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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/i386/i686
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/i386/i686')
-rw-r--r--sysdeps/i386/i686/multiarch/memchr-sse2.S14
1 files changed, 6 insertions, 8 deletions
diff --git a/sysdeps/i386/i686/multiarch/memchr-sse2.S b/sysdeps/i386/i686/multiarch/memchr-sse2.S
index e41f324a77..172d70de13 100644
--- a/sysdeps/i386/i686/multiarch/memchr-sse2.S
+++ b/sysdeps/i386/i686/multiarch/memchr-sse2.S
@@ -117,14 +117,12 @@ L(crosscache):
 
 # ifndef USE_AS_RAWMEMCHR
 	jnz	L(match_case2_prolog1)
-        /* Calculate the last acceptable address and check for possible
-           addition overflow by using satured math:
-           edx = ecx + edx
-           edx |= -(edx < ecx)  */
-	add	%ecx, %edx
-	sbb	%eax, %eax
-	or	%eax, %edx
-	sub	$16, %edx
+        /* "ecx" is less than 16.  Calculate "edx + ecx - 16" by using
+	   "edx - (16 - ecx)" instead of "(edx + ecx) - 16" to void
+	   possible addition overflow.  */
+	neg	%ecx
+	add	$16, %ecx
+	sub	%ecx, %edx
 	jbe	L(return_null)
 	lea	16(%edi), %edi
 # else