[ConstantFolding] Constant-fold llvm.sqrt(x) like other intrinsics.

Summary:
Currently we return undef, but we're in the process of changing the
LangRef so that llvm.sqrt behaves like the other math intrinsics,
matching the return value of the standard libcall but not setting errno.

This change is legal even without the LangRef change because currently
calling llvm.sqrt(x) where x is negative is spec'ed to be UB.  But in
practice it's also safe because we're simply constant-folding fewer
inputs: Inputs >= -0 get constant-folded as before, but inputs < -0 now
aren't constant-folded, because ConstantFoldFP aborts if the host math
function raises an fp exception.

Reviewers: hfinkel, efriedma, sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28929

llvm-svn: 292692

diff --git a/llvm/lib/Analysis/ConstantFolding.cpp b/llvm/lib/Analysis/ConstantFolding.cpp
index 7386727..db8ac82 100644 (file)
--- a/llvm/lib/Analysis/ConstantFolding.cpp
+++ b/llvm/lib/Analysis/ConstantFolding.cpp
@@ -1630,6 +1630,8 @@ Constant *ConstantFoldScalarCall(StringRef Name, unsigned IntrinsicID, Type *Ty,
           return ConstantFoldFP(sin, V, Ty);
         case Intrinsic::cos:
           return ConstantFoldFP(cos, V, Ty);
+        case Intrinsic::sqrt:
+          return ConstantFoldFP(sqrt, V, Ty);
       }
 
       if (!TLI)
@@ -1683,19 +1685,6 @@ Constant *ConstantFoldScalarCall(StringRef Name, unsigned IntrinsicID, Type *Ty,
         else if ((Name == "log10" && V > 0 && TLI->has(LibFunc::log10)) ||
                  (Name == "log10f" && V > 0 && TLI->has(LibFunc::log10f)))
           return ConstantFoldFP(log10, V, Ty);
-        else if (IntrinsicID == Intrinsic::sqrt &&
-                 (Ty->isHalfTy() || Ty->isFloatTy() || Ty->isDoubleTy())) {
-          if (V >= -0.0)
-            return ConstantFoldFP(sqrt, V, Ty);
-          else {
-            // Unlike the sqrt definitions in C/C++, POSIX, and IEEE-754 - which
-            // all guarantee or favor returning NaN - the square root of a
-            // negative number is not defined for the LLVM sqrt intrinsic.
-            // This is because the intrinsic should only be emitted in place of
-            // libm's sqrt function when using "no-nans-fp-math".
-            return UndefValue::get(Ty);
-          }
-        }
         break;
       case 'r':
         if ((Name == "round" && TLI->has(LibFunc::round)) ||

diff --git a/llvm/test/Transforms/InstCombine/constant-fold-math.ll b/llvm/test/Transforms/InstCombine/constant-fold-math.ll
index ce8d337..6eb371a 100644 (file)
--- a/llvm/test/Transforms/InstCombine/constant-fold-math.ll
+++ b/llvm/test/Transforms/InstCombine/constant-fold-math.ll
@@ -45,9 +45,10 @@ define double @constant_fold_fmuladd_f64() #0 {
   ret double %x
 }
 
-; The sqrt intrinsic is undefined for negative inputs besides -0.0.
+; Currently we don't constant-fold intrinsics whose corresponding libcalls
+; raise an fp exception.
 ; CHECK-LABEL: @bad_sqrt
-; CHECK-NEXT: ret double undef
+; CHECK-NEXT: call double @llvm.sqrt.f64(double -2
 define double @bad_sqrt() {
   %x = call double @llvm.sqrt.f64(double -2.000000e+00)
   ret double %x