On 6/5/23 12:17, Stefan Kanthak wrote:
--- failure.c ---
int _clz(unsigned long long argument) {
return __builtin_clzll(argument);
}
int _ctz(unsigned long long argument) {
return __builtin_ctzll(argument);
}
--- EOF ---
GCC 13.1 -m32 -mabm -mbmi -mlzcnt -O3 failure.c
<https://godbolt.org/z/MMf11hKch>
_clz(unsigned long long):
mov edx, DWORD PTR [esp+8]
xor ecx, ecx
xor eax, eax
lzcnt eax, DWORD PTR [esp+4]
add eax, 32
lzcnt ecx, edx
test edx, edx
cmovne eax, ecx
ret
_ctz(unsigned long long):
sub esp, 20
push DWORD PTR [esp+28]
push DWORD PTR [esp+28]
call __ctzdi2
add esp, 28
ret
OUCH: although EXPLICITLY enabled via -mabm (for AMD processors) and -mbmi
(for Intel processors), GCC generates slowmotion code calling __ctzdi2()
instead of TZCNT instructions available since 10 (in words: TEN) years.
GCC 13.1 -m32 -march=i386 -O3 failure.c
<https://godbolt.org/z/16ezfaexb>
_clz(unsigned long long):
mov edx, DWORD PTR [esp+4]
mov eax, DWORD PTR [esp+8]
test eax, eax
je .L2
bsr eax, eax
xor eax, 31
ret
.L2:
bsr eax, edx
xor eax, 31
lea eax, [eax+32]
ret
_ctz(unsigned long long):
sub esp, 20
push DWORD PTR [esp+28]
push DWORD PTR [esp+28]
call __ctzdi2
add esp, 28
ret
OUCH²: the BSF/BSR instructions were introduced 38 (in words: THIRTY-EIGHT)
years ago with the i386 processor, but GCC fails to know/use BSF --
a real shame!
OUCH³: an optimising compiler would of course generate "JMP __ctzdi2" instead
of code fiddling with the stack!
Stefan Kanthak
Hi,
While the optimization concerns your email raises appear to be genuine
to some degree, I must admit to being quite clueless as to why, exactly,
after at least 2 years of intermittently sending various seemingly quite
incensed emails to this list, you appears not to have, at any point,
questioned whether this method of action is in any way effective.
I myself have, too, made a hobby of investigating the capabilities of
GCC's optimizer on various pieces of code, and trying to get it
improved, by filing various appropriately formulated bugs in GCC's
bugzilla whenever it seemed appropriate to do so (see e.g.
https://gcc.gnu.org/PR94795, https://gcc.gnu.org/PR98957,
https://gcc.gnu.org/PR102224 or https://gcc.gnu.org/PR104371 for what I
believe to be pretty good examples of how to file such a bug).
I have not spent all this time sending various inflammatory emails to
mailing lists not made for that purpose, and I am quite puzzled by the
fact you appear to have concluded that doing so is appropriate. Should
the page on GCC's website about the mailing lists perhaps contain a
mention that the main mailing list is not made for spamming complaints
about minor bugs ? I'd have figured it'd be self-evident, but evidently
it seems this might perhaps be necessary...
Anyway, this leaves me with two more things I'd like to ask you about:
1. Why exactly are you so mad at GCC's optimizer and constantly belittle
its capacities ? Did a GCC developer working on the optimizer destroy
your hometown and salt the earth or something ? I also don't get why you
put "optimizer" between quotes like that - is this supposed to imply
calling GCC's optimizer as such is some insult to some hypothetical
better optimizer ? (note: I say hypothetical because I haven't seen any
optimizer that would be so much better than GCC's as to make it seem
insultingly bad compared to it - Clang is comparable to GCC in its
capacities but you also appear to hate it, given what I've seen
elsewhere, and I'd expect other compilers like ICC and MSVC to be the
ones that would be the butt of the joke here, given how poor they are
compared to GCC and Clang...)
2. Are you aware that these emails are not only pretty useless, but
potentially actively counterproductive ? I'd personally expect GCC
developers, who are rightfully not particularly happy at the thought of
having to constantly suffer your belittling emails, to at best actively
ignore them - but even worse from what I presume would be your POV (that
of someone who wants GCC's optimizer to improve), this constant spam
might give rise to what would be a pretty reasonable response: to be
biased against any proposed improvement that comes from or derives from
your emails. Arguably, to not be biased against them would encourage you
to keep sending more emails, which people do not want as of now.
PS: I hope you understand that I'm being somewhat generous when assuming
you genuinely want to get GCC's optimizer to improve, and are simply
failing to work effectively towards this goal. Someone else might
instead think you're just a troll that uses this as a pretext to send
inflammatory emails everywhere and waste the time of people like me.
