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nir: document how nir_opt_dce works

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Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38385>
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Marek Olšák 2025-11-11 14:06:40 -05:00 • committed by Marge Bot
parent e372365cf4
commit 9247a78925
1 changed files with 31 additions and 19 deletions
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50
src/compiler/nir/nir_opt_dce.c
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@ -1,30 +1,42 @@
/*
* Copyright © 2014 Intel Corporation
*
* Permission is hereby granted, free of charge, to any person obtaining a
* copy of this software and associated documentation files (the "Software"),
* to deal in the Software without restriction, including without limitation
* the rights to use, copy, modify, merge, publish, distribute, sublicense,
* and/or sell copies of the Software, and to permit persons to whom the
* Software is furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice (including the next
* paragraph) shall be included in all copies or substantial portions of the
* Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
* THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
* IN THE SOFTWARE.
* SPDX-License-Identifier: MIT
*
* Authors:
* Connor Abbott (cwabbott0@gmail.com)
*
*/
/* Dead Code Elimination
*
* It eliminates those instructions whose results are unused and those that
* have no effect on instructions with side effects (like stores).
* To be precise, instructions are not dead if their result is needed by
* non-removable instructions.
*
* The algorithm walks the shader backwards and:
* - marks sources of non-removable instructions as live
* - if an instruction is live, mark all its sources as live as well
* - if a loop header phi source is marked as live and the source is not in
* the block preceding the loop, the loop is walked backwards again
* (beucase the phi source is likely after the phi)
*
* Why looking only at the number of uses of each instruction doesn't work:
* There could be a loop in the def-use graph where every instructions has
* non-zero uses, yet all of them are dead. Example:
*
* loop {
* %1 = phi %0, %2
*
* loop {
* %2 = phi %1, %0
* }
* }
*
* In this case, both phis have non-zero uses because they use each other,
* but they have no effect on the shader and can be removed.
*/
#include "nir.h"
static bool