Hi,
compile the following function on a system with Core2 processor
(released January 2008) for the 32-bit execution environment:
--- demo.c ---
int ispowerof2(unsigned long long argument)
{
return (argument & argument - 1) == 0;
}
--- EOF ---
GCC 13.3: gcc -m32 -O3 demo.c
NOTE: -mtune=native is the default!
# https://godbolt.org/z/b43cjGdY9
ispowerof2(unsigned long long):
movq xmm1, [esp+4]
pcmpeqd xmm0, xmm0
paddq xmm0, xmm1
pand xmm0, xmm1
movd edx, xmm0 # pxor xmm1, xmm1
psrlq xmm0, 32 # pcmpeqb xmm0, xmm1
movd eax, xmm0 # pmovmskb eax, xmm0
or edx, eax # cmp al, 255
sete al # sete al
movzx eax, al #
ret
11 instructions in 40 bytes # 10 instructions in 36 bytes
OOPS: why does GCC (ab)use the SSE2 alias "Willamette New Instruction Set"
here instead of the native SSE4.1 alias "Penryn New Instruction Set"
of the Core2 (and all later processors)?
OUCH: why does it FAIL to REALLY use SSE2, as shown in the comments on the
right side?
Now add the -mtune=core2 option to EXPLICITLY enable the NATIVE SSE4.1
alias "Penryn New Instruction Set" of the Core2 processor:
GCC 13.3: gcc -m32 -mtune=core2 -O3 demo.c
# https://godbolt.org/z/svhEoYT11
ispowerof2(unsigned long long):
# xor eax, eax
movq xmm1, [esp+4] # movq xmm1, [esp+4]
pcmpeqd xmm0, xmm0 # pcmpeqq xmm0, xmm0
paddq xmm0, xmm1 # paddq xmm0, xmm1
pand xmm0, xmm1 # ptest xmm0, xmm1
movd edx, xmm0 #
psrlq xmm0, 32 #
movd eax, xmm0 #
or edx, eax #
sete al # sete al
movzx eax, al #
ret # ret
11 instructions in 40 bytes # 7 instructions in 26 bytes
OUCH: GCC FAILS to use SSE4.1 as shown in the comments on the right side.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Last compile with -mtune=i386 for the i386 processor:
GCC 13.3: gcc -m32 -mtune=i386 -O3 demo.c
# https://godbolt.org/z/e76W6dsMj
ispowerof2(unsigned long long):
push ebx #
mov ecx, [esp+8] # mov eax, [esp+4]
mov ebx, [esp+12] # mov edx, [esp+8]
mov eax, ecx #
mov edx, ebx #
add eax, -1 # add eax, -1
adc edx, -1 # adc edx, -1
and eax, ecx # and eax, [esp+4]
and edx, ebx # and edx, [esp+8]
or eax, edx # or eax, edx
sete al # neg eax
movzx eax, al # sbb eax, eax
pop ebx # inc eax
ret # ret
14 instructions in 33 bytes # 11 instructions in 32 bytes
OUCH: why does GCC abuse EBX (and ECX too) and performs a superfluous
memory write?
Stefan Kanthak
