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Re: [PATCH] Fix x86_64 memchr for large input sizes
- From: Adhemerval Zanella <adhemerval dot zanella at linaro dot org>
- To: libc-alpha at sourceware dot org
- Date: Tue, 20 Dec 2016 12:03:28 -0200
- Subject: Re: [PATCH] Fix x86_64 memchr for large input sizes
- Authentication-results: sourceware.org; auth=none
- References: <1481834313-5626-1-git-send-email-adhemerval.zanella@linaro.org>
Ping.
On 15/12/2016 18:38, Adhemerval Zanella wrote:
> Current optimized memchr for x86_64 does for input arguments pointers
> module 64 in range of [49,63] if there is no searchr char in the rest
> of 64-byte block a pointer addition which might overflow:
>
> * sysdeps/x86_64/memchr.S
>
> 77 .p2align 4
> 78 L(unaligned_no_match):
> 79 add %rcx, %rdx
>
> Add (uintptr_t)s % 16 to n in %rdx.
>
> 80 sub $16, %rdx
> 81 jbe L(return_null)
>
> This patch fixes by adding a saturated math that sets a maximum pointer
> value if it overflows (UINTPTR_MAX). This is similar of the fix for
> powerpc64/power7 version [1] and rely on for test-memchr.c changes.
>
> Checked on x86_64-linux-gnu and powerpc64-linux-gnu.
>
> [1] https://sourceware.org/ml/libc-alpha/2016-12/msg00576.html
>
> [BZ# 19387]
> * sysdeps/x86_64/memchr.S (memchr): Avoid overflow in pointer
> addition.
> * string/test-memchr.c (do_test): Remove alignment limitation.
> (test_main): Add test that trigger BZ# 19387.
> ---
> ChangeLog | 6 ++++++
> string/test-memchr.c | 9 ++++-----
> sysdeps/x86_64/memchr.S | 8 ++++++++
> 3 files changed, 18 insertions(+), 5 deletions(-)
>
> diff --git a/string/test-memchr.c b/string/test-memchr.c
> index 7431386..6901711 100644
> --- a/string/test-memchr.c
> +++ b/string/test-memchr.c
> @@ -76,7 +76,6 @@ do_test (size_t align, size_t pos, size_t len, size_t n, int seek_char)
> size_t i;
> CHAR *result;
>
> - align &= 7;
> if ((align + len) * sizeof (CHAR) >= page_size)
> return;
>
> @@ -194,12 +193,12 @@ test_main (void)
> do_test (i, 64, 256, SIZE_MAX, 0);
> }
>
> - for (i = 1; i < 16; ++i)
> + for (i = 1; i < 64; ++i)
> {
> - for (j = 1; j < 16; j++)
> + for (j = 1; j < 64; j++)
> {
> - do_test (0, 16 - j, 32, SIZE_MAX, 23);
> - do_test (i, 16 - j, 32, SIZE_MAX, 23);
> + do_test (0, 64 - j, 64, SIZE_MAX, 23);
> + do_test (i, 64 - j, 64, SIZE_MAX, 23);
> }
> }
>
> diff --git a/sysdeps/x86_64/memchr.S b/sysdeps/x86_64/memchr.S
> index 132eacb..b140de1 100644
> --- a/sysdeps/x86_64/memchr.S
> +++ b/sysdeps/x86_64/memchr.S
> @@ -76,7 +76,15 @@ L(crosscache):
>
> .p2align 4
> L(unaligned_no_match):
> + /* Calculate the last acceptable address and check for possible
> + addition overflow by using satured math:
> + rdx = rcx + rdx
> + rdx |= -(rdx < x) */
> add %rcx, %rdx
> + sbb %eax, %eax
> + movslq %eax, %rax
> + orq %rdx, %rax
> + mov %rax, %rdx
> sub $16, %rdx
> jbe L(return_null)
> add $16, %rdi
>