propagate: do not copy-propagate MOV srcs with source modifiers

[Iago Toral Quiroga]

If a source operand in a MOV has source modifiers, then we cannot
copy-propagate it from the parent instruction and remove the MOV.
---

I noticed this while debugging some regressions introduced with the fp64
code. Basically, I had code similar to this:

vec4 ssa1 = intrincisc1 (...) (...)
vec2 ssa2 = vec2 ssa1 -ssa1.y
vec4 ssa3 = intrinsic2 (ssa2) (...)

that would be turned into this by copy propagation:

vec4 ssa1 = some intrincisc
vec4 ssa2 = some intrinsic (ssa1)

which is obviously not correct. This was happening because 
is_swizzleless_move checked that the MOV/vecN operation that generates
the value we want to copy-propagate does not incorporate swizzling, but
it ignored the case where it also added source modifiers, in which case
we can't copy-propagate either.

Of course, now that we have made vecN operations unsigned again, that example
can't happen because lower_to_source_mods won't produce things like that, but
I figured that the patch would still make sense, since it fixes a case where
copy-propagation won't work as intended, even if we are not currently triggering
it (at least not with vecN operations).

 src/glsl/nir/nir_opt_copy_propagate.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/glsl/nir/nir_opt_copy_propagate.c b/src/glsl/nir/nir_opt_copy_propagate.c
index 7d8bdd7..7caa4b7 100644
--- a/src/glsl/nir/nir_opt_copy_propagate.c
+++ b/src/glsl/nir/nir_opt_copy_propagate.c
@@ -65,9 +65,12 @@ static bool is_vec(nir_alu_instr *instr)
 }
 
 static bool
-is_swizzleless_move(nir_alu_instr *instr)
+is_simple_move(nir_alu_instr *instr)
 {
    if (is_move(instr)) {
+      if (instr->src[0].negate || instr->src[0].abs)
+         return false;
+
       for (unsigned i = 0; i < 4; i++) {
          if (!((instr->dest.write_mask >> i) & 1))
             break;
@@ -81,6 +84,9 @@ is_swizzleless_move(nir_alu_instr *instr)
          if (instr->src[i].swizzle[0] != i)
             return false;
 
+         if (instr->src[i].negate || instr->src[i].abs)
+            return false;
+
          if (def == NULL) {
             def = instr->src[i].src.ssa;
          } else if (instr->src[i].src.ssa != def) {
@@ -107,7 +113,7 @@ copy_prop_src(nir_src *src, nir_instr *parent_instr, nir_if *parent_if)
       return false;
 
    nir_alu_instr *alu_instr = nir_instr_as_alu(src_instr);
-   if (!is_swizzleless_move(alu_instr))
+   if (!is_simple_move(alu_instr))
       return false;
 
    /* Don't let copy propagation land us with a phi that has more
-- 
1.9.1