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nir/opt_if: Simplify if's with general conditions

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Dolphin ubershaders have a pattern:

   if (x && y) {
   } else {
      discard;
   }

The current code to simplify if's will bail on this pattern, since the condition
is not a comparison. However, if that check is dropped and we allow NIR to
invert this, we get:

   if (!(x && y)) {
      discard;
   } else {
   }

which is now in a form for nir_opt_conditional_discard to turn into it

   discard_if(!(x && y))

which may be substantially cheaper than the original code.

In general, I see no reason to restrict to conditionals. Assuming the backend is
clever enough to delete empty else blocks (I think most are), then this patch is
a strict win as long as inot instructions are cheaper than empty else blocks.
This matches my intuition for typical GPUs, where simple ALU instructions are
cheaper than control flow. Furthermore, it may be possible in practice for
backends to fold the inot into a richer set of instructions. For example, most
GPUs have a NAND instructions which would fold in the inot in the above code.

So just drop the check, simplify the pass, get the win.

---

Also, to avoid inflating register pressure, make sure we put the inot right
before the if. Android shader-db on is uninspiring due to terrible
coalescing decisions in the current RA. But it does fix the Dolphin smell.

   total instructions in shared programs: 1756571 -> 1756568 (<.01%)
   instructions in affected programs: 1600 -> 1597 (-0.19%)
   helped: 1
   HURT: 4
   Inconclusive result (value mean confidence interval includes 0).

   total bytes in shared programs: 11521172 -> 11521156 (<.01%)
   bytes in affected programs: 10080 -> 10064 (-0.16%)
   helped: 1
   HURT: 4
   Inconclusive result (value mean confidence interval includes 0).

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24965>
This commit is contained in:
Alyssa Rosenzweig
2023-08-30 18:10:28 -04:00
committed by Marge Bot
parent 977ef3b388
commit 31b5f5a51f
1 changed files with 2 additions and 15 deletions
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17
src/compiler/nir/nir_opt_if.c
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@@ -861,22 +861,9 @@ opt_if_simplification(nir_builder *b, nir_if *nif)
is_block_empty(nir_if_first_else_block(nif)))
return false;
/* Make sure the condition is a comparison operation. */
nir_instr *src_instr = nif->condition.ssa->parent_instr;
if (src_instr->type != nir_instr_type_alu)
return false;
nir_alu_instr *alu_instr = nir_instr_as_alu(src_instr);
if (!nir_alu_instr_is_comparison(alu_instr))
return false;
/* Insert the inverted instruction and rewrite the condition. */
b->cursor = nir_after_instr(&alu_instr->instr);
nir_def *new_condition =
nir_inot(b, &alu_instr->def);
nir_src_rewrite(&nif->condition, new_condition);
b->cursor = nir_before_src(&nif->condition);
nir_src_rewrite(&nif->condition, nir_inot(b, nif->condition.ssa));
/* Grab pointers to the last then/else blocks for fixing up the phis. */
nir_block *then_block = nir_if_last_then_block(nif);