[WebAssembly] Fix fneg lowering

Summary:
Subtraction from zero and floating point negation do not have the same
semantics, so fix lowering.

Reviewers: aheejin, dschuff

Subscribers: sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D52948

llvm-svn: 344107

1 files changed, 4 insertions, 2 deletions

diff --git a/llvm/test/CodeGen/WebAssembly/simd-arith.ll b/llvm/test/CodeGen/WebAssembly/simd-arith.ll
index 0bda334cd98..f3e70156d8b 100644
--- a/llvm/test/CodeGen/WebAssembly/simd-arith.ll
+++ b/llvm/test/CodeGen/WebAssembly/simd-arith.ll
@@ -748,7 +748,8 @@ define <2 x i64> @bitselect_v2i64(<2 x i64> %c, <2 x i64> %v1, <2 x i64> %v2) {
 ; SIMD128-NEXT: f32x4.neg $push[[R:[0-9]+]]=, $0{{$}}
 ; SIMD128-NEXT: return $pop[[R]]{{$}}
 define <4 x float> @neg_v4f32(<4 x float> %x) {
-  %a = fsub <4 x float> <float 0., float 0., float 0., float 0.>, %x
+  ; nsz makes this semantically equivalent to flipping sign bit
+  %a = fsub nsz <4 x float> <float 0.0, float 0.0, float 0.0, float 0.0>, %x
   ret <4 x float> %a
 }
 
@@ -830,7 +831,8 @@ define <4 x float> @sqrt_v4f32(<4 x float> %x) {
 ; SIMD128-NEXT: f64x2.neg $push[[R:[0-9]+]]=, $0{{$}}
 ; SIMD128-NEXT: return $pop[[R]]{{$}}
 define <2 x double> @neg_v2f64(<2 x double> %x) {
-  %a = fsub <2 x double> <double 0., double 0.>, %x
+  ; nsz makes this semantically equivalent to flipping sign bit
+  %a = fsub nsz <2 x double> <double 0., double 0.>, %x
   ret <2 x double> %a
 }