[llvm] d6467e0 - AMDGPU: Fix infinite loop in DAG combine with fneg + fma

[Matt Arsenault via llvm-commits]

Author: Matt Arsenault
Date: 2021-06-18T19:09:03-04:00
New Revision: d6467e00dfccf10aa91e5a1d55d981a9b13642a3

URL: https://github.com/llvm/llvm-project/commit/d6467e00dfccf10aa91e5a1d55d981a9b13642a3
DIFF: https://github.com/llvm/llvm-project/commit/d6467e00dfccf10aa91e5a1d55d981a9b13642a3.diff

LOG: AMDGPU: Fix infinite loop in DAG combine with fneg + fma

We were not reporting isFNegFree for v2f32, although it is effectively
free after legalization. The generic combine was pulling fneg out of
the fma source operands, and the AMDGPU combine was doing the
opposite.

Added: 
    

Modified: 
    llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
    llvm/test/CodeGen/AMDGPU/fneg-combines.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp b/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
index 895ccefd020f..50873a252e1e 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
@@ -848,9 +848,9 @@ bool AMDGPUTargetLowering::isFAbsFree(EVT VT) const {
 
 bool AMDGPUTargetLowering::isFNegFree(EVT VT) const {
   assert(VT.isFloatingPoint());
-  return VT == MVT::f32 || VT == MVT::f64 ||
-         (Subtarget->has16BitInsts() && VT == MVT::f16) ||
-         (Subtarget->hasVOP3PInsts() && VT == MVT::v2f16);
+  // Report this based on the end legalized type.
+  VT = VT.getScalarType();
+  return VT == MVT::f32 || VT == MVT::f64 || VT == MVT::f16;
 }
 
 bool AMDGPUTargetLowering:: storeOfVectorConstantIsCheap(EVT MemVT,

diff  --git a/llvm/test/CodeGen/AMDGPU/fneg-combines.ll b/llvm/test/CodeGen/AMDGPU/fneg-combines.ll
index 2ed74ac1722f..900d07c2d7ce 100644
--- a/llvm/test/CodeGen/AMDGPU/fneg-combines.ll
+++ b/llvm/test/CodeGen/AMDGPU/fneg-combines.ll
@@ -2575,8 +2575,31 @@ define amdgpu_kernel void @multi_use_cost_to_fold_into_src(float addrspace(1)* %
   ret void
 }
 
+; The AMDGPU combine to pull fneg into the FMA operands was being
+; undone by the generic combine to pull the fneg out of the fma if
+; !isFNegFree. We were reporting false for v2f32 even though it will
+; be split into f32 where it will be free.
+; GCN-LABEL: {{^}}fneg_fma_fneg_dagcombine_loop:
+; GCN: s_brev_b32 [[NEGZERO:s[0-9]+]], 1{{$}}
+; GCN-DAG: v_fma_f32 [[FMA0:v[0-9]+]], v2, -v4, [[NEGZERO]]
+; GCN-DAG: v_fma_f32 [[FMA1:v[0-9]+]], v3, -v5, [[NEGZERO]]
+; GCN-DAG: v_sub_f32_e32 [[SUB0:v[0-9]+]], [[FMA0]], v0
+; GCN-DAG: v_sub_f32_e32 [[SUB1:v[0-9]+]], [[FMA1]], v1
+; GCN-DAG: v_mul_f32_e32 v0, [[SUB0]], v4
+; GCN-DAG: v_mul_f32_e32 v1, [[SUB1]], v5
+; GCN: s_setpc_b64
+define <2 x float> @fneg_fma_fneg_dagcombine_loop(<2 x float> %arg, <2 x float> %arg1, <2 x float> %arg2) #0 {
+bb:
+  %i3 = call fast <2 x float> @llvm.fma.v2f32(<2 x float> %arg1, <2 x float> %arg2, <2 x float> zeroinitializer)
+  %i4 = fadd fast <2 x float> %i3, %arg
+  %i5 = fneg <2 x float> %i4
+  %i6 = fmul fast <2 x float> %i5, %arg2
+  ret <2 x float> %i6
+}
+
 declare i32 @llvm.amdgcn.workitem.id.x() #1
 declare float @llvm.fma.f32(float, float, float) #1
+declare <2 x float> @llvm.fma.v2f32(<2 x float>, <2 x float>, <2 x float>)
 declare float @llvm.fmuladd.f32(float, float, float) #1
 declare <4 x float> @llvm.fmuladd.v4f32(<4 x float>, <4 x float>, <4 x float>) #1
 declare float @llvm.sin.f32(float) #1
@@ -2601,3 +2624,4 @@ declare float @llvm.amdgcn.interp.p2(float, float, i32, i32, i32) #0
 attributes #0 = { nounwind "denormal-fp-math-f32"="preserve-sign,preserve-sign" }
 attributes #1 = { nounwind readnone }
 attributes #2 = { nounwind "unsafe-fp-math"="true" }
+attributes #3 = { nounwind "no-signed-zeros-fp-math"="true" }