[PATCH] Restore the sqrt -> llvm.sqrt mapping in fast-math mode

[Eli Friedman]

On Thu, Sep 12, 2013 at 2:03 PM, Hal Finkel <hfinkel at anl.gov> wrote:

> Hello,
>
> Please review the attached patch which restores the libm sqrt* ->
> @llvm.sqrt* mapping, but only in fast-math mode (specifically, when the
> UnsafeFPMath or NoNaNsFPMath CodeGen options are enabled). The @llvm.sqrt*
> intrinsics have slightly different semantics from the libm call,
> specifically, they are undefined when given a non-zero negative number (the
> libm calls will always return NaN for any negative number).
>
> This mapping was removed in r100613, and replaced with a TODO, but at that
> time the fast-math flags were not yet implemented. Now that we have these,
> restoring this mapping is important because it will enable
> autovectorization of sqrt calls in loops (at least in fast-math mode).
>
>
This is dangerous, if LangRef is actually correct.  People don't associate
-ffast-math with "my program will crash at random". :)  Of course, LangRef
is probably overstating the issue.

That said, there's actually a general issue here: if we map the LLVM
intrinsics to libc functions, and the libc functions set errno, we could
break code that depends on errno for non-math calls (e.g. fopen().)

-Eli
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