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| author | David Gibson <david@gibson.dropbear.id.au> | 2023-09-28 11:21:01 +1000 |
|---|---|---|
| committer | Stefano Brivio <sbrivio@redhat.com> | 2023-09-30 12:40:51 +0200 |
| commit | 04b10a8d907d8ceb09e3adb4885c02f546841bbc (patch) | |
| tree | 4b2b473f9f690165a1b40d10d9d99248c5bdc3ba /checksum.c | |
| parent | ceae8422c1ca997b80436e9b574f48146e92d12c (diff) | |
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siphash, checksum: Move TBAA explanation to checksum.c
A number of checksum and hash functions require workarounds for the odd
behaviour of Type-Baased Alias Analysis. We have a detailed comment about
this on siphash_8b() and other functions reference that.
Move the main comment to csume_16b() instead, because we're going to
reorganise things in siphash.c.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Diffstat (limited to 'checksum.c')
| -rw-r--r-- | checksum.c | 19 |
1 files changed, 14 insertions, 5 deletions
@@ -69,8 +69,17 @@ * * Return: 32-bit sum of 16-bit words */ +/* Type-Based Alias Analysis (TBAA) optimisation in gcc 11 and 12 (-flto -O2) + * makes these functions essentially useless by allowing reordering of stores of + * input data across function calls. Not even declaring @in as char pointer is + * enough: disable gcc's interpretation of strict aliasing altogether. See also: + * + * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106706 + * https://stackoverflow.com/questions/2958633/gcc-strict-aliasing-and-horror-stories + * https://lore.kernel.org/all/alpine.LFD.2.00.0901121128080.6528__33422.5328093909$1232291247$gmane$org@localhost.localdomain/ + */ /* NOLINTNEXTLINE(clang-diagnostic-unknown-attributes) */ -__attribute__((optimize("-fno-strict-aliasing"))) /* See siphash_8b() */ +__attribute__((optimize("-fno-strict-aliasing"))) uint32_t sum_16b(const void *buf, size_t len) { const uint16_t *p = buf; @@ -110,7 +119,7 @@ uint16_t csum_fold(uint32_t sum) * Return: 16-bit IPv4-style checksum */ /* NOLINTNEXTLINE(clang-diagnostic-unknown-attributes) */ -__attribute__((optimize("-fno-strict-aliasing"))) /* See siphash_8b() */ +__attribute__((optimize("-fno-strict-aliasing"))) /* See csum_16b() */ uint16_t csum_unaligned(const void *buf, size_t len, uint32_t init) { return (uint16_t)~csum_fold(sum_16b(buf, len) + init); @@ -247,7 +256,7 @@ void csum_icmp6(struct icmp6hdr *icmp6hr, * - coding style adaptation */ /* NOLINTNEXTLINE(clang-diagnostic-unknown-attributes) */ -__attribute__((optimize("-fno-strict-aliasing"))) /* See siphash_8b() */ +__attribute__((optimize("-fno-strict-aliasing"))) /* See csum_16b() */ static uint32_t csum_avx2(const void *buf, size_t len, uint32_t init) { __m256i a, b, sum256, sum_a_hi, sum_a_lo, sum_b_hi, sum_b_lo, c, d; @@ -395,7 +404,7 @@ less_than_128_bytes: * Return: 16-bit folded, complemented checksum sum */ /* NOLINTNEXTLINE(clang-diagnostic-unknown-attributes) */ -__attribute__((optimize("-fno-strict-aliasing"))) /* See siphash_8b() */ +__attribute__((optimize("-fno-strict-aliasing"))) /* See csum_16b() */ uint16_t csum(const void *buf, size_t len, uint32_t init) { return (uint16_t)~csum_fold(csum_avx2(buf, len, init)); @@ -412,7 +421,7 @@ uint16_t csum(const void *buf, size_t len, uint32_t init) * Return: 16-bit folded, complemented checksum */ /* NOLINTNEXTLINE(clang-diagnostic-unknown-attributes) */ -__attribute__((optimize("-fno-strict-aliasing"))) /* See siphash_8b() */ +__attribute__((optimize("-fno-strict-aliasing"))) /* See csum_16b() */ uint16_t csum(const void *buf, size_t len, uint32_t init) { return csum_unaligned(buf, len, init); |