Re: [PATCH] PR middle-end/111701: signbit(x*x) vs -fsignaling-nans

[Joseph Myers <josmyers@redhat.com>]

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
josmyers@redhat.com