builtins: Force SAVE_EXPR for __builtin_{add,sub,mul}_overflow and __builtin{add,sub}c [PR108789]

The following testcase is miscompiled, because we use save_expr
on the .{ADD,SUB,MUL}_OVERFLOW call we are creating, but if the first
two operands are not INTEGER_CSTs (in that case we just fold it right away)
but are TREE_READONLY/!TREE_SIDE_EFFECTS, save_expr doesn't actually
create a SAVE_EXPR at all and so we lower it to
*arg2 = REALPART_EXPR (.ADD_OVERFLOW (arg0, arg1)), \
IMAGPART_EXPR (.ADD_OVERFLOW (arg0, arg1))
which evaluates the ifn twice and just hope it will be CSEd back.
As *arg2 aliases *arg0, that is not the case.
The builtins are really never const/pure as they store into what
the third arguments points to, so after handling the INTEGER_CST+INTEGER_CST
case, I think we should just always use SAVE_EXPR.  Just building SAVE_EXPR
by hand and setting TREE_SIDE_EFFECTS on it doesn't work, because
c_fully_fold optimizes it away again, so the following patch marks the
ifn calls as TREE_SIDE_EFFECTS (but doesn't do it for the
__builtin_{add,sub,mul}_overflow_p case which were designed for use
especially in constant expressions and don't really evaluate the
realpart side, so we don't really need a SAVE_EXPR in that case).

2024-06-04  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/108789
	* builtins.cc (fold_builtin_arith_overflow): For ovf_only,
	don't call save_expr and don't build REALPART_EXPR, otherwise
	set TREE_SIDE_EFFECTS on call before calling save_expr.
	(fold_builtin_addc_subc): Set TREE_SIDE_EFFECTS on call before
	calling save_expr.

	* gcc.c-torture/execute/pr108789.c: New test.

1 files changed, 39 insertions, 0 deletions

diff --git a/gcc/testsuite/gcc.c-torture/execute/pr108789.c b/gcc/testsuite/gcc.c-torture/execute/pr108789.c
new file mode 100644
index 0000000..32ee19b
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/execute/pr108789.c
@@ -0,0 +1,39 @@
+/* PR middle-end/108789 */
+
+int
+add (unsigned *r, const unsigned *a, const unsigned *b)
+{
+  return __builtin_add_overflow (*a, *b, r);
+}
+
+int
+mul (unsigned *r, const unsigned *a, const unsigned *b)
+{
+  return __builtin_mul_overflow (*a, *b, r);
+}
+
+int
+main ()
+{
+  unsigned x;
+
+  /* 1073741824U + 1073741824U should not overflow.  */
+  x = (__INT_MAX__ + 1U) / 2;
+  if (add (&x, &x, &x))
+    __builtin_abort ();
+
+  /* 256U * 256U should not overflow */
+  x = 1U << (sizeof (int) * __CHAR_BIT__ / 4);
+  if (mul (&x, &x, &x))
+    __builtin_abort ();
+
+  /* 2147483648U + 2147483648U should overflow */
+  x = __INT_MAX__ + 1U;
+  if (!add (&x, &x, &x))
+    __builtin_abort ();
+
+  /* 65536U * 65536U should overflow */
+  x = 1U << (sizeof (int) * __CHAR_BIT__ / 2);
+  if (!mul (&x, &x, &x))
+    __builtin_abort ();
+}