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aco: consider exec empty after divergent continue then divergent break

Browse source 
For:
loop {
   if (divergent)
      continue
   if (divergent)
      break
   //exec is potentially empty here
   loop {
      if (divergent)
         break
   }
}
If a subset of invocations take the continue and then the rest take the
break, then exec will be empty after the break.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30320>
This commit is contained in:
Rhys Perry 2024-07-19 18:14:24 +01:00 • committed by Marge Bot
parent e863acb318
commit 39264a90c3
2 changed files with 52 additions and 9 deletions
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51
src/amd/compiler/aco_instruction_selection.cpp
View file

@ -56,11 +56,13 @@ struct if_context {
bool divergent_old;
bool exec_potentially_empty_discard_old;
bool exec_potentially_empty_break_old;
bool exec_potentially_empty_continue_old;
bool had_divergent_discard_old;
bool had_divergent_discard_then;
bool has_divergent_continue_old;
bool has_divergent_continue_then;
uint16_t exec_potentially_empty_break_depth_old;
uint16_t exec_potentially_empty_continue_depth_old;
unsigned BB_if_idx;
unsigned invert_idx;
@ -10284,8 +10286,14 @@ end_loop(isel_context* ctx, loop_context* lc)
Builder bld(ctx->program, ctx->block);
append_logical_end(ctx->block);
/* No need to check exec_potentially_empty_break/continue originating inside the loop. In the
* only case where it's possible at this point (divergent break after divergent continue), we
* should continue anyway. */
if (ctx->cf_info.exec_potentially_empty_discard ||
ctx->cf_info.exec_potentially_empty_break) {
(ctx->cf_info.exec_potentially_empty_break &&
ctx->cf_info.exec_potentially_empty_break_depth < ctx->block->loop_nest_depth) ||
(ctx->cf_info.exec_potentially_empty_continue &&
ctx->cf_info.exec_potentially_empty_continue_depth < ctx->block->loop_nest_depth)) {
/* Discards can result in code running with an empty exec mask.
* This would result in divergent breaks not ever being taken. As a
* workaround, break the loop when the loop mask is empty instead of
@ -10337,6 +10345,10 @@ end_loop(isel_context* ctx, loop_context* lc)
ctx->cf_info.parent_if.is_divergent = lc->divergent_if_old;
if (!ctx->block->loop_nest_depth && !ctx->cf_info.parent_if.is_divergent)
ctx->cf_info.exec_potentially_empty_discard = false;
if (ctx->block->loop_nest_depth < ctx->cf_info.exec_potentially_empty_break_depth) {
ctx->cf_info.exec_potentially_empty_break = false;
ctx->cf_info.exec_potentially_empty_break_depth = UINT16_MAX;
}
}
void
@ -10362,6 +10374,11 @@ emit_loop_jump(isel_context* ctx, bool is_break)
return;
}
ctx->cf_info.parent_loop.has_divergent_branch = true;
if (!ctx->cf_info.exec_potentially_empty_break) {
ctx->cf_info.exec_potentially_empty_break = true;
ctx->cf_info.exec_potentially_empty_break_depth = ctx->block->loop_nest_depth;
}
} else {
logical_target = &ctx->program->blocks[ctx->cf_info.parent_loop.header_idx];
add_logical_edge(idx, logical_target);
@ -10380,11 +10397,11 @@ emit_loop_jump(isel_context* ctx, bool is_break)
we must ensure that they are handled correctly */
ctx->cf_info.parent_loop.has_divergent_continue = true;
ctx->cf_info.parent_loop.has_divergent_branch = true;
}
if (ctx->cf_info.parent_if.is_divergent && !ctx->cf_info.exec_potentially_empty_break) {
ctx->cf_info.exec_potentially_empty_break = true;
ctx->cf_info.exec_potentially_empty_break_depth = ctx->block->loop_nest_depth;
if (!ctx->cf_info.exec_potentially_empty_continue) {
ctx->cf_info.exec_potentially_empty_continue = true;
ctx->cf_info.exec_potentially_empty_continue_depth = ctx->block->loop_nest_depth;
}
}
/* remove critical edges from linear CFG */
@ -10595,7 +10612,10 @@ begin_divergent_if_then(isel_context* ctx, if_context* ic, Temp cond,
ic->exec_potentially_empty_discard_old = ctx->cf_info.exec_potentially_empty_discard;
ic->exec_potentially_empty_break_old = ctx->cf_info.exec_potentially_empty_break;
ic->exec_potentially_empty_continue_old = ctx->cf_info.exec_potentially_empty_continue;
ic->exec_potentially_empty_break_depth_old = ctx->cf_info.exec_potentially_empty_break_depth;
ic->exec_potentially_empty_continue_depth_old =
ctx->cf_info.exec_potentially_empty_continue_depth;
ic->divergent_old = ctx->cf_info.parent_if.is_divergent;
ic->had_divergent_discard_old = ctx->cf_info.had_divergent_discard;
ctx->cf_info.parent_if.is_divergent = true;
@ -10604,6 +10624,8 @@ begin_divergent_if_then(isel_context* ctx, if_context* ic, Temp cond,
ctx->cf_info.exec_potentially_empty_discard = false;
ctx->cf_info.exec_potentially_empty_break = false;
ctx->cf_info.exec_potentially_empty_break_depth = UINT16_MAX;
ctx->cf_info.exec_potentially_empty_continue = false;
ctx->cf_info.exec_potentially_empty_continue_depth = UINT16_MAX;
/** emit logical then block */
ctx->program->next_divergent_if_logical_depth++;
@ -10658,10 +10680,16 @@ begin_divergent_if_else(isel_context* ctx, if_context* ic,
ic->exec_potentially_empty_break_old |= ctx->cf_info.exec_potentially_empty_break;
ic->exec_potentially_empty_break_depth_old = std::min(
ic->exec_potentially_empty_break_depth_old, ctx->cf_info.exec_potentially_empty_break_depth);
ic->exec_potentially_empty_continue_old |= ctx->cf_info.exec_potentially_empty_continue;
ic->exec_potentially_empty_continue_depth_old =
std::min(ic->exec_potentially_empty_continue_depth_old,
ctx->cf_info.exec_potentially_empty_continue_depth);
/* divergent branches use cbranch_execz */
ctx->cf_info.exec_potentially_empty_discard = false;
ctx->cf_info.exec_potentially_empty_break = false;
ctx->cf_info.exec_potentially_empty_break_depth = UINT16_MAX;
ctx->cf_info.exec_potentially_empty_continue = false;
ctx->cf_info.exec_potentially_empty_continue_depth = UINT16_MAX;
ic->had_divergent_discard_then = ctx->cf_info.had_divergent_discard;
ctx->cf_info.had_divergent_discard = ic->had_divergent_discard_old;
@ -10715,16 +10743,27 @@ end_divergent_if(isel_context* ctx, if_context* ic)
ctx->cf_info.exec_potentially_empty_break |= ic->exec_potentially_empty_break_old;
ctx->cf_info.exec_potentially_empty_break_depth = std::min(
ic->exec_potentially_empty_break_depth_old, ctx->cf_info.exec_potentially_empty_break_depth);
ctx->cf_info.exec_potentially_empty_continue |= ic->exec_potentially_empty_continue_old;
ctx->cf_info.exec_potentially_empty_continue_depth =
std::min(ic->exec_potentially_empty_continue_depth_old,
ctx->cf_info.exec_potentially_empty_continue_depth);
if (ctx->block->loop_nest_depth == ctx->cf_info.exec_potentially_empty_break_depth &&
!ctx->cf_info.parent_if.is_divergent) {
!ctx->cf_info.parent_if.is_divergent && !ctx->cf_info.parent_loop.has_divergent_continue) {
ctx->cf_info.exec_potentially_empty_break = false;
ctx->cf_info.exec_potentially_empty_break_depth = UINT16_MAX;
}
if (ctx->block->loop_nest_depth == ctx->cf_info.exec_potentially_empty_continue_depth &&
!ctx->cf_info.parent_if.is_divergent) {
ctx->cf_info.exec_potentially_empty_continue = false;
ctx->cf_info.exec_potentially_empty_continue_depth = UINT16_MAX;
}
/* uniform control flow never has an empty exec-mask */
if (!ctx->block->loop_nest_depth && !ctx->cf_info.parent_if.is_divergent) {
ctx->cf_info.exec_potentially_empty_discard = false;
ctx->cf_info.exec_potentially_empty_break = false;
ctx->cf_info.exec_potentially_empty_break_depth = UINT16_MAX;
ctx->cf_info.exec_potentially_empty_continue = false;
ctx->cf_info.exec_potentially_empty_continue_depth = UINT16_MAX;
}
ctx->cf_info.had_divergent_discard |= ic->had_divergent_discard_then;

10
src/amd/compiler/aco_instruction_selection.h
View file

@ -61,10 +61,14 @@ struct isel_context {
bool exec_potentially_empty_discard =
false; /* set to false when loop_nest_depth==0 && parent_if.is_divergent==false */
uint16_t exec_potentially_empty_break_depth = UINT16_MAX;
/* Set to false when loop_nest_depth==exec_potentially_empty_break_depth
* and parent_if.is_divergent==false. Called _break but it's also used for
* loop continues. */
/* Set to false when loop_nest_depth==exec_potentially_empty_break_depth,
* parent_if.is_divergent==false and parent_loop.has_divergent_continue==false. Also set to
* false if loop_nest_depth<exec_potentially_empty_break_depth */
bool exec_potentially_empty_break = false;
uint16_t exec_potentially_empty_continue_depth = UINT16_MAX;
/* Set to false when loop_nest_depth==exec_potentially_empty_break_depth
* and parent_if.is_divergent==false. */
bool exec_potentially_empty_continue = false;
} cf_info;
/* NIR range analysis. */