https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112962
--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
On the other side, maybe we want some of the diagnostics that is only done at
expansion time (argument xyz must be such and such immediate). Though, at -O2
it isn't diagnosed anyway because it is DCEd.
Anyway, with just -O0 -mssse3 this works fine because of expr.cc:
11097 /* If we are going to ignore this result, we need only do something
11098 if there is a side-effect somewhere in the expression. If there
11099 is, short-circuit the most common cases here. Note that we must
11100 not call expand_expr with anything but const0_rtx in case this
11101 is an initial expansion of a size that contains a
PLACEHOLDER_EXPR. */
11102
11103 if (ignore)
11104 {
11105 if (! TREE_SIDE_EFFECTS (exp))
11106 return const0_rtx;
but with -fexceptions (and probably because we incorrectly don't mark the
builtins nothrow?) this doesn't happen.
I was thinking about something like
--- gcc/i386-expand.cc.jj 2023-12-07 08:31:59.855850982 +0100
+++ gcc/i386-expand.cc 2023-12-12 11:02:54.524733315 +0100
@@ -11842,6 +11842,12 @@ ix86_expand_args_builtin (const struct b
xops[i] = op;
}
+ if (icode == CODE_FOR_nothing)
+ {
+ gcc_assert (target == const0_rtx);
+ return const0_rtx;
+ }
+
switch (nargs)
{
case 1:
but of course, your choice...
