Re: [PATCH] Implement __builtin_issignaling

[Jakub Jelinek <jakub@redhat.com>]

On Mon, Aug 15, 2022 at 11:24:14AM +0000, Richard Biener wrote:
> Unlike the issignalling macro from glibc the builtin will return
> false for sNaN arguments when -fno-signalling-nans is used (similar
> to isinf, isnan, etc.).  I think this deserves mentioning in the
> documentation (and I have my reservations about this long-time
> behavior of FP classification builtins we have).

I have actually tried to make the builtin working even with
-fno-signaling-nans (i.e. the default).
That is why the folding is done only if the argument is REAL_CST
or if !tree_expr_maybe_nan_p (arg).
At one point I was doing the folding when
tree_expr_signaling_nan_p (arg) (to true) or
!tree_expr_maybe_signaling_nan_p (arg) (to false) and in that
case indeed -fsignaling-nans was a requirement.
-fsignaling-nans is used in the tests nevertheless because the
tests really do care about sNaNs, so I've turned on the option
that says they should be honored.

> Generally it looks OK - what does it do to size optimized code?

The problem is that except for the glibc __issignaling{f,,l,f128}
entrypoints, other C libraries don't implement it, so there is nothing to
fallback to (unless we want to also implement it in libgcc.a).

For float/double, it is relatively short:
        movd    %xmm0, %eax
        xorl    $4194304, %eax
        andl    $2147483647, %eax
        cmpl    $2143289344, %eax
        seta    %al
        movzbl  %al, %eax
which e.g. for if (__builtin_issignaling (arg)) could be even simplified
further by just doing ja or jna, resp.
        movabsq $9221120237041090560, %rdx
        movq    %xmm0, %rax
        btcq    $51, %rax
        btrq    $63, %rax
        cmpq    %rax, %rdx
        setb    %al
        movzbl  %al, %eax
For long double (especially Intel) / _Float128 it is larger (26 insns for XFmode,
15 for _Float128), sure.

> glibc 2.31 seems to silently accept
> 
> #include <tgmath.h>
> 
> int foo(_Complex double x)
> {
>   return issignaling (x);
> }

That seems like a glibc bug/weird feature in the __MATH_TG macro
or _Generic.
When compiled with C++ it is rejected.

	Jakub