kgess-mirror/freebsd-src
1
0
Fork 0
mirror of https://git.freebsd.org/src.git synced 2026-01-11 19:57:22 +00:00
Code Issues Projects Releases Packages Wiki Activity Actions 1

libc/amd64: fix stpncpy.S again
Some checks are pending
Cross-build Kernel / aarch64 macos-latest (clang-18) (push) Waiting to run
Details
Cross-build Kernel / amd64 ubuntu-22.04 (clang-15) (push) Waiting to run
Details
Cross-build Kernel / aarch64 ubuntu-22.04 (clang-15) (push) Waiting to run
Details
Cross-build Kernel / amd64 ubuntu-24.04 (clang-18) (push) Waiting to run
Details
Cross-build Kernel / aarch64 ubuntu-24.04 (clang-18) (push) Waiting to run
Details
Cross-build Kernel / amd64 macos-latest (clang-18) (push) Waiting to run
Details

Browse source 
The previous fix introduced a regression on machines without the BMI1
instruction set extension.  The TZCNT instruction used in this function
behaves different on old machines when the source operand is zero, but
the code was originally designed to never trigger this case.  The bug
fix caused this case to be possible, leading to a regression on
sufficiently old hardware.

Fix the code by messing with things such that the source operand is
never zero.

PR:		291720
Fixes:		66eb78377b
Tested by:	cy
Approved by:	markj (mentor)
Differential Revision:	https://reviews.freebsd.org/D54303
This commit is contained in:
Robert Clausecker 2025-12-19 00:37:33 +01:00
parent 9595055ae7
commit 2f83319214
1 changed files with 3 additions and 6 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

9
lib/libc/amd64/string/stpncpy.S
View file

@ -36,9 +36,7 @@
.set stpncpy, __stpncpy
ARCHFUNCS(__stpncpy)
ARCHFUNC(__stpncpy, scalar)
#if 0 /* temporarily disabled cf. PR 291720 */
ARCHFUNC(__stpncpy, baseline)
#endif
ENDARCHFUNCS(__stpncpy)
ARCHENTRY(__stpncpy, scalar)
@ -93,7 +91,6 @@ ARCHEND(__stpncpy, scalar)
/* stpncpy(char *restrict rdi, const char *rsi, size_t rdx) */
ARCHENTRY(__stpncpy, baseline)
#define bounce (-3*16-8) /* location of on-stack bounce buffer */
test %rdx, %rdx # no bytes to copy?
jz .L0
@ -225,8 +222,8 @@ ARCHENTRY(__stpncpy, baseline)
/* 1--32 bytes to copy, bounce through the stack */
.Lrunt: movdqa %xmm1, bounce+16(%rsp) # clear out rest of on-stack copy
bts %r10, %r8 # treat end of buffer as end of string
and %r9d, %r8d # mask out head before string
bts %r10, %r8 # treat end of buffer as end of string
test $0x1ffff, %r8d # end of string within first chunk or right after?
jnz 0f # if yes, do not inspect second buffer
@ -235,10 +232,10 @@ ARCHENTRY(__stpncpy, baseline)
pcmpeqb %xmm1, %xmm0 # NUL in second chunk?
pmovmskb %xmm0, %r9d
shl $16, %r9d
or %r9d, %r8d # merge found NUL bytes into NUL mask
or %r9, %r8 # merge found NUL bytes into NUL mask
/* end of string after one buffer */
0: tzcnt %r8d, %r8d # location of last char in string
0: tzcnt %r8, %r8 # location of last char in string
movdqu %xmm1, bounce(%rsp, %r8, 1) # clear bytes behind string
lea bounce(%rsp, %rcx, 1), %rsi # start of string copy on stack
lea (%rdi, %r8, 1), %rax # return pointer to NUL byte