[glibc/ibm/2.28/master] Fix avx2 strncmp offset compare condition check [BZ #25933]

[glibc/ibm/2.28/master] Fix avx2 strncmp offset compare condition check [BZ #25933]

[Tulio Magno Quites Machado Filho]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=23db5555eb914c6a0228ecf5dc13c13be5d8dbd7

commit 23db5555eb914c6a0228ecf5dc13c13be5d8dbd7
Author: Sunil K Pandey <skpgkp1@gmail.com>
Date:   Fri Jun 12 08:57:16 2020 -0700

    Fix avx2 strncmp offset compare condition check [BZ #25933]
    
    strcmp-avx2.S: In avx2 strncmp function, strings are compared in
    chunks of 4 vector size(i.e. 32x4=128 byte for avx2). After first 4
    vector size comparison, code must check whether it already passed
    the given offset. This patch implement avx2 offset check condition
    for strncmp function, if both string compare same for first 4 vector
    size.
    
    (cherry picked from commit 75870237ff3bb363447b03f4b0af100227570910)

Diff:
---
 sysdeps/x86_64/multiarch/strcmp-avx2.S | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/sysdeps/x86_64/multiarch/strcmp-avx2.S b/sysdeps/x86_64/multiarch/strcmp-avx2.S
index 78fc116877..156c19496a 100644
--- a/sysdeps/x86_64/multiarch/strcmp-avx2.S
+++ b/sysdeps/x86_64/multiarch/strcmp-avx2.S
@@ -591,7 +591,14 @@ L(loop_cross_page_2_vec):
 	movl	$(PAGE_SIZE / (VEC_SIZE * 4) - 1), %esi
 
 	testq	%rdi, %rdi
+# ifdef USE_AS_STRNCMP
+	/* At this point, if %rdi value is 0, it already tested
+	   VEC_SIZE*4+%r10 byte starting from %rax. This label
+	   checks whether strncmp maximum offset reached or not.  */
+	je	L(string_nbyte_offset_check)
+# else
 	je	L(back_to_loop)
+# endif
 	tzcntq	%rdi, %rcx
 	addq	%r10, %rcx
 	/* Adjust for number of bytes skipped.  */
@@ -627,6 +634,14 @@ L(loop_cross_page_2_vec):
 	VZEROUPPER
 	ret
 
+# ifdef USE_AS_STRNCMP
+L(string_nbyte_offset_check):
+	leaq	(VEC_SIZE * 4)(%r10), %r10
+	cmpq	%r10, %r11
+	jbe	L(zero)
+	jmp	L(back_to_loop)
+# endif
+
 	.p2align 4
 L(cross_page_loop):
 	/* Check one byte/dword at a time.  */