Make the sqrt intrinsic return undef for a negative input.

[spatel]

Summary

As discussed here:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140609/220598.html

And again here:
http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-September/077168.html

The sqrt of a negative number when using the llvm intrinsic is undefined. We should return undef rather than 0.0 to match the definition in the LLVM IR lang ref.

This change should not affect any code that isn't using "no-nans-fp-math"; ie, no-nans is a requirement for generating the llvm intrinsic in place of a sqrt function call.

Unfortunately, the behavior introduced by this patch will not match current gcc, xlc, icc, and possibly other compilers. The current clang/llvm behavior of returning 0.0 doesn't either. We knowingly approve of this difference with the other compilers in an attempt to flag code that is invoking undefined behavior.

A front-end warning should also try to convince the user that the program will fail:
http://llvm.org/bugs/show_bug.cgi?id=21093

[hfinkel]

LGTM.

lib/Analysis/ConstantFolding.cpp

1527

I'd say, "... should only be emitted in place of a call to libm's sqrt function when using..."

[spatel]

Closed by commit rL218803 (authored by @spatel).

lib/Analysis/ConstantFolding.cpp

                  (Ty->isHalfTy() || Ty->isFloatTy() || Ty->isDoubleTy())) {
           if (V >= -0.0)
             return ConstantFoldFP(sqrt, V, Ty);
-          else // Undefined
-            return Constant::getNullValue(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 generated when using
+            // "no-nans-fp-math".
+            return UndefValue::get(Ty);
+          }
         }
         break;
       case 's':

test/Transforms/InstCombine/constant-fold-math.ll

 declare double @llvm.fma.f64(double, double, double) #0
 declare double @llvm.fmuladd.f64(double, double, double) #0
 
+declare double @llvm.sqrt.f64(double) #0
 
 
 ; CHECK-LABEL: @constant_fold_fma_f32

   ret double %x
 }
 
+; The sqrt intrinsic is undefined for negative inputs besides -0.0.
+; CHECK-LABEL: @bad_sqrt
+; CHECK-NEXT: ret double undef
+define double @bad_sqrt() {
+  %x = call double @llvm.sqrt.f64(double -2.000000e+00)
+  ret double %x
+}
+
 attributes #0 = { nounwind readnone }