"Andrew Pinski" <pins...@gmail.com> wrote:
> On Sat, May 27, 2023 at 2:25 PM Stefan Kanthak <stefan.kant...@nexgo.de>
> wrote:
>>
>> Just to show how SLOPPY, INCONSEQUENTIAL and INCOMPETENT GCC's developers
>> are:
>>
>> --- dontcare.c ---
>> int ispowerof2(unsigned __int128 argument) {
>> return __builtin_popcountll(argument) + __builtin_popcountll(argument >>
>> 64) == 1;
>> }
>> --- EOF ---
>>
>> GCC 13.3 gcc -march=haswell -O3
>>
>> https://gcc.godbolt.org/z/PPzYsPzMc
>> ispowerof2(unsigned __int128):
>> popcnt rdi, rdi
>> popcnt rsi, rsi
>> add esi, edi
>> xor eax, eax
>> cmp esi, 1
>> sete al
>> ret
>>
>> OOPS: what about Intel's CPU errata regarding the false dependency on
>> POPCNTs output?
>
> Because the popcount is going to the same register, there is no false
> dependency ....
> The false dependency errata only applies if the result of the popcnt
> is going to a different register, the processor thinks it depends on
> the result in that register from a previous instruction but it does
> not (which is why it is called a false dependency). In this case it
> actually does depend on the previous result since the input is the
> same as the input.
OUCH, my fault; sorry for the confusion and the wrong accusation.
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 # superfluous
ret
5 bytes and 1 instruction saved; 5 bytes here and there accumulate to
kilo- or even megabytes, and they can extend code to cross a cache line
or a 16-byte alignment boundary.
JFTR: same for "__builtin_popcount(argument) == 1;" and 32-bit argument
JFTR: GCC is notorious for generating superfluous MOVZX instructions
where its optimiser SHOULD be able see that the value is already
less than 256!
Stefan
