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authorH.J. Lu <hjl.tools@gmail.com>2017-05-19 10:46:29 -0700
committerH.J. Lu <hjl.tools@gmail.com>2017-05-19 10:48:45 -0700
commit402bf0695218bbe290418b9486b1dd5fe284d903 (patch)
tree0107d383f8a38c75076dae69996b15b46e13b04a /sysdeps/x86_64
parent1d71a6315396f6e1cc79a1d7ecca0a559929230a (diff)
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x86: Optimize SSE2 memchr overflow calculation
SSE2 memchr computes "edx + ecx - 16" where ecx is less than 16. Use "edx - (16 - ecx)", instead of satured math, to avoid possible addition overflow. This replaces add %ecx, %edx sbb %eax, %eax or %eax, %edx sub $16, %edx with neg %ecx add $16, %ecx sub %ecx, %edx It is the same for x86_64, except for rcx/rdx, instead of ecx/edx. * sysdeps/i386/i686/multiarch/memchr-sse2.S (MEMCHR): Use "edx + ecx - 16" to avoid possible addition overflow. * sysdeps/x86_64/memchr.S (memchr): Likewise.
Diffstat (limited to 'sysdeps/x86_64')
-rw-r--r--sysdeps/x86_64/memchr.S14
1 files changed, 6 insertions, 8 deletions
diff --git a/sysdeps/x86_64/memchr.S b/sysdeps/x86_64/memchr.S
index a205a25998..f82e1c5bf7 100644
--- a/sysdeps/x86_64/memchr.S
+++ b/sysdeps/x86_64/memchr.S
@@ -76,14 +76,12 @@ 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 < rcx) */
- add %rcx, %rdx
- sbb %rax, %rax
- or %rax, %rdx
- sub $16, %rdx
+ /* "rcx" is less than 16. Calculate "rdx + rcx - 16" by using
+ "rdx - (16 - rcx)" instead of "(rdx + rcx) - 16" to void
+ possible addition overflow. */
+ neg %rcx
+ add $16, %rcx
+ sub %rcx, %rdx
jbe L(return_null)
add $16, %rdi
sub $64, %rdx