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| author | Florian Weimer <fweimer@redhat.com> | 2017-06-23 17:23:44 +0200 |
|---|---|---|
| committer | Florian Weimer <fweimer@redhat.com> | 2017-06-23 17:24:40 +0200 |
| commit | 3ec7c02cc3e922b9364dc8cfd1d4546671b91003 (patch) | |
| tree | 283d54448fe89359272093156316884e61992c9a | |
| parent | 7fa1d9462baabc5a1058efc13a48444af4678acf (diff) | |
| download | glibc-3ec7c02cc3e922b9364dc8cfd1d4546671b91003.tar glibc-3ec7c02cc3e922b9364dc8cfd1d4546671b91003.tar.gz glibc-3ec7c02cc3e922b9364dc8cfd1d4546671b91003.tar.bz2 glibc-3ec7c02cc3e922b9364dc8cfd1d4546671b91003.zip | |
x86-64: memcmp-avx2-movbe.S needs saturating subtraction [BZ #21662]
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.
| -rw-r--r-- | ChangeLog | 7 | ||||
| -rw-r--r-- | string/test-memcmp.c | 11 | ||||
| -rw-r--r-- | sysdeps/x86_64/multiarch/memcmp-avx2-movbe.S | 20 |
3 files changed, 23 insertions, 15 deletions
@@ -1,3 +1,10 @@ +2017-06-23 Florian Weimer <fweimer@redhat.com> + + [BZ #21662] + * sysdeps/x86_64/multiarch/memcmp-avx2-movbe.S (between_2_3): + Use only 24 bits of the register before the subtraction. + * string/test-memcmp.c (check1): Check with different lengths. + 2017-06-23 Gabriel F. T. Gomes <gftg@linux.vnet.ibm.com> * sysdeps/ieee754/float128/Makefile (CFLAGS-strfromf128.c): Add diff --git a/string/test-memcmp.c b/string/test-memcmp.c index a7969edaea..1538930534 100644 --- a/string/test-memcmp.c +++ b/string/test-memcmp.c @@ -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. */ diff --git a/sysdeps/x86_64/multiarch/memcmp-avx2-movbe.S b/sysdeps/x86_64/multiarch/memcmp-avx2-movbe.S index 47630dd97b..9d1921033e 100644 --- a/sysdeps/x86_64/multiarch/memcmp-avx2-movbe.S +++ b/sysdeps/x86_64/multiarch/memcmp-avx2-movbe.S @@ -137,18 +137,18 @@ L(exit): .p2align 4 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 + /* Load as big endian to avoid branches. */ + movzwl (%rdi), %eax + movzwl (%rsi), %ecx + shll $8, %eax + shll $8, %ecx bswap %eax bswap %ecx + movzbl -1(%rdi, %rdx), %edi + movzbl -1(%rsi, %rdx), %esi + orl %edi, %eax + orl %esi, %ecx + /* Subtraction is okay because the upper 8 bits a zero. */ subl %ecx, %eax ret |