[PATCH] Returns NaN for sqrt with negative fp argument

[James Molloy]

Hi Tim,

> I think that might be an orthogonal issue. What you want is an
> IEEE-compliant sqrt, except that it doesn't necessarily set errno.
> LLVM already has this concept: it's represented by a call to @sqrt
> with the "readnone" attribute. Clang produces it with -fno-math-errno
> (via SemaDecl.cpp:10290 ultimately)

Lovely, thanks! I didn't know about that. A quick test proves that call produces the expected fsqrt intrinsic. Now I just need a small pass to emit it with a fallback.

Cheers,

James

> -----Original Message-----
> From: Tim Northover [mailto:t.p.northover at gmail.com]
> Sent: 11 June 2014 08:48
> To: James Molloy
> Cc: Jiangning Liu; llvm-commits
> Subject: Re: [PATCH] Returns NaN for sqrt with negative fp argument
>
> Hi James,
>
> On 11 June 2014 08:14, James Molloy <james.molloy at arm.com> wrote:
> > Could I please weigh in here? I’d like the behaviour to be NaN too, but
> for a different reason.
>
> I think that might be an orthogonal issue. What you want is an
> IEEE-compliant sqrt, except that it doesn't necessarily set errno.
> LLVM already has this concept: it's represented by a call to @sqrt
> with the "readnone" attribute. Clang produces it with -fno-math-errno
> (via SemaDecl.cpp:10290 ultimately)
>
> From Jiangning:
> > I think it is not OK, because undef means an undefined value, and for
> well-defined program, this undef shouldn't affect semantic
>
> @llvm.sqrt seems to be for true -ffast-math situations, where calling
> with a negative number is user error; the program is not well-defined
> if that happens in -ffast-math. Which is why I still think undef is
> the right choice.
>
> Cheers.
>
> Tim.


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