Who cares about performance (or Intel's CPU errata)?

Stefan Kanthak stefan.kanthak@nexgo.de
Sat May 27 22:52:30 GMT 2023

"Andrew Pinski" <pinskia@gmail.com> wrote:

> On Sat, May 27, 2023 at 2:25 PM Stefan Kanthak <stefan.kanthak@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

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

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!


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