[FPEnv][InstSimplify] Fold constrained X + -0.0 ==> X

Currently the fadd optimizations in InstSimplify don't know how to do this
"X + -0.0 ==> X" fold when using the constrained intrinsics. This adds the
support.

This commit is derived from D106362 with some improvements from D107285.

Differential Revision: https://reviews.llvm.org/D111085

diff --git a/llvm/include/llvm/IR/FPEnv.h b/llvm/include/llvm/IR/FPEnv.h
index 6215400..81c5f37 100644 (file)
--- a/llvm/include/llvm/IR/FPEnv.h
+++ b/llvm/include/llvm/IR/FPEnv.h
@@ -58,5 +58,11 @@ Optional<StringRef> ExceptionBehaviorToStr(fp::ExceptionBehavior);
 inline bool isDefaultFPEnvironment(fp::ExceptionBehavior EB, RoundingMode RM) {
   return EB == fp::ebIgnore && RM == RoundingMode::NearestTiesToEven;
 }
+
+/// Returns true if the rounding mode RM may be QRM at compile time or
+/// at run time.
+inline bool canRoundingModeBe(RoundingMode RM, RoundingMode QRM) {
+  return RM == QRM || RM == RoundingMode::Dynamic;
+}
 }
 #endif

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 556250e..0f114b5 100644 (file)
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4935,6 +4935,11 @@ static Constant *simplifyFPOp(ArrayRef<Value *> Ops, FastMathFlags FMF,
   return nullptr;
 }
 
+// TODO: Move this out to a header file:
+static inline bool canIgnoreSNaN(fp::ExceptionBehavior EB, FastMathFlags FMF) {
+  return (EB == fp::ebIgnore || FMF.noNaNs());
+}
+
 /// Given operands for an FAdd, see if we can fold the result.  If not, this
 /// returns null.
 static Value *
@@ -4949,13 +4954,20 @@ SimplifyFAddInst(Value *Op0, Value *Op1, FastMathFlags FMF,
   if (Constant *C = simplifyFPOp({Op0, Op1}, FMF, Q, ExBehavior, Rounding))
     return C;
 
+  // fadd X, -0 ==> X
+  // With strict/constrained FP, we have these possible edge cases that do
+  // not simplify to Op0:
+  // fadd SNaN, -0.0 --> QNaN
+  // fadd +0.0, -0.0 --> -0.0 (but only with round toward negative)
+  if (canIgnoreSNaN(ExBehavior, FMF) &&
+      (!canRoundingModeBe(Rounding, RoundingMode::TowardNegative) ||
+       FMF.noSignedZeros()))
+    if (match(Op1, m_NegZeroFP()))
+      return Op0;
+
   if (!isDefaultFPEnvironment(ExBehavior, Rounding))
     return nullptr;
 
-  // fadd X, -0 ==> X
-  if (match(Op1, m_NegZeroFP()))
-    return Op0;
-
   // fadd X, 0 ==> X, when we know X is not -0
   if (match(Op1, m_PosZeroFP()) &&
       (FMF.noSignedZeros() || CannotBeNegativeZero(Op0, Q.TLI)))

diff --git a/llvm/test/Transforms/InstSimplify/strictfp-fadd.ll b/llvm/test/Transforms/InstSimplify/strictfp-fadd.ll
index 8f917ba..0d836b6 100644 (file)
--- a/llvm/test/Transforms/InstSimplify/strictfp-fadd.ll
+++ b/llvm/test/Transforms/InstSimplify/strictfp-fadd.ll
@@ -101,8 +101,7 @@ define <2 x float> @fadd_vec_x_n0_dynamic(<2 x float> %a) #0 {
 ; Test one of the remaining rounding modes and the rest will be fine.
 define float @fadd_x_n0_towardzero(float %a) #0 {
 ; CHECK-LABEL: @fadd_x_n0_towardzero(
-; CHECK-NEXT:    [[RET:%.*]] = call float @llvm.experimental.constrained.fadd.f32(float [[A:%.*]], float -0.000000e+00, metadata !"round.towardzero", metadata !"fpexcept.ignore") #[[ATTR0]]
-; CHECK-NEXT:    ret float [[RET]]
+; CHECK-NEXT:    ret float [[A:%.*]]
 ;
   %ret = call float @llvm.experimental.constrained.fadd.f32(float %a, float -0.0, metadata !"round.towardzero", metadata !"fpexcept.ignore") #0
   ret float %ret
@@ -112,8 +111,7 @@ define float @fadd_x_n0_towardzero(float %a) #0 {
 ; Test one of the remaining rounding modes and the rest will be fine.
 define <2 x float> @fadd_vec_x_n0_towardzero(<2 x float> %a) #0 {
 ; CHECK-LABEL: @fadd_vec_x_n0_towardzero(
-; CHECK-NEXT:    [[RET:%.*]] = call <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> [[A:%.*]], <2 x float> <float -0.000000e+00, float -0.000000e+00>, metadata !"round.towardzero", metadata !"fpexcept.ignore") #[[ATTR0]]
-; CHECK-NEXT:    ret <2 x float> [[RET]]
+; CHECK-NEXT:    ret <2 x float> [[A:%.*]]
 ;
   %ret = call <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> %a, <2 x float><float -0.0, float -0.0>, metadata !"round.towardzero", metadata !"fpexcept.ignore") #0
   ret <2 x float> %ret
@@ -121,8 +119,7 @@ define <2 x float> @fadd_vec_x_n0_towardzero(<2 x float> %a) #0 {
 
 define float @fadd_nnan_x_n0_ebmaytrap(float %a) #0 {
 ; CHECK-LABEL: @fadd_nnan_x_n0_ebmaytrap(
-; CHECK-NEXT:    [[RET:%.*]] = call nnan float @llvm.experimental.constrained.fadd.f32(float [[A:%.*]], float -0.000000e+00, metadata !"round.tonearest", metadata !"fpexcept.maytrap") #[[ATTR0]]
-; CHECK-NEXT:    ret float [[RET]]
+; CHECK-NEXT:    ret float [[A:%.*]]
 ;
   %ret = call nnan float @llvm.experimental.constrained.fadd.f32(float %a, float -0.0, metadata !"round.tonearest", metadata !"fpexcept.maytrap") #0
   ret float %ret
@@ -130,8 +127,7 @@ define float @fadd_nnan_x_n0_ebmaytrap(float %a) #0 {
 
 define <2 x float> @fadd_vec_nnan_x_n0_ebmaytrap(<2 x float> %a) #0 {
 ; CHECK-LABEL: @fadd_vec_nnan_x_n0_ebmaytrap(
-; CHECK-NEXT:    [[RET:%.*]] = call nnan <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> [[A:%.*]], <2 x float> <float -0.000000e+00, float -0.000000e+00>, metadata !"round.tonearest", metadata !"fpexcept.maytrap") #[[ATTR0]]
-; CHECK-NEXT:    ret <2 x float> [[RET]]
+; CHECK-NEXT:    ret <2 x float> [[A:%.*]]
 ;
   %ret = call nnan <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> %a, <2 x float><float -0.0, float -0.0>, metadata !"round.tonearest", metadata !"fpexcept.maytrap") #0
   ret <2 x float> %ret
@@ -140,7 +136,7 @@ define <2 x float> @fadd_vec_nnan_x_n0_ebmaytrap(<2 x float> %a) #0 {
 define float @fadd_nnan_x_n0_ebstrict(float %a) #0 {
 ; CHECK-LABEL: @fadd_nnan_x_n0_ebstrict(
 ; CHECK-NEXT:    [[RET:%.*]] = call nnan float @llvm.experimental.constrained.fadd.f32(float [[A:%.*]], float -0.000000e+00, metadata !"round.tonearest", metadata !"fpexcept.strict") #[[ATTR0]]
-; CHECK-NEXT:    ret float [[RET]]
+; CHECK-NEXT:    ret float [[A]]
 ;
   %ret = call nnan float @llvm.experimental.constrained.fadd.f32(float %a, float -0.0, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
   ret float %ret
@@ -149,7 +145,7 @@ define float @fadd_nnan_x_n0_ebstrict(float %a) #0 {
 define <2 x float> @fadd_vec_nnan_x_n0_ebstrict(<2 x float> %a) #0 {
 ; CHECK-LABEL: @fadd_vec_nnan_x_n0_ebstrict(
 ; CHECK-NEXT:    [[RET:%.*]] = call nnan <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> [[A:%.*]], <2 x float> <float -0.000000e+00, float -0.000000e+00>, metadata !"round.tonearest", metadata !"fpexcept.strict") #[[ATTR0]]
-; CHECK-NEXT:    ret <2 x float> [[RET]]
+; CHECK-NEXT:    ret <2 x float> [[A]]
 ;
   %ret = call nnan <2 x float> @llvm.experimental.constrained.fadd.v2f32(<2 x float> %a, <2 x float><float -0.0, float -0.0>, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
   ret <2 x float> %ret