[PATCH] D19747: AMDGPU: llvm.SI.fs.constant is a source of divergence

[Nicolai Hähnle via llvm-commits]

nhaehnle created this revision.
nhaehnle added reviewers: arsenm, tstellarAMD.
nhaehnle added a subscriber: llvm-commits.
Herald added a subscriber: arsenm.

This intrinsic is used to get flat-shaded fragment shader inputs. Those are
uniform across a primitive, but a fragment shader wave may process pixels from
multiple primitives (as indicated by the prim_mask), and so that's where
divergence can arise.

http://reviews.llvm.org/D19747

Files:
  lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp
  test/Analysis/DivergenceAnalysis/AMDGPU/interp-intrinsics.ll

Index: test/Analysis/DivergenceAnalysis/AMDGPU/interp-intrinsics.ll
===================================================================
--- /dev/null
+++ test/Analysis/DivergenceAnalysis/AMDGPU/interp-intrinsics.ll
@@ -0,0 +1,22 @@
+; RUN: opt  -mtriple amdgcn--- -analyze -divergence %s | FileCheck %s
+
+; CHECK-LABEL: 'fs_interp'
+; CHECK: DIVERGENT: %v = call float @llvm.SI.fs.interp(
+define amdgpu_ps void @fs_interp(i32 inreg %prim_mask, <2 x i32> %interp_param) #1 {
+  %v = call float @llvm.SI.fs.interp(i32 0, i32 0, i32 %prim_mask, <2 x i32> %interp_param)
+  store volatile float %v, float addrspace(1)* undef
+  ret void
+}
+
+; CHECK-LABEL: 'fs_constant'
+; CHECK: DIVERGENT: %v = call float @llvm.SI.fs.constant(
+define amdgpu_ps void @fs_constant(i32 inreg %prim_mask, <2 x i32> %interp_param) #1 {
+  %v = call float @llvm.SI.fs.constant(i32 0, i32 0, i32 %prim_mask)
+  store volatile float %v, float addrspace(1)* undef
+  ret void
+}
+
+declare float @llvm.SI.fs.interp(i32, i32, i32, <2 x i32>) #0
+declare float @llvm.SI.fs.constant(i32, i32, i32) #0
+
+attributes #0 = { nounwind readnone }
Index: lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp
===================================================================
--- lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp
+++ lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp
@@ -260,6 +260,7 @@
     return false;
   case AMDGPUIntrinsic::SI_tid:
   case AMDGPUIntrinsic::SI_fs_interp:
+  case AMDGPUIntrinsic::SI_fs_constant:
     return true;
   }
 }


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