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siphash, checksum: Move TBAA explanation to checksum.c

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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>
This commit is contained in:
David Gibson 2023-09-28 11:21:01 +10:00 committed by Stefano Brivio
parent ceae8422c1
commit 04b10a8d90
2 changed files with 19 additions and 19 deletions
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19
checksum.c
View file

@ -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);

19
siphash.c
View file

@ -24,17 +24,8 @@
*
* Return: the 64-bit hash output
*/
/* 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")))
__attribute__((optimize("-fno-strict-aliasing"))) /* See csum_16b() */
/* cppcheck-suppress unusedFunction */
uint64_t siphash_8b(const uint8_t *in, const uint64_t *k)
{
@ -53,7 +44,7 @@ uint64_t siphash_8b(const uint8_t *in, const uint64_t *k)
* Return: the 64-bit hash output
*/
/* NOLINTNEXTLINE(clang-diagnostic-unknown-attributes) */
__attribute__((optimize("-fno-strict-aliasing"))) /* See siphash_8b() */
__attribute__((optimize("-fno-strict-aliasing"))) /* See csum_16b() */
/* cppcheck-suppress unusedFunction */
uint64_t siphash_12b(const uint8_t *in, const uint64_t *k)
{
@ -73,7 +64,7 @@ uint64_t siphash_12b(const uint8_t *in, const uint64_t *k)
* Return: the 64-bit hash output
*/
/* NOLINTNEXTLINE(clang-diagnostic-unknown-attributes) */
__attribute__((optimize("-fno-strict-aliasing"))) /* See siphash_8b() */
__attribute__((optimize("-fno-strict-aliasing"))) /* See csum_16b() */
uint64_t siphash_20b(const uint8_t *in, const uint64_t *k)
{
struct siphash_state state = SIPHASH_INIT(k);
@ -94,7 +85,7 @@ uint64_t siphash_20b(const uint8_t *in, const uint64_t *k)
* Return: the 64-bit hash output
*/
/* NOLINTNEXTLINE(clang-diagnostic-unknown-attributes) */
__attribute__((optimize("-fno-strict-aliasing"))) /* See siphash_8b() */
__attribute__((optimize("-fno-strict-aliasing"))) /* See csum_16b() */
/* cppcheck-suppress unusedFunction */
uint64_t siphash_32b(const uint8_t *in, const uint64_t *k)
{
@ -116,7 +107,7 @@ uint64_t siphash_32b(const uint8_t *in, const uint64_t *k)
* Return: the 64-bit hash output
*/
/* NOLINTNEXTLINE(clang-diagnostic-unknown-attributes) */
__attribute__((optimize("-fno-strict-aliasing"))) /* See siphash_8b() */
__attribute__((optimize("-fno-strict-aliasing"))) /* See csum_16b() */
uint64_t siphash_36b(const uint8_t *in, const uint64_t *k)
{
struct siphash_state state = SIPHASH_INIT(k);