On Mon, 15 Aug 2022, Jakub Jelinek wrote:
> 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.
Ah, I misread
+static rtx
+expand_builtin_issignaling (tree exp, rtx target)
+{
+ if (!validate_arglist (exp, REAL_TYPE, VOID_TYPE))
+ return NULL_RTX;
+
+ tree arg = CALL_EXPR_ARG (exp, 0);
+ scalar_float_mode fmode = SCALAR_FLOAT_TYPE_MODE (TREE_TYPE (arg));
+ const struct real_format *fmt = REAL_MODE_FORMAT (fmode);
+
+ /* Expand the argument yielding a RTX expression. */
+ rtx temp = expand_normal (arg);
+
+ /* If mode doesn't support NaN, always return 0. */
+ if (!HONOR_NANS (fmode))
+ {
+ emit_move_insn (target, const0_rtx);
+ return target;
which doesn't use HONOR_SNANS but still HONOR_NANS and thus
-ffinite-math-only. You possibly want MODE_HAS_NANS instead here?
> > 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.
So what about __builtin_issignaling then? Do we want to silently
ignore errors there?
Richard.
