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radeonsi: optimization barriers to work around LLVM deficiencies

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Notably, llvm.amdgcn.readfirstlane and llvm.amdgcn.icmp may be hoisted
out of loops or if/else branches in cases like

  if (cond) {
    v = readFirstInvocationARB(x);
    ... use v ...
  } else {
    v = readFirstInvocationARB(x);
    ... use v ...
  }
===>
  v = readFirstInvocationARB(x);
  if (cond) {
    ... use v ...
  } else {
    ... use v ...
  }

The optimization barrier is a heavy hammer to stop that until LLVM
is taught the semantics of the intrinsic properly.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
This commit is contained in:
Nicolai Hähnle 2017-03-31 18:42:51 +02:00
parent 24d4fbe226
commit 8b13b11f11
1 changed files with 12 additions and 4 deletions
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16
src/gallium/drivers/radeonsi/si_shader.c
View file

@ -3131,7 +3131,6 @@ static void build_tex_intrinsic(const struct lp_build_tgsi_action *action,
* Optionally, a value can be passed through the inline assembly to prevent
* LLVM from hoisting calls to ReadNone functions.
*/
#if 0 /* unused currently */
static void emit_optimization_barrier(struct si_shader_context *ctx,
LLVMValueRef *pvgpr)
{
@ -3165,7 +3164,6 @@ static void emit_optimization_barrier(struct si_shader_context *ctx,
*pvgpr = vgpr;
}
}
#endif
/* Combine these with & instead of |. */
#define NOOP_WAITCNT 0xf7f
@ -5176,8 +5174,13 @@ static LLVMValueRef si_emit_ballot(struct si_shader_context *ctx,
LLVMConstInt(ctx->i32, LLVMIntNE, 0)
};
if (LLVMTypeOf(value) != ctx->i32)
args[0] = LLVMBuildBitCast(gallivm->builder, value, ctx->i32, "");
/* We currently have no other way to prevent LLVM from lifting the icmp
* calls to a dominating basic block.
*/
emit_optimization_barrier(ctx, &args[0]);
if (LLVMTypeOf(args[0]) != ctx->i32)
args[0] = LLVMBuildBitCast(gallivm->builder, args[0], ctx->i32, "");
return lp_build_intrinsic(gallivm->builder,
"llvm.amdgcn.icmp.i32",
@ -5282,6 +5285,11 @@ static void read_lane_emit(
struct si_shader_context *ctx = si_shader_context(bld_base);
LLVMBuilderRef builder = ctx->gallivm.builder;
/* We currently have no other way to prevent LLVM from lifting the icmp
* calls to a dominating basic block.
*/
emit_optimization_barrier(ctx, &emit_data->args[0]);
for (unsigned i = 0; i < emit_data->arg_count; ++i) {
emit_data->args[i] = LLVMBuildBitCast(builder, emit_data->args[i],
ctx->i32, "");