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nir/opt_loop: Don't peel initial break from do-while loops

Browse source 
As the main purpose of this optimization is to transform
while- into do-while loops, don't apply on loops which are
already in do-while form. Also set nir_loop::do_while after
this transformation, so that it is only applied once.

Totals from 576 (0.28% of 202440) affected shaders: (Navi48)
Instrs: 1337529 -> 1253438 (-6.29%); split: -6.36%, +0.07%
CodeSize: 8390852 -> 7837328 (-6.60%); split: -6.61%, +0.01%
VGPRs: 50856 -> 50844 (-0.02%)
SpillSGPRs: 42198 -> 35395 (-16.12%); split: -16.13%, +0.01%
SpillVGPRs: 47608 -> 44620 (-6.28%)
Latency: 31043828 -> 44143753 (+42.20%); split: -0.06%, +42.26%
InvThroughput: 6973433 -> 10079000 (+44.53%); split: -0.08%, +44.61%
VClause: 26839 -> 24718 (-7.90%); split: -7.91%, +0.00%
SClause: 21831 -> 21583 (-1.14%); split: -1.52%, +0.38%
Copies: 183503 -> 150040 (-18.24%); split: -18.84%, +0.61%
Branches: 27738 -> 26848 (-3.21%); split: -5.12%, +1.91%
PreSGPRs: 40233 -> 39083 (-2.86%); split: -2.88%, +0.02%
PreVGPRs: 38745 -> 38903 (+0.41%); split: -0.02%, +0.43%
VALU: 688396 -> 645948 (-6.17%); split: -6.17%, +0.01%
SALU: 189792 -> 177642 (-6.40%); split: -6.97%, +0.57%
VMEM: 121500 -> 112748 (-7.20%)
SMEM: 38765 -> 37767 (-2.57%); split: -2.58%, +0.00%
VOPD: 102488 -> 89071 (-13.09%); split: +0.24%, -13.33%

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/40349>
This commit is contained in:
Daniel Schürmann 2026-03-05 22:26:19 +01:00 • committed by Marge Bot
parent a9a2edfbb6
commit 1f9a0490c6
2 changed files with 35 additions and 14 deletions
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48
src/compiler/nir/nir_opt_loop.c
View file

@ -339,6 +339,37 @@ has_phi_with_constant_src(nir_block *block, nir_block *pred)
return false;
}
/* For now, we restrict this optimization to cases where the outer IF
* 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.
*/
static bool
should_peel_initial_break(nir_loop *loop)
{
nir_block *header_block = nir_loop_first_block(loop);
nir_cf_node *if_node = nir_cf_node_next(&header_block->cf_node);
nir_if *nif = nir_cf_node_as_if(if_node);
/* This loop is already in do-while form. */
if (loop->do_while)
return false;
/* If there is a phi with a constant loop-carried value, we can remove
* the phi by peeling one iteration.
*/
if (has_phi_with_constant_src(header_block, nir_loop_last_block(loop)))
return true;
return nir_block_contains_work(nir_cf_node_cf_tree_next(if_node)) &&
can_constant_fold(nir_get_scalar(nif->condition.ssa, 0), header_block);
}
/**
* This optimization tries to peel the first loop break.
*
@ -399,19 +430,7 @@ opt_loop_peel_initial_break(nir_loop *loop)
if (nir_block_ends_in_jump(nir_loop_last_block(loop)))
return false;
/* For now, we restrict this optimization to cases where the outer IF
* 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 (!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)))
if (!should_peel_initial_break(loop))
return false;
/* Even though this if statement has a jump on one side, we may still have
@ -456,6 +475,9 @@ opt_loop_peel_initial_break(nir_loop *loop)
nir_after_cf_node(&loop->cf_node));
nir_cf_reinsert(&tmp, nir_after_block(nir_if_first_else_block(nif)));
/* This loop is now in do-while form. */
loop->do_while = true;
return true;
}

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

@ -12,7 +12,6 @@ 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.