| H A D | xfrm.h | 6c786bcb Sat Mar 25 11:41:17 CDT 2017 Alexey Dobriyan <adobriyan@gmail.com> xfrm: branchless addr4_match() on 64-bit
Current addr4_match() code has special test for /0 prefixes because of
standard required undefined behaviour. However, it is possible to omit
it on 64-bit because shifting can be done within a 64-bit register and
then truncated to the expected value (which is 0 mask).
Implicit truncation by htonl() fits nicely into R32-within-R64 model
on x86-64.
Space savings: none (coincidence)
Branch savings: 1
Before:
movzx eax,BYTE PTR [rdi+0x2a] # ->prefixlen_d
test al,al
jne xfrm_selector_match + 0x23f
...
movzx eax,BYTE PTR [rbx+0x2b] # ->prefixlen_s
test al,al
je xfrm_selector_match + 0x1c7
After (no branches):
mov r8d,0x20
mov rdx,0xffffffffffffffff
mov esi,DWORD PTR [rsi+0x2c]
mov ecx,r8d
sub cl,BYTE PTR [rdi+0x2a]
xor esi,DWORD PTR [rbx]
mov rdi,rdx
xor eax,eax
shl rdi,cl
bswap edi
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
6c786bcb Sat Mar 25 11:41:17 CDT 2017 Alexey Dobriyan <adobriyan@gmail.com> xfrm: branchless addr4_match() on 64-bit
Current addr4_match() code has special test for /0 prefixes because of
standard required undefined behaviour. However, it is possible to omit
it on 64-bit because shifting can be done within a 64-bit register and
then truncated to the expected value (which is 0 mask).
Implicit truncation by htonl() fits nicely into R32-within-R64 model
on x86-64.
Space savings: none (coincidence)
Branch savings: 1
Before:
movzx eax,BYTE PTR [rdi+0x2a] # ->prefixlen_d
test al,al
jne xfrm_selector_match + 0x23f
...
movzx eax,BYTE PTR [rbx+0x2b] # ->prefixlen_s
test al,al
je xfrm_selector_match + 0x1c7
After (no branches):
mov r8d,0x20
mov rdx,0xffffffffffffffff
mov esi,DWORD PTR [rsi+0x2c]
mov ecx,r8d
sub cl,BYTE PTR [rdi+0x2a]
xor esi,DWORD PTR [rbx]
mov rdi,rdx
xor eax,eax
shl rdi,cl
bswap edi
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
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