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combine: Fix up simplify_shift_const_1 for nested ROTATEs [PR97386]

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The following testcases are miscompiled (the first one since my improvements
to rotate discovery on GIMPLE, the other one for many years) because
combiner optimizes nested ROTATEs with narrowing SUBREG in between (i.e.
the outer rotate is performed in shorter precision than the inner one) to
just one ROTATE of the rotated constant.  While that (under certain
conditions) can work for shifts, it can't work for rotates where we can only
do that with rotates of the same precision.

2020-10-13  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/97386
	* combine.c (simplify_shift_const_1): Don't optimize nested ROTATEs if
	they have different modes.

	* gcc.c-torture/execute/pr97386-1.c: New test.
	* gcc.c-torture/execute/pr97386-2.c: New test.
This commit is contained in:
Jakub Jelinek 2020-10-13 19:13:26 +02:00
parent 0e0beddd7f
commit f76949cee9
3 changed files with 41 additions and 2 deletions
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7
gcc/combine.c
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@ -11003,8 +11003,11 @@ simplify_shift_const_1 (enum rtx_code code, machine_mode result_mode,
break;
/* For ((int) (cstLL >> count)) >> cst2 just give up. Queuing
up outer sign extension (often left and right shift) is
hardly more efficient than the original. See PR70429. */
if (code == ASHIFTRT && int_mode != int_result_mode)
hardly more efficient than the original. See PR70429.
Similarly punt for rotates with different modes.
See PR97386. */
if ((code == ASHIFTRT || code == ROTATE)
&& int_mode != int_result_mode)
break;
rtx count_rtx = gen_int_shift_amount (int_result_mode, count);

16
gcc/testsuite/gcc.c-torture/execute/pr97386-1.c Normal file
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@ -0,0 +1,16 @@
/* PR rtl-optimization/97386 */
__attribute__((noipa)) unsigned char
foo (unsigned int c)
{
return __builtin_bswap16 ((unsigned long long) (0xccccLLU << c | 0xccccLLU >> ((-c) & 63)));
}
int
main ()
{
unsigned char x = foo (0);
if (__CHAR_BIT__ == 8 && __SIZEOF_SHORT__ == 2 && x != 0xcc)
__builtin_abort ();
return 0;
}

20
gcc/testsuite/gcc.c-torture/execute/pr97386-2.c Normal file
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@ -0,0 +1,20 @@
/* PR rtl-optimization/97386 */
__attribute__((noipa)) unsigned
foo (int x)
{
unsigned long long a = (0x800000000000ccccULL << x) | (0x800000000000ccccULL >> (64 - x));
unsigned int b = a;
return (b << 24) | (b >> 8);
}
int
main ()
{
if (__CHAR_BIT__ == 8
&& __SIZEOF_INT__ == 4
&& __SIZEOF_LONG_LONG__ == 8
&& foo (1) != 0x99000199U)
__builtin_abort ();
return 0;
}