From 3ec7c02cc3e922b9364dc8cfd1d4546671b91003 Mon Sep 17 00:00:00 2001 From: Florian Weimer Date: Fri, 23 Jun 2017 17:23:44 +0200 Subject: 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. --- ChangeLog | 7 +++++++ string/test-memcmp.c | 11 ++++++----- sysdeps/x86_64/multiarch/memcmp-avx2-movbe.S | 20 ++++++++++---------- 3 files changed, 23 insertions(+), 15 deletions(-) diff --git a/ChangeLog b/ChangeLog index 4f1ef82c51..12f1e3bce6 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,3 +1,10 @@ +2017-06-23 Florian Weimer + + [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 * 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 -- cgit 1.4.1