[PATCH] Handle sqrt() shrinking in SimplifyLibCalls like any other call

[Hal Finkel]

Hrmm... I'm not entirely comfortable with this. Steve, do you have an opinion?

 -Hal

----- Original Message -----
> From: "Sanjay Patel" <spatel at rotateright.com>
> To: spatel at rotateright.com, beanz at apple.com, hfinkel at anl.gov
> Cc: llvm-commits at cs.uiuc.edu
> Sent: Thursday, October 23, 2014 2:24:50 PM
> Subject: Re: [PATCH] Handle sqrt() shrinking in SimplifyLibCalls like any other call
> 
> >>! In D5919#4, @hfinkel wrote:
> > Please don't loosen this for the function call (although leave it
> > as-is for the intrinsic). The intrinsic is different here,
> > especially because it is defined to have different semantics than
> > the function call, but we should not change the function call
> > itself without fast-math enabled. The underlying issue is that
> > (float) sqrt(x) != sqrtf(x) in general because of rounding issues
> > (and on some systems, the sqrtf is not exactly 1ulp accurate), and
> > we should not alter that without fast-math enabled.
> 
> Let me make sure we're on the same page. The existing behavior is to
> transform this function call:
>    float x;
>    float y = (float) sqrt ((double) x)
> 
> or in IR:
>    %conv = fpext float %f to double
>    %call = call double @sqrt(double %conv)
>    %conv1 = fptrunc double %call to float
>    ret float %conv1
> 
> into:
>   float y = sqrtf (x);
> 
> There's are 2 existing positive test cases in
> test/Transform/InstCombine/sqrt.ll that check for this pattern and 1
> negative test case to make sure we don't optimize in the event the
> result is used as a double.
> 
> Are you saying those existing test cases are invalid? I think that
> the sqrt function call test that I'm proposing to modify in
> double-float-shrink-1.ll would actually become redundant with the
> second test case in sqrt.ll; that was added for:
> http://llvm.org/bugs/show_bug.cgi?id=8096
> 
> http://reviews.llvm.org/D5919
> 
> 
> 

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