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nir: Make nir_opt_remove_phis see through moves.

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I found a shader in Tales of Maj'Eyal that contains:

        if ssa_21 {
                block block_1:
                /* preds: block_0 */
                ...instructions that prevent the select peephole...
                vec1 32 ssa_23 = imov ssa_4
                vec1 32 ssa_24 = imov ssa_4.y
                vec1 32 ssa_25 = imov ssa_4.z
                /* succs: block_3 */
        } else {
                block block_2:
                /* preds: block_0 */
                vec1 32 ssa_26 = imov ssa_4
                vec1 32 ssa_27 = imov ssa_4.y
                vec1 32 ssa_28 = imov ssa_4.z
                /* succs: block_3 */
        }
        block block_3:
        /* preds: block_1 block_2 */
        vec1 32 ssa_29 = phi block_1: ssa_23, block_2: ssa_26
        vec1 32 ssa_30 = phi block_1: ssa_24, block_2: ssa_27
        vec1 32 ssa_31 = phi block_1: ssa_25, block_2: ssa_28

Here, copy propagation will bail because phis cannot perform swizzles,
and CSE won't do anything because there is no dominance relationship
between the imovs.  By making nir_opt_remove_phis handle identical moves,
we can eliminate the phis and rewrite everything to use ssa_4 directly,
so all the moves become dead and get eliminated.

I don't think we need to check "exact" - just the alu sources.
Presumably phi sources should match in their exactness.

On Broadwell:

total instructions in shared programs: 11639872 -> 11638535 (-0.01%)
instructions in affected programs: 134222 -> 132885 (-1.00%)
helped: 338
HURT: 0

v2: Fix return value to be NULL, not false (caught by Iago).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
This commit is contained in:
Kenneth Graunke 2016-07-29 01:29:12 -07:00
parent 7603b4d3a1
commit 144cbf8987
1 changed files with 23 additions and 1 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

24
src/compiler/nir/nir_opt_remove_phis.c
View file

@ -27,6 +27,26 @@
#include "nir.h"
static nir_alu_instr *
get_parent_mov(nir_ssa_def *ssa)
{
if (ssa->parent_instr->type != nir_instr_type_alu)
return NULL;
nir_alu_instr *alu = nir_instr_as_alu(ssa->parent_instr);
return (alu->op == nir_op_imov || alu->op == nir_op_fmov) ? alu : NULL;
}
static bool
matching_mov(nir_alu_instr *mov1, nir_ssa_def *ssa)
{
if (!mov1)
return false;
nir_alu_instr *mov2 = get_parent_mov(ssa);
return mov2 && nir_alu_srcs_equal(mov1, mov2, 0, 0);
}
/*
* This is a pass for removing phi nodes that look like:
* a = phi(b, b, b, ...)
@ -54,6 +74,7 @@ remove_phis_block(nir_block *block)
nir_phi_instr *phi = nir_instr_as_phi(instr);
nir_ssa_def *def = NULL;
nir_alu_instr *mov = NULL;
bool srcs_same = true;
nir_foreach_phi_src(src, phi) {
@ -75,8 +96,9 @@ remove_phis_block(nir_block *block)
if (def == NULL) {
def = src->src.ssa;
mov = get_parent_mov(def);
} else {
if (src->src.ssa != def) {
if (src->src.ssa != def && !matching_mov(mov, src->src.ssa)) {
srcs_same = false;
break;
}