Optimization bug with floating-point?

[Steve Kargl]

On Wed, Mar 13, 2019 at 09:32:57AM -0700, John Baldwin wrote:
> On 3/13/19 8:16 AM, Steve Kargl wrote:
> > On Tue, Mar 12, 2019 at 07:45:41PM -0700, Steve Kargl wrote:
> >>
> >> gcc8 --version
> >> gcc8 (FreeBSD Ports Collection) 8.3.0
> >>
> >> gcc8 -fno-builtin -o z a.c -lm && ./z
> >> gcc8 -O -fno-builtin -o z a.c -lm && ./z
> >> gcc8 -O2 -fno-builtin -o z a.c -lm && ./z
> >> gcc8 -O3 -fno-builtin -o z a.c -lm && ./z
> >>
> >> Max ULP: 2.297073
> >> Count: 0           (# of ULP that exceed 21)
> >>
> > 
> > clang agrees with gcc8 if one changes ...
> > 
> >> int
> >> main(void)
> >> {
> >>    double re, im, u, ur, ui;
> >>    float complex f;
> >>    float x, y;
> > 
> > this line to "volatile float x, y".
> 
> So it seems to be a regression in clang 7 vs clang 6?
> 

/usr/local/bin/clang60 has the same problem.  

% /usr/local/bin/clang60 -o z -O2 a.c -lm && ./z
  Maximum ULP: 23.061242
# of ULP > 21: 39

Adding volatile as in the above "fixes" the problem.

AFAICT, this a i386/387 code generation problem.  Perhaps,
an alignment issue?

-- 
Steve
20170425 https://www.youtube.com/watch?v=VWUpyCsUKR4
20161221 https://www.youtube.com/watch?v=IbCHE-hONow