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nir/opt_loop: Relax restrictions on opt_loop_peel_initial_break() for more loops

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In addition to loops where the break condition can be constant-folded,
we also allow to peel the initial break from loops which have at least
one phi with a constant loop-carried source, effectively removing that
phi from the loop.

Totals from 172 (0.22% of 79377) affected shaders: (Navi31)
Instrs: 372798 -> 369181 (-0.97%); split: -1.07%, +0.10%
CodeSize: 1907312 -> 1891948 (-0.81%); split: -0.89%, +0.09%
VGPRs: 8436 -> 8460 (+0.28%)
Latency: 3646016 -> 3396657 (-6.84%)
InvThroughput: 434848 -> 389079 (-10.53%)
Copies: 28436 -> 27118 (-4.63%); split: -4.79%, +0.15%
Branches: 26504 -> 25344 (-4.38%); split: -4.44%, +0.06%
PreSGPRs: 8585 -> 8603 (+0.21%)
VALU: 148291 -> 148355 (+0.04%); split: -0.01%, +0.06%
SALU: 95625 -> 92649 (-3.11%); split: -3.22%, +0.11%

Reviewed-by: Georg Lehmann <dadschoorse@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33666>
This commit is contained in:
Daniel Schürmann 2025-02-21 11:27:15 +01:00 • committed by Marge Bot
parent 71d68d9166
commit 6313e9f549
2 changed files with 21 additions and 6 deletions
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26
src/compiler/nir/nir_opt_loop.c
View file

@ -327,6 +327,18 @@ can_constant_fold(nir_scalar scalar, nir_block *loop_header)
return false;
}
static bool
has_phi_with_constant_src(nir_block *block, nir_block *pred)
{
nir_foreach_phi(phi, block) {
nir_phi_src *src = nir_phi_get_src_from_block(phi, pred);
if (nir_src_is_const(src->src))
return true;
}
return false;
}
/**
* This optimization tries to peel the first loop break.
*
@ -387,17 +399,19 @@ opt_loop_peel_initial_break(nir_loop *loop)
if (nir_block_ends_in_jump(nir_loop_last_block(loop)))
return false;
/* Check that there is actual work to be done after the initial break. */
if (!nir_block_contains_work(nir_cf_node_cf_tree_next(if_node)))
return false;
/* For now, we restrict this optimization to cases where the outer IF
* can be constant-folded.
* can be constant-folded or where at least one phi at the loop-header
* has a constant loop-carried source. If it can be constant-folded,
* we additionally require that there is actual work to be done after
* the initial break. This is to avoid unconditionally unrolling long
* loops.
*
* Note: If this restriction is lifted, it might recurse infinitely.
* Prevent by e.g. restricting to single-exit loops.
*/
if (!can_constant_fold(nir_get_scalar(nif->condition.ssa, 0), header_block))
if (!has_phi_with_constant_src(header_block, nir_loop_last_block(loop)) &&
(!nir_block_contains_work(nir_cf_node_cf_tree_next(if_node)) ||
!can_constant_fold(nir_get_scalar(nif->condition.ssa, 0), header_block)))
return false;
/* Even though this if statement has a jump on one side, we may still have

1
src/gallium/drivers/d3d12/ci/d3d12-quick_shader.txt
View file

@ -12,6 +12,7 @@ spec@glsl-1.50@execution@primitive-id-no-gs-quad-strip,Fail
spec@glsl-1.50@execution@primitive-id-no-gs-quads,Fail
spec@glsl-1.50@execution@primitive-id-no-gs-point,Fail
spec@glsl-1.50@execution@variable-indexing@gs-output-array-vec4-index-wr,Crash
shaders@ssa@fs-if-def-else-break,Fail
# These tests use a TCS output variable only as temporary storage. Since the output
# is unused by the TES, we remove it.