On Fri, 3 May 2024, Richard Biener wrote:
> So what I do not necessarily agree with is that we need to preserve
> the multiplication with -fsignaling-nans. Do we consider a program doing
>
> handler() { exit(0); }
>
> x = sNaN;
> ...
> sigaction(SIGFPE, ... handler)
> x*x;
> format_hard_drive();
>
> and expecting the program to exit(0) rather than formating the hard-disk
> to be expecting something the C standard guarantees? And is it enough
> for the program to enable -fsignaling-nans for this?
>
> If so then the first and foremost bug is that 'x*x' doesn't have
> TREE_SIDE_EFFECTS
> set and thus we do not preserve it when optimizing __builtin_signbit () of it.
Signaling NaNs don't seem relevant here. "Signal" means "set the
exception flag" - and 0 * Inf raises the same "invalid" exception flag as
sNaN * sNaN. Changing flow of control on an exception is outside the
scope of standard C and requires nonstandard extensions such as
feenableexcept. (At present -ftrapping-math covers both kinds of
exception handling - the default setting of a flag, and the nonstandard
change of flow of control.)
--
Joseph S. Myers
josmy...@redhat.com
