Mesa (staging/21.3): nir/loop_unroll: Always unroll loops that iterate at most once
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Thu Oct 14 20:38:37 UTC 2021
Module: Mesa
Branch: staging/21.3
Commit: d3492921ab3821cb50511b56ec4f01deb93c7e4c
URL: http://cgit.freedesktop.org/mesa/mesa/commit/?id=d3492921ab3821cb50511b56ec4f01deb93c7e4c
Author: Ian Romanick <ian.d.romanick at intel.com>
Date: Tue Oct 12 13:06:04 2021 -0700
nir/loop_unroll: Always unroll loops that iterate at most once
Two carchase compute shaders (shader-db) and two Fallout 4 fragment
shaders (fossil-db) were helped. Based on the NIR of the shaders, all
four had structures like
for (i = 0; i < 1; i++) {
...
for (...) {
...
}
}
All HSW+ platforms had similar results. (Ice Lake shown)
total loops in shared programs: 6033 -> 6031 (-0.03%)
loops in affected programs: 4 -> 2 (-50.00%)
helped: 2
HURT: 0
All Intel platforms had similar results. (Ice Lake shown)
Instructions in all programs: 143692018 -> 143692006 (-0.0%)
SENDs in all programs: 6947154 -> 6947154 (+0.0%)
Loops in all programs: 38285 -> 38283 (-0.0%)
Cycles in all programs: 8434822225 -> 8434476815 (-0.0%)
Spills in all programs: 191665 -> 191665 (+0.0%)
Fills in all programs: 298822 -> 298822 (+0.0%)
In the presense of loop unrolling like this, the change in cycles is not
accurate.
v2: Rearrange the logic in the if-condition to read a little better.
Suggested by Tim.
Closes: #5089
Reviewed-by: Timothy Arceri <tarceri at itsqueeze.com>
(cherry picked from commit ae99ea6f4ddb90f7120b4986709a9e9064037f94)
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13366>
---
src/compiler/nir/nir_opt_loop_unroll.c | 22 +++++++++++++++++++++-
1 file changed, 21 insertions(+), 1 deletion(-)
diff --git a/src/compiler/nir/nir_opt_loop_unroll.c b/src/compiler/nir/nir_opt_loop_unroll.c
index 3dec5e15e56..e81cebbb338 100644
--- a/src/compiler/nir/nir_opt_loop_unroll.c
+++ b/src/compiler/nir/nir_opt_loop_unroll.c
@@ -976,7 +976,27 @@ process_loops(nir_shader *sh, nir_cf_node *cf_node, bool *has_nested_loop_out,
}
}
- if (has_nested_loop || !loop->info->limiting_terminator)
+ /* Intentionally don't consider exact_trip_count_known here. When
+ * max_trip_count is non-zero, it is the upper bound on the number of
+ * times the loop will iterate, but the loop may iterate less. For
+ * example, the following loop will iterate 0 or 1 time:
+ *
+ * for (i = 0; i < min(x, 1); i++) { ... }
+ *
+ * Trivial single-interation loops (e.g., do { ... } while (false)) and
+ * trivial zero-iteration loops (e.g., while (false) { ... }) will have
+ * already been handled.
+ *
+ * If the loop is known to execute at most once and meets the other
+ * unrolling criteria, unroll it even if it has nested loops.
+ *
+ * It is unlikely that such loops exist in real shaders. GraphicsFuzz is
+ * known to generate spurious loops that iterate exactly once. It is
+ * plausible that it could eventually start generating loops like the
+ * example above, so it seems logical to defend against it now.
+ */
+ if (!loop->info->limiting_terminator ||
+ (loop->info->max_trip_count != 1 && has_nested_loop))
goto exit;
if (!check_unrolling_restrictions(sh, loop))
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