"Andrew Pinski" <pins...@gmail.com> wrote:
> On Sat, May 27, 2023 at 3:54 PM Stefan Kanthak <stefan.kant...@nexgo.de>
> wrote:
>> Nevertheless GCC fails to optimise code properly:
>>
>> --- .c ---
>> int ispowerof2(unsigned long long argument) {
>> return __builtin_popcountll(argument) == 1;
>> }
>> --- EOF ---
>>
>> GCC 13.3 gcc -m32 -mpopcnt -O3
>>
>> https://godbolt.org/z/fT7a7jP4e
>> ispowerof2(unsigned long long):
>> xor eax, eax
>> xor edx, edx
>> popcnt eax, [esp+4]
>> popcnt edx, [esp+8]
>> add eax, edx # eax is less than 64!
>> cmp eax, 1 -> dec eax # 2 bytes shorter
sete al
movzx eax, al
ret
>
> dec eax is done for -Os already.
> -O2 means performance, it does not mean decrease size.
But -Os is supposed to optimise for size? REALITY CHECK:
GCC 13.3 gcc -m32 -mpopcnt -Os
https://godbolt.org/z/41Ed6rr6r
ispowerof2(unsigned long long):
push ebp
mov ebp, esp
sub esp, 16
push [ebp+12]
push [ebp+8]
call __popcountdi2
add esp, 16 # superfluous!
leave
dec eax
sete al
movzx eax, al
ret
While -O3 generates 9 instructions in a total of 24 bytes,
-Os[lowmotion] but generates 12 instructions in 29 bytes,
PLUS the uncounted instructions/bytes of __popcountdi2()!
This is what I call an EPIC FAILURE!
Stefan
