[PATCH] These builtin functions set errno. Mark them accordingly.

[Hal Finkel]

----- Original Message -----
> From: "Chris Lattner" <clattner at apple.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: rjmccall at gmail.com, cfe-commits at cs.uiuc.edu, reviews+D3806+public+8377178d9ac8d89d at reviews.llvm.org
> Sent: Monday, May 19, 2014 6:37:46 PM
> Subject: Re: [PATCH] These builtin functions set errno. Mark them accordingly.
> 
> On May 19, 2014, at 10:05 AM, Hal Finkel <hfinkel at anl.gov> wrote:
> >>> 
> >>> I'm not going to push this patch further because it's not the
> >>> right
> >>> solution.  In theory, I believe it is correct, but it breaks the
> >>> __buitlin escape hatch, which would likely result in serious
> >>> performance degradations.
> >> 
> >> I don’t think this is a good solution.  The better option here is
> >> to
> >> tell people to use -fno-math-errno if they don’t care about libm
> >> functions setting errno.
> > 
> > But this is the chicken and egg problem: is __builtin_sqrt a libm
> > function? The problem that we currently have, IIUC, is that the
> > answer is *sometimes*.
> 
> I don’t really care about the builtin,

We need to decide on the semantics of the builtin when -fmath-errno is in effect: might the builtin clobber errno? The current answer is sometimes, the useful answer is no (IMHO) -- at least the useful answer is no when the target really can implement the function without clobbering errno.

> it is a transient part of the
> AST and having the attributes on it be changed when -fno-math-errno
> is set is natural and probably already done.

We currently do this for the libm calls themselves, but not for the builtins. This allows us to use the __builtin version to get the 'readnone' version of the function, even when -fmath-errno is in effect. Presumably, this permits the target to directly lower to some instruction (something not normally possible with -fmath-errno and the libm function).

> 
> At the IR level, we already have attributes to model both states:
> llvm.sqrt when set to readnone doesn’t modify errno.

Yes.

 -Hal

> 
> -Chris
> 
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory