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diff --git a/lib/libc/amd64/string/memccpy.S b/lib/libc/amd64/string/memccpy.S
--- a/lib/libc/amd64/string/memccpy.S
+++ b/lib/libc/amd64/string/memccpy.S
@@ -83,22 +83,24 @@
pshufd $0, %xmm4, %xmm4 # cccc -> cccccccccccccccc
and $~0xf, %rsi
movdqa %xmm4, %xmm1
- pcmpeqb (%rsi), %xmm1 # NUL found in head?
+ pcmpeqb (%rsi), %xmm1 # c found in head?
mov $-1, %r8d
and $0xf, %ecx
shl %cl, %r8d # mask of bytes in the string
pmovmskb %xmm1, %eax
+ lea (%rcx, %rdx), %r11 # dist. from alignment boundary to buffer end
+ lea 0x10000(%rax), %r10d # match mask with match induced behind buf
+ cmp $16, %r11
+ cmovb %r10d, %eax # induce that match if string ends early
and %r8d, %eax
jnz .Lhead_nul
movdqa 16(%rsi), %xmm3 # load second string chunk
movdqu (%r9), %xmm2 # load unaligned string head
- mov $32, %r8d
- sub %ecx, %r8d # head length + length of second chunk
movdqa %xmm4, %xmm1
pcmpeqb %xmm3, %xmm1 # NUL found in second chunk?
- sub %r8, %rdx # enough space left for the second chunk?
+ cmp $32, %r11
jb .Lhead_buf_end
/* process second chunk */
@@ -111,10 +113,11 @@
movdqa %xmm4, %xmm1
movdqu %xmm2, (%rdi) # deposit head into buffer
sub %rcx, %rdi # adjust RDI to correspond to RSI
+ mov %r11, %rdx
movdqu %xmm3, 16(%rdi) # deposit second chunk
sub %rsi, %rdi # express RDI as distance from RSI
add $32, %rsi # advance RSI past first two chunks
- sub $16, %rdx # enough left for another round?
+ sub $48, %rdx # enough left for another round?
jb 1f
/* main loop unrolled twice */
@@ -174,10 +177,9 @@
.Lhead_buf_end:
pmovmskb %xmm1, %r8d
- add $32, %edx # restore edx to (len-1) + ecx
shl $16, %r8d # place 2nd chunk NUL mask into bits 16--31
mov %r8d, %r10d
- bts %rdx, %r8 # treat end of buffer as if terminator present
+ bts %r11, %r8 # treat end of buffer as if terminator present
xor %eax, %eax # return value if terminator not found
tzcnt %r8, %rdx # find string/buffer len from alignment boundary
lea 1(%rdi, %rdx, 1), %r8 # return value if terminator found + rcx
@@ -188,7 +190,6 @@
jmp .L0132
.Lsecond_nul:
- add %r8, %rdx # restore buffer length
tzcnt %eax, %r8d # where is the NUL byte?
lea -16(%rcx), %eax
sub %eax, %r8d # string length

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