[SimplifyLibCalls] Restore the old behaviour, emit a libcall.

Hal pointed out that the semantic of our intrinsic and the libc
call are slightly different. Add a comment while I'm here to
explain why we can't emit an intrinsic. Thanks Hal!

llvm-svn: 278200

1 files changed, 5 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
index a958eee..c10a1ff 100644
--- a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -1052,9 +1052,11 @@ Value *LibCallSimplifier::optimizePow(CallInst *CI, IRBuilder<> &B) {
     if (CI->hasUnsafeAlgebra()) {
       IRBuilder<>::FastMathFlagGuard Guard(B);
       B.setFastMathFlags(CI->getFastMathFlags());
-      Value *Sqrt = Intrinsic::getDeclaration(CI->getModule(), Intrinsic::sqrt,
-                                              Op1->getType());
-      return B.CreateCall(Sqrt, Op1, "sqrt");
+
+      // Unlike other math intrinsics, sqrt has differerent semantics
+      // from the libc function. See LangRef for details.
+      return emitUnaryFloatFnCall(Op1, TLI->getName(LibFunc::sqrt), B,
+                                  Callee->getAttributes());
     }
 
     // Expand pow(x, 0.5) to (x == -infinity ? +infinity : fabs(sqrt(x))).