[LangRef] Make @llvm.sqrt(x) return undef, rather than have UB, for n…

…egative x.

Summary:
Some frontends emit a speculate-and-select idiom for sqrt, wherein they compute
sqrt(x), check if x is negative, and select NaN if it is:

  %cmp = fcmp olt double %a, -0.000000e+00
  %sqrt = call double @llvm.sqrt.f64(double %a)
  %ret = select i1 %cmp, double 0x7FF8000000000000, double %sqrt

This is technically UB as the LangRef is written today if %a is ever less than
-0.  But emitting code that's compliant with the current definition of sqrt
would require a branch, which would then prevent us from matching this idiom in
SelectionDAG (which we do today -- ISD::FSQRT has defined behavior on negative
inputs), because SelectionDAG looks at one BB at a time.

Nothing in LLVM takes advantage of this undefined behavior, as far as we can
tell, and the fact that llvm.sqrt has UB dates from its initial addition to the
LangRef.

Reviewers: arsenm, mehdi_amini, hfinkel

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D28797

llvm-svn: 293242

llvm/docs/LangRef.rst

@@ -10076,11 +10076,8 @@ Overview:
 """""""""
 
 The '``llvm.sqrt``' intrinsics return the sqrt of the specified operand,
-returning the same value as the libm '``sqrt``' functions would. Unlike
-``sqrt`` in libm, however, ``llvm.sqrt`` has undefined behavior for
-negative numbers other than -0.0 (which allows for better optimization,
-because there is no need to worry about errno being set).
-``llvm.sqrt(-0.0)`` is defined to return -0.0 like IEEE sqrt.
+returning the same value as the libm '``sqrt``' functions would, but without
+trapping or setting ``errno``.
 
 Arguments:
 """"""""""

llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp

@@ -1097,8 +1097,11 @@ Value *LibCallSimplifier::optimizePow(CallInst *CI, IRBuilder<> &B) {
       IRBuilder<>::FastMathFlagGuard Guard(B);
       B.setFastMathFlags(CI->getFastMathFlags());
 
-      // Here we cannot lower to an intrinsic because C99 sqrt() and llvm.sqrt
-      // are not guaranteed to have the same semantics.
+      // TODO: If the pow call is an intrinsic, we should lower to the sqrt
+      // intrinsic, so we match errno semantics.  We also should check that the
+      // target can in fact lower the sqrt intrinsic -- we currently have no way
+      // to ask this question other than asking whether the target has a sqrt
+      // libcall, which is a sufficient but not necessary condition.
       Value *Sqrt = emitUnaryFloatFnCall(Op1, TLI->getName(LibFunc_sqrt), B,
                                          Callee->getAttributes());
 
@@ -1115,8 +1118,8 @@ Value *LibCallSimplifier::optimizePow(CallInst *CI, IRBuilder<> &B) {
       IRBuilder<>::FastMathFlagGuard Guard(B);
       B.setFastMathFlags(CI->getFastMathFlags());
 
-      // Unlike other math intrinsics, sqrt has differerent semantics
-      // from the libc function. See LangRef for details.
+      // TODO: As above, we should lower to the sqrt intrinsic if the pow is an
+      // intrinsic, to match errno semantics.
       return emitUnaryFloatFnCall(Op1, TLI->getName(LibFunc_sqrt), B,
                                   Callee->getAttributes());
     }
@@ -1127,6 +1130,9 @@ Value *LibCallSimplifier::optimizePow(CallInst *CI, IRBuilder<> &B) {
     // TODO: In finite-only mode, this could be just fabs(sqrt(x)).
     Value *Inf = ConstantFP::getInfinity(CI->getType());
     Value *NegInf = ConstantFP::getInfinity(CI->getType(), true);
+
+    // TODO: As above, we should lower to the sqrt intrinsic if the pow is an
+    // intrinsic, to match errno semantics.
     Value *Sqrt = emitUnaryFloatFnCall(Op1, "sqrt", B, Callee->getAttributes());
 
     Module *M = Callee->getParent();
@@ -1309,8 +1315,11 @@ Value *LibCallSimplifier::optimizeLog(CallInst *CI, IRBuilder<> &B) {
 Value *LibCallSimplifier::optimizeSqrt(CallInst *CI, IRBuilder<> &B) {
   Function *Callee = CI->getCalledFunction();
   Value *Ret = nullptr;
+  // TODO: Once we have a way (other than checking for the existince of the
+  // libcall) to tell whether our target can lower @llvm.sqrt, relax the
+  // condition below.
   if (TLI->has(LibFunc_sqrtf) && (Callee->getName() == "sqrt" ||
-                             Callee->getIntrinsicID() == Intrinsic::sqrt))
+                                  Callee->getIntrinsicID() == Intrinsic::sqrt))
     Ret = optimizeUnaryDoubleFP(CI, B, true);
 
   if (!CI->hasUnsafeAlgebra())