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overhaul optimized x86_64 memset asm

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on most cpu models, "rep stosq" has high overhead that makes it
undesirable for small memset sizes. the new code extends the
minimal-branch fast path for short memsets from size 15 up to size
126, and shrink-wraps this code path. in addition, "rep stosq" is
sensitive to misalignment. the cost varies with size and with cpu
model, but it has been observed performing 1.5 times slower when the
destination address is not aligned mod 16. the new code thus ensures
alignment mod 16, but also preserves any existing additional
alignment, in case there are cpu models where it is beneficial.

this version is based in part on changes proposed by Denys Vlasenko.
This commit is contained in:
Rich Felker 2015-02-26 02:07:08 -05:00
parent 69858fa931
commit e346ff86c8
1 changed files with 60 additions and 31 deletions
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91
src/string/x86_64/memset.s
View File

@ -1,43 +1,72 @@
.global memset
.type memset,@function
memset:
movzbl %sil,%esi
mov $0x101010101010101,%rax
# 64-bit imul has 3-7 cycles latency, launch early
imul %rsi,%rax
movzbq %sil,%rax
mov $0x101010101010101,%r8
imul %r8,%rax
cmp $16,%rdx
jb 1f
cmp $126,%rdx
ja 2f
lea -1(%rdx),%rcx
test %edx,%edx
jz 1f
mov %sil,(%rdi)
mov %sil,-1(%rdi,%rdx)
cmp $2,%edx
jbe 1f
mov %ax,1(%rdi)
mov %ax,(-1-2)(%rdi,%rdx)
cmp $6,%edx
jbe 1f
mov %eax,(1+2)(%rdi)
mov %eax,(-1-2-4)(%rdi,%rdx)
cmp $14,%edx
jbe 1f
mov %rax,(1+2+4)(%rdi)
mov %rax,(-1-2-4-8)(%rdi,%rdx)
cmp $30,%edx
jbe 1f
mov %rax,(1+2+4+8)(%rdi)
mov %rax,(1+2+4+8+8)(%rdi)
mov %rax,(-1-2-4-8-16)(%rdi,%rdx)
mov %rax,(-1-2-4-8-8)(%rdi,%rdx)
cmp $62,%edx
jbe 1f
mov %rax,(1+2+4+8+16)(%rdi)
mov %rax,(1+2+4+8+16+8)(%rdi)
mov %rax,(1+2+4+8+16+16)(%rdi)
mov %rax,(1+2+4+8+16+24)(%rdi)
mov %rax,(-1-2-4-8-16-32)(%rdi,%rdx)
mov %rax,(-1-2-4-8-16-24)(%rdi,%rdx)
mov %rax,(-1-2-4-8-16-16)(%rdi,%rdx)
mov %rax,(-1-2-4-8-16-8)(%rdi,%rdx)
1: mov %rdi,%rax
ret
2: test $15,%edi
mov %rdi,%r8
shr $3,%rcx
mov %rax,-8(%rdi,%rdx)
mov %rdx,%rcx
jnz 2f
1: shr $3,%rcx
rep
stosq
mov %r8,%rax
ret
1: test %edx,%edx
jz 1f
mov %al,(%rdi)
mov %al,-1(%rdi,%rdx)
cmp $2,%edx
jbe 1f
mov %al,1(%rdi)
mov %al,-2(%rdi,%rdx)
cmp $4,%edx
jbe 1f
mov %eax,(%rdi)
mov %eax,-4(%rdi,%rdx)
cmp $8,%edx
jbe 1f
mov %eax,4(%rdi)
mov %eax,-8(%rdi,%rdx)
1: mov %rdi,%rax
ret
2: xor %edx,%edx
sub %edi,%edx
and $15,%edx
mov %rax,(%rdi)
mov %rax,8(%rdi)
sub %rdx,%rcx
add %rdx,%rdi
jmp 1b