[PATCH] D28797: [LangRef] Make @llvm.sqrt(x) return undef, rather than have UB, for negative x.

Hal Finkel via Phabricator via llvm-commits llvm-commits at lists.llvm.org
Tue Jan 17 12:00:54 PST 2017

hfinkel added a comment.

In https://reviews.llvm.org/D28797#648535, @efriedma wrote:

> > 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.
> This is false: we take advantage of this to lower @llvm.sqrt() to libm sqrt() on platforms which don't have a native sqrt instruction, and that can have side-effects.  See also https://reviews.llvm.org/D28335 .

We do, but this seems somewhat accidental. Most of the libm intrinsics have this problem, and sqrt is the only one for which we have this undefined behavior.


More information about the llvm-commits mailing list