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nak/lower_cf: Flag phis as convergent when possible

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Because we go in and out of SSA, all the phis get re-created and the new
phis will default to divergent.  This little pass attempts to prove as
many of the phis convergent as possible.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29591>
This commit is contained in:
Faith Ekstrand 2024-05-29 11:06:46 -05:00 • committed by Marge Bot
parent 06902bf52e
commit b013d54e4f
1 changed files with 50 additions and 0 deletions
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50
src/nouveau/compiler/nak_nir_lower_cf.c
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@ -373,6 +373,55 @@ lower_cf_list(nir_builder *b, nir_def *esc_reg, struct scope *parent_scope,
}
}
static void
recompute_phi_divergence_impl(nir_function_impl *impl)
{
bool progress;
do {
progress = false;
nir_foreach_block_unstructured(block, impl) {
nir_foreach_instr(instr, block) {
if (instr->type != nir_instr_type_phi)
break;
nir_phi_instr *phi = nir_instr_as_phi(instr);
bool divergent = false;
nir_foreach_phi_src(phi_src, phi) {
/* There is a tricky case we need to care about here where a
* convergent block has a divergent dominator. This can happen
* if, for instance, you have the following loop:
*
* loop {
* if (div) {
* %20 = load_ubo(0, 0);
* } else {
* terminate;
* }
* }
* use(%20);
*
* In this case, the load_ubo() dominates the use() even though
* the load_ubo() exists in divergent control-flow. In this
* case, we simply flag the whole phi divergent because we
* don't want to deal with inserting a r2ur somewhere.
*/
if (phi_src->pred->divergent || phi_src->src.ssa->divergent ||
phi_src->src.ssa->parent_instr->block->divergent) {
divergent = true;
break;
}
}
if (divergent != phi->def.divergent) {
phi->def.divergent = divergent;
progress = true;
}
}
}
} while(progress);
}
static bool
lower_cf_func(nir_function *func)
{
@ -422,6 +471,7 @@ lower_cf_func(nir_function *func)
nir_sort_unstructured_blocks(new_impl);
nir_repair_ssa_impl(new_impl);
nir_lower_reg_intrinsics_to_ssa_impl(new_impl);
recompute_phi_divergence_impl(new_impl);
return true;
}