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nir: Add an opt pass for phis after if choosing between true/false.

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This pattern almost always gets peephole-selected out anyway, but I
noticed it once I removed glsl opt_conditional_discard.

iris shader-db:
total instructions in shared programs: 8933934 -> 8933158 (<.01%)
instructions in affected programs: 75575 -> 74799 (-1.03%)
helped: 179
HURT: 15

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17664>
This commit is contained in:
Emma Anholt 2022-07-27 12:38:38 -07:00 • committed by Marge Bot
parent a2397e6378
commit 3714c89d0e
1 changed files with 55 additions and 0 deletions
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55
src/compiler/nir/nir_opt_if.c
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@ -934,6 +934,60 @@ opt_if_simplification(nir_builder *b, nir_if *nif)
return true;
}
/* Find phi statements after an if that choose between true and false, and
* replace them with the if statement's condition (or an inot of it).
*/
static bool
opt_if_phi_is_condition(nir_builder *b, nir_if *nif)
{
/* Grab pointers to the last then/else blocks for looking in the phis. */
nir_block *then_block = nir_if_last_then_block(nif);
nir_block *else_block = nir_if_last_else_block(nif);
nir_ssa_def *cond = nif->condition.ssa;
bool progress = false;
nir_block *after_if_block = nir_cf_node_as_block(nir_cf_node_next(&nif->cf_node));
nir_foreach_instr_safe(instr, after_if_block) {
if (instr->type != nir_instr_type_phi)
break;
nir_phi_instr *phi = nir_instr_as_phi(instr);
if (phi->dest.ssa.bit_size != cond->bit_size ||
phi->dest.ssa.num_components != 1)
continue;
enum opt_bool {
T, F, UNKNOWN
} then_val = UNKNOWN, else_val = UNKNOWN;
nir_foreach_phi_src(src, phi) {
assert(src->pred == then_block || src->pred == else_block);
enum opt_bool *pred_val = src->pred == then_block ? &then_val : &else_val;
nir_ssa_scalar val = nir_ssa_scalar_resolved(src->src.ssa, 0);
if (!nir_ssa_scalar_is_const(val))
break;
if (nir_ssa_scalar_as_int(val) == -1)
*pred_val = T;
else if (nir_ssa_scalar_as_uint(val) == 0)
*pred_val = F;
else
break;
}
if (then_val == T && else_val == F) {
nir_ssa_def_rewrite_uses(&phi->dest.ssa, cond);
progress = true;
} else if (then_val == F && else_val == T) {
b->cursor = nir_before_cf_node(&nif->cf_node);
nir_ssa_def_rewrite_uses(&phi->dest.ssa, nir_inot(b, cond));
progress = true;
}
}
return progress;
}
/**
* This optimization tries to merge two break statements into a single break.
* For this purpose, it checks if both branch legs end in a break or
@ -1560,6 +1614,7 @@ opt_if_cf_list(nir_builder *b, struct exec_list *cf_list,
progress |= opt_if_loop_terminator(nif);
progress |= opt_if_merge(nif);
progress |= opt_if_simplification(b, nif);
progress |= opt_if_phi_is_condition(b, nif);
break;
}