Re: [PATCH] Disable -ftrapping-math by default

[Marc Glisse <marc dot glisse at inria dot fr>]

On Thu, 16 Nov 2017, Richard Biener wrote:

> On Thu, Nov 16, 2017 at 3:33 PM, Wilco Dijkstra <Wilco.Dijkstra@arm.com> wrote:
> > GCC currently defaults to -ftrapping-math.  This is supposed to generate
> > code for correct user-visible traps and FP status flags.
> >
> > However it doesn't work as expected since it doesn't block any floating
> > point optimizations.  For example it continues to perform CSE, moves FP
> > operations across calls, moves FP operations out of loops, constant folds
> > and removes dead floating point operations that cause exceptions.
> >
> > Given the majority of code doesn't contain user trap handlers or inspects
> > FP status flags, there is no point in enabling it even if it worked as expected.
> >
> > Simple case that should cause a FP exception:
> >
> > void f(void)
> > {
> >   0.0 / 0.0;
> > }
> >
> > Compiles to:
> >
> > f:
> >         ret
>
> We are generally not preserving traps but we guard any transform that
> might introduce traps with -ftrapping-math.  That's similar to how we treat
> -ftrapv and pointer dereferences.

Joseph seems to have a different opinion in PR 53805. Splitting the option 
(one strict version that preserves floating point status changes, one weak 
version that only avoids introducing new traps but may remove existing 
ones) would help clarify the situation.

> We're mitigating the "bad" effect of the -ftrapping-math default
> by defaulting to -fno-signalling-nans.
>
> If it doesn't block any optimizations what's the point of the patch?
>
> Richard.
>
> > OK for commit?
> >
> > 2017-11-16  Wilco Dijkstra  <wdijkstr@arm.com>

You could mention PR 54192 here.

> >         * common.opt (ftrapping-math): Change default to 0.
> >         * doc/invoke.texi (-ftrapping-math): Update documentation.

I am surprised that no testsuite change is needed.

--
Marc Glisse