On Thu, May 25, 2023 at 11:56 PM Stefan Kanthak <stefan.kant...@nexgo.de> wrote:
>
> 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!
You need to use -march=native and not -mtune=native .... to turn on
the architecture features.
Thanks,
Andrew
>
> # 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
