From c005d1bd6f0e88ab4b822844d75d10c8978f5404 Mon Sep 17 00:00:00 2001 From: Noah Goldstein Date: Tue, 13 Aug 2024 23:29:14 +0800 Subject: x86: Fix bug in strchrnul-evex512 [BZ #32078] Issue was we were expecting not matches with CHAR before the start of the string in the page cross case. The check code in the page cross case: ``` and $0xffffffffffffffc0,%rax vmovdqa64 (%rax),%zmm17 vpcmpneqb %zmm17,%zmm16,%k1 vptestmb %zmm17,%zmm17,%k0{%k1} kmovq %k0,%rax inc %rax shr %cl,%rax je L(continue) ``` expects that all characters that neither match null nor CHAR will be 1s in `rax` prior to the `inc`. Then the `inc` will overflow all of the 1s where no relevant match was found. This is incorrect in the page-cross case, as the `vmovdqa64 (%rax),%zmm17` loads from before the start of the input string. If there are matches with CHAR before the start of the string, `rax` won't properly overflow. The fix is quite simple. Just replace: ``` inc %rax shr %cl,%rax ``` With: ``` sar %cl,%rax inc %rax ``` The arithmetic shift will clear any matches prior to the start of the string while maintaining the signbit so the 1s can properly overflow to zero in the case of no matches. Reviewed-by: H.J. Lu (cherry picked from commit 7da08862471dfec6fdae731c2a5f351ad485c71f) --- sysdeps/x86_64/multiarch/strchr-evex-base.S | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) (limited to 'sysdeps') diff --git a/sysdeps/x86_64/multiarch/strchr-evex-base.S b/sysdeps/x86_64/multiarch/strchr-evex-base.S index 7209435..da6d0ea 100644 --- a/sysdeps/x86_64/multiarch/strchr-evex-base.S +++ b/sysdeps/x86_64/multiarch/strchr-evex-base.S @@ -124,13 +124,13 @@ L(page_cross): VPCMPNE %VMM(1), %VMM(0), %k1 VPTEST %VMM(1), %VMM(1), %k0{%k1} KMOV %k0, %VRAX -# ifdef USE_AS_WCSCHR + sar %cl, %VRAX +#ifdef USE_AS_WCSCHR sub $VEC_MATCH_MASK, %VRAX -# else +#else inc %VRAX -# endif +#endif /* Ignore number of character for alignment adjustment. */ - shr %cl, %VRAX jz L(align_more) bsf %VRAX, %VRAX -- cgit v1.1