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nir/lower_continue_constructs: special-case Continue Constructs with zero or one predecessors

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If a loop has only a single continue, the control flow is already
converged and we can inline the continue construct.
If a loop has no continue statement at all, the Continue Construct
is unreachable and can simply be deleted.

Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13962>
This commit is contained in:
Daniel Schürmann 2021-12-02 11:08:31 +01:00 • committed by Marge Bot
parent f42eb71df9
commit 7fba5abfd7
1 changed files with 55 additions and 24 deletions
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79
src/compiler/nir/nir_lower_continue_constructs.c
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@ -36,35 +36,66 @@ lower_loop_continue_block(nir_builder *b, nir_loop *loop)
nir_block *header = nir_loop_first_block(loop);
nir_block *cont = nir_loop_first_continue_block(loop);
/* count continue statements excluding unreachable ones */
unsigned num_continue = 0;
nir_block *single_predecessor = NULL;
set_foreach(cont->predecessors, entry) {
nir_block *pred = (nir_block*) entry->key;
/* If the continue block has no predecessors, it is unreachable. */
if (pred->predecessors->entries == 0)
continue;
single_predecessor = pred;
if (num_continue++)
break;
}
nir_lower_phis_to_regs_block(header);
nir_lower_phis_to_regs_block(cont);
/* As control flow has to re-converge before executing the continue
* construct, we insert it at the beginning of the loop with a flag
* to ensure that it doesn't get executed in the first iteration:
*
* loop {
* if (i != 0) {
* continue construct
* }
* loop body
* }
*/
nir_variable *do_cont =
nir_local_variable_create(b->impl, glsl_bool_type(), "cont");
b->cursor = nir_before_cf_node(&loop->cf_node);
nir_store_var(b, do_cont, nir_imm_false(b), 1);
b->cursor = nir_before_block(header);
nir_if *cont_if = nir_push_if(b, nir_load_var(b, do_cont));
{
if (num_continue == 0) {
/* this loop doesn't continue at all. delete the continue construct */
nir_cf_list extracted;
nir_cf_list_extract(&extracted, &loop->continue_list);
nir_cf_reinsert(&extracted, nir_before_cf_list(&cont_if->then_list));
nir_cf_delete(&extracted);
} else if (num_continue == 1) {
/* inline the continue construct */
assert(single_predecessor->successors[0] == cont);
assert(single_predecessor->successors[1] == NULL);
nir_cf_list extracted;
nir_cf_list_extract(&extracted, &loop->continue_list);
nir_cf_reinsert(&extracted,
nir_after_block_before_jump(single_predecessor));
} else {
nir_lower_phis_to_regs_block(cont);
/* As control flow has to re-converge before executing the continue
* construct, we insert it at the beginning of the loop with a flag
* to ensure that it doesn't get executed in the first iteration:
*
* loop {
* if (i != 0) {
* continue construct
* }
* loop body
* }
*/
nir_variable *do_cont =
nir_local_variable_create(b->impl, glsl_bool_type(), "cont");
b->cursor = nir_before_cf_node(&loop->cf_node);
nir_store_var(b, do_cont, nir_imm_false(b), 1);
b->cursor = nir_before_block(header);
nir_if *cont_if = nir_push_if(b, nir_load_var(b, do_cont));
{
nir_cf_list extracted;
nir_cf_list_extract(&extracted, &loop->continue_list);
nir_cf_reinsert(&extracted, nir_before_cf_list(&cont_if->then_list));
}
nir_pop_if(b, cont_if);
nir_store_var(b, do_cont, nir_imm_true(b), 1);
}
nir_pop_if(b, cont_if);
nir_store_var(b, do_cont, nir_imm_true(b), 1);
nir_loop_remove_continue_construct(loop);
return true;