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

[Eli Friedman]

On Thu, Sep 12, 2013 at 2:44 PM, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
> >
> > 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.
>
> I agree, and the LangRef does indeed say "undefined behavior", but I
> assume that should really mean, "returns an undefined value." Do you agree?
>

Well, if we map llvm.sqrt to sqrt and sqrt sets errno, we really do mean
"undefined behavior"... or at least something more that "returns an
undefined value".

>
> >
> > 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().)
>
> Perhaps, but I'm not changing that here. For one thing, if the mapping
> does, in effect, sqrt -> llvm.sqrt -> sqrt, and only when -fmath-errno=0.
> Are you worried about cases where the libm functions actually do set errno
> (even though we have -fmath-errno=0)?
>
>
I'm not sure our implementation of -fno-math-errno is safe: according to
the gcc manual, it isn't equivalent to marking the math functions with
attribute((const)).  (For example, the gcc manual's definition allows
transforming a call to sqrt() into the SSE sqrt instruction, but it doesn't
allow hoisting a call to sqrt out of arbitrary loops on a machine where the
sqrt() call could set errno.)

-Eli
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