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

[Richard Biener <richard dot guenther at gmail dot com>]

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.

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>
>
>         * common.opt (ftrapping-math): Change default to 0.
>         * doc/invoke.texi (-ftrapping-math): Update documentation.
> --
>
> diff --git a/gcc/common.opt b/gcc/common.opt
> index 1bb87353f760d7c60c39de8b9de4311c1ec3d892..59940c64356964f8f9b9d842ad3f1a1c02548bab 100644
> --- a/gcc/common.opt
> +++ b/gcc/common.opt
> @@ -2470,7 +2470,7 @@ generate them instead of using descriptors.
>  ; (user-visible) trap.  This is the case, for example, in nonstop
>  ; IEEE 754 arithmetic.
>  ftrapping-math
> -Common Report Var(flag_trapping_math) Init(1) Optimization SetByCombined
> +Common Report Var(flag_trapping_math) Init(0) Optimization SetByCombined
>  Assume floating-point operations can trap.
>
>  ftrapv
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index 74c33ea35b9f320b419a3417e6007d2391536f1b..3673b34b3b7f7b57cfa6375b5316f9f282a9e9bb 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -9479,20 +9479,21 @@ This option implies that the sign of a zero result isn't significant.
>
>  The default is @option{-fsigned-zeros}.
>
> -@item -fno-trapping-math
> -@opindex fno-trapping-math
> -Compile code assuming that floating-point operations cannot generate
> +@item -ftrapping-math
> +@opindex ftrapping-math
> +Compile code assuming that floating-point operations can generate
>  user-visible traps.  These traps include division by zero, overflow,
> -underflow, inexact result and invalid operation.  This option requires
> -that @option{-fno-signaling-nans} be in effect.  Setting this option may
> -allow faster code if one relies on ``non-stop'' IEEE arithmetic, for example.
> +underflow, inexact result and invalid operation.
>
> -This option should never be turned on by any @option{-O} option since
> -it can result in incorrect output for programs that depend on
> -an exact implementation of IEEE or ISO rules/specifications for
> -math functions.
> +Note this option has only been partially implemented and does not work
> +as expected.  For example @option{-ftrapping-math} performs floating
> +point optimizations such as loop invariant motion, constant folding
> +and scheduling across function calls which have user-visible effects
> +on FP exception flags.
> +
> +This option is turned on when using @option{-fsignaling-nans}.
>
> -The default is @option{-ftrapping-math}.
> +The default is @option{-fno-trapping-math}.
>
>  @item -frounding-math
>  @opindex frounding-math