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expmed: Fix store_integral_bit_field [PR101562]

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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.
This commit is contained in:
Jakub Jelinek 2021-07-23 19:55:16 +02:00
parent 435f90187e
commit 8408d34570
2 changed files with 25 additions and 1 deletions
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5
gcc/expmed.c
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@ -921,7 +921,10 @@ store_integral_bit_field (rtx op0, opt_scalar_int_mode op0_mode,
}
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);

21
gcc/testsuite/gcc.c-torture/compile/pr101562.c Normal file
View File

@ -0,0 +1,21 @@
/* 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]);
}