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x86-64: memcmp-avx2-movbe.S needs saturating subtraction [BZ #21662]

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This code:

L(between_2_3):
	/* Load as big endian with overlapping loads and bswap to avoid
	   branches.  */
	movzwl	-2(%rdi, %rdx), %eax
	movzwl	-2(%rsi, %rdx), %ecx
	shll	$16, %eax
	shll	$16, %ecx
	movzwl	(%rdi), %edi
	movzwl	(%rsi), %esi
	orl	%edi, %eax
	orl	%esi, %ecx
	bswap	%eax
	bswap	%ecx
	subl	%ecx, %eax
	ret

needs a saturating subtract because the full register is used.
With this commit, only the lower 24 bits of the register are used,
so a regular subtraction suffices.

The test case change adds coverage for these kinds of bugs.
This commit is contained in:
Florian Weimer
2017-06-23 17:23:44 +02:00
parent 7fa1d9462b
commit 3ec7c02cc3
3 changed files with 23 additions and 15 deletions
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11
string/test-memcmp.c
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@ -441,11 +441,12 @@ check1 (void)
n = 116;
for (size_t i = 0; i < n; i++)
{
exp_result = SIMPLE_MEMCMP (s1 + i, s2 + i, n - i);
FOR_EACH_IMPL (impl, 0)
check_result (impl, s1 + i, s2 + i, n - i, exp_result);
}
for (size_t len = 0; len <= n - i; ++len)
{
exp_result = SIMPLE_MEMCMP (s1 + i, s2 + i, len);
FOR_EACH_IMPL (impl, 0)
check_result (impl, s1 + i, s2 + i, len, exp_result);
}
}
/* This test checks that memcmp doesn't overrun buffers. */