Re: [PATCH] Implement __builtin_issignaling

[Jakub Jelinek <jakub@redhat.com>]

On Mon, Aug 15, 2022 at 03:06:16PM +0200, Jakub Jelinek via Gcc-patches wrote:
> On Mon, Aug 15, 2022 at 12:07:38PM +0000, Richard Biener wrote:
> > 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;
> 
> I think I can expand on the comment why HONOR_NANS instead of HONOR_SNANS
> and also add comment to the folding case.

So what about like in this incremental patch:

--- gcc/builtins.cc.jj	2022-08-16 13:23:04.220103861 +0200
+++ gcc/builtins.cc	2022-08-16 13:32:03.411257574 +0200
@@ -2765,7 +2765,13 @@ expand_builtin_issignaling (tree exp, rt
   /* Expand the argument yielding a RTX expression. */
   rtx temp = expand_normal (arg);
 
-  /* If mode doesn't support NaN, always return 0.  */
+  /* If mode doesn't support NaN, always return 0.
+     Don't use !HONOR_SNANS (fmode) here, so there is some possibility of
+     __builtin_issignaling working without -fsignaling-nans.  Especially
+     when -fno-signaling-nans is the default.
+     On the other side, MODE_HAS_NANS (fmode) is unnecessary, with
+     -ffinite-math-only even __builtin_isnan or __builtin_fpclassify
+     fold to 0 or non-NaN/Inf classification.  */
   if (!HONOR_NANS (fmode))
     {
       emit_move_insn (target, const0_rtx);
@@ -9259,6 +9265,12 @@ fold_builtin_classify (location_t loc, t
       return fold_build2_loc (loc, UNORDERED_EXPR, type, arg, arg);
 
     case BUILT_IN_ISSIGNALING:
+      /* Folding to true for REAL_CST is done in fold_const_call_ss.
+	 Don't use tree_expr_signaling_nan_p (arg) -> integer_one_node
+	 and !tree_expr_maybe_signaling_nan_p (arg) -> integer_zero_node
+	 here, so there is some possibility of __builtin_issignaling working
+	 without -fsignaling-nans.  Especially when -fno-signaling-nans is
+	 the default.  */
       if (!tree_expr_maybe_nan_p (arg))
 	return omit_one_operand_loc (loc, type, integer_zero_node, arg);
       return NULL_TREE;

> > > 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?
> 
> I think we should just restrict it to the scalar floating point types.
> After all, other typegeneric builtins that are or can be used similarly
> do the same thing.

Note, that is what the patch does currently (rejecting _Complex
{float,double,long double} etc. arguments).

	Jakub