Our documentation says that paradoxical subregs shouldn't appear
in strict_low_part:
'(strict_low_part (subreg:M (reg:N R) 0))'
This expression code is used in only one context: as the
destination operand of a 'set' expression. In addition, the
operand of this expression must be a non-paradoxical 'subreg'
expression.
but on the testcase below that triggers UB at runtime
store_integral_bit_field emits exactly that.
The following patch fixes it by ensuring the requirement is satisfied.
2021-07-23 Jakub Jelinek <jakub@redhat.com>
PR rtl-optimization/101562
* expmed.c (store_integral_bit_field): Only use movstrict_optab
if the operand isn't paradoxical.
* gcc.c-torture/compile/pr101562.c: New test.
(cherry picked from commit
8408d34570c9fe9f3d22a25a76df2a4c64f08477)
}
subreg_off = bitnum / BITS_PER_UNIT;
- if (validate_subreg (fieldmode, GET_MODE (arg0), arg0, subreg_off))
+ if (validate_subreg (fieldmode, GET_MODE (arg0), arg0, subreg_off)
+ /* STRICT_LOW_PART must have a non-paradoxical subreg as
+ operand. */
+ && !paradoxical_subreg_p (fieldmode, GET_MODE (arg0)))
{
arg0 = gen_rtx_SUBREG (fieldmode, arg0, subreg_off);
--- /dev/null
+/* PR rtl-optimization/101562 */
+
+struct S { char c; };
+void baz (struct S a, struct S b);
+
+void
+foo (void)
+{
+ struct S x[1];
+ *(short *)&x[0] = 256;
+ baz (x[0], x[1]);
+}
+
+void
+bar (void)
+{
+ struct S x[1];
+ x[0].c = 0;
+ x[1].c = 1;
+ baz (x[0], x[1]);
+}