git: 3d8ef251aa9d - main - lib/libc/amd64/string/strchrnul.S: fix edge case in scalar code
Date: Fri, 25 Aug 2023 19:23:06 UTC
The branch main has been updated by fuz:
URL: https://cgit.FreeBSD.org/src/commit/?id=3d8ef251aa9dceabd57f7821a0e6749d35317db3
commit 3d8ef251aa9dceabd57f7821a0e6749d35317db3
Author: Robert Clausecker <fuz@FreeBSD.org>
AuthorDate: 2023-08-25 16:22:22 +0000
Commit: Robert Clausecker <fuz@FreeBSD.org>
CommitDate: 2023-08-25 19:21:54 +0000
lib/libc/amd64/string/strchrnul.S: fix edge case in scalar code
When the buffer is immediately preceeded by the character we
are looking for and begins with one higher than that character,
and the buffer is misaligned, a match was errorneously detected
in the first character. Fix this by changing the way we prevent
matches before the buffer from being detected: instead of
removing the corresponding bit from the 0x80..80 mask, set the
LSB of bytes before the buffer after xoring with the character we
look for.
The bug only affects amd64 with ARCHLEVEL=scalar (cf. simd(7)).
The change comes at a 2% performance impact for short strings
if ARCHLEVEL is set to scalar. The default configuration is not
affected.
os: FreeBSD
arch: amd64
cpu: 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz
│ strchrnul.scalar.0.out │ strchrnul.scalar.2.out │
│ sec/op │ sec/op vs base │
Short 57.89µ ± 2% 59.08µ ± 1% +2.07% (p=0.030 n=20)
Mid 19.24µ ± 0% 19.73µ ± 0% +2.53% (p=0.000 n=20)
Long 11.03µ ± 0% 11.03µ ± 0% ~ (p=0.547 n=20)
geomean 23.07µ 23.43µ +1.53%
│ strchrnul.scalar.0.out │ strchrnul.scalar.2.out │
│ B/s │ B/s vs base │
Short 2.011Gi ± 2% 1.970Gi ± 1% -2.02% (p=0.030 n=20)
Mid 6.049Gi ± 0% 5.900Gi ± 0% -2.47% (p=0.000 n=20)
Long 10.56Gi ± 0% 10.56Gi ± 0% ~ (p=0.547 n=20)
geomean 5.045Gi 4.969Gi -1.50%
MFC to: stable/14
MFC after: 3 days
Approved by: mjg (blanket, via IRC)
Sponsored by: The FreeBSD Foundation
---
lib/libc/amd64/string/strchrnul.S | 17 ++++++++++-------
1 file changed, 10 insertions(+), 7 deletions(-)
diff --git a/lib/libc/amd64/string/strchrnul.S b/lib/libc/amd64/string/strchrnul.S
index 9aa9591b07ca..0e70b02311d7 100644
--- a/lib/libc/amd64/string/strchrnul.S
+++ b/lib/libc/amd64/string/strchrnul.S
@@ -52,22 +52,25 @@ ARCHENTRY(__strchrnul, scalar)
movabs $0x0101010101010101, %r8
mov (%rdi), %rax # load first word
imul %r8, %rsi # replicate char 8 times
- movabs $0x8080808080808080, %r9
/*
* Unaligned input: align to 8 bytes. Then proceed the same
- * way as with aligned input, but ignore matches before the
- * beginning of the string. This is achieved by shifting r9
- * into r10 to have 0x00 bytes before the string begins.
+ * way as with aligned input, but prevent matches before the
+ * beginning of the string. This is achieved by oring 0x01
+ * into each byte of the buffer before the string
*/
shl $3, %ecx
- mov %r9, %r10
+ mov %r8, %r10
add $8, %rdi
- shl %cl, %r10 # 0x80 where the string is
+ shl %cl, %r10 # 0x01 where the string is
+ xor %r8, %r10 # 0x01 where it is not
neg %r8 # negate 01..01 so we can use lea
+ movabs $0x8080808080808080, %r9
mov %rsi, %rcx
xor %rax, %rcx # str ^ c
+ or %r10, %rax # str without NUL bytes before it
+ or %r10, %rcx # (str ^ c) without matches before it
lea (%rax, %r8, 1), %rdx # str - 0x01..01
lea (%rcx, %r8, 1), %r11 # (str ^ c) - 0x01..01
not %rax # ~str
@@ -75,7 +78,7 @@ ARCHENTRY(__strchrnul, scalar)
and %rdx, %rax # (str - 0x01..01) & ~str
and %r11, %rcx # ((str ^ c - 0x01..01) & ~(str ^ c)
or %rcx, %rax # matches for both
- and %r10, %rax # not including junk bytes or bytes before the string
+ and %r9, %rax # not including junk bytes
jnz 1f
/* main loop unrolled twice */