Dávid Bolvanský via llvm-dev
2018-May-09 17:57 UTC
[llvm-dev] Ignored branch predictor hints
Hello,
#define likely(x) __builtin_expect((x),1)
// switch like
char * b(int e) {
if (likely(e == 0))
return "0";
else if (e == 1)
return "1";
else return "f";
}
GCC correctly prefers the first case:
b(int):
mov eax, OFFSET FLAT:.LC0
test edi, edi
jne .L7
ret
But Clang seems to ignore _builtin_expect hints in this case.
b(int): # @b(int)
cmp edi, 1
mov eax, offset .L.str.1
mov ecx, offset .L.str.2
cmove rcx, rax
test edi, edi
mov eax, offset .L.str
cmovne rax, rcx
ret
https://godbolt.org/g/tuAVT7
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David Zarzycki via llvm-dev
2018-May-09 18:29 UTC
[llvm-dev] Ignored branch predictor hints
I’d wager that the if-else chain is being converted to a "switch statement” during an optimization pass and the __builtin_expect() hint is lost. Can you file a bug? https://bugs.llvm.org> On May 9, 2018, at 1:57 PM, Dávid Bolvanský via llvm-dev <llvm-dev at lists.llvm.org> wrote: > > Hello, > #define likely(x) __builtin_expect((x),1) > > // switch like > char * b(int e) { > if (likely(e == 0)) > return "0"; > else if (e == 1) > return "1"; > else return "f"; > } > GCC correctly prefers the first case: > b(int): > mov eax, OFFSET FLAT:.LC0 > test edi, edi > jne .L7 > ret > > But Clang seems to ignore _builtin_expect hints in this case. > b(int): # @b(int) > cmp edi, 1 > mov eax, offset .L.str.1 > mov ecx, offset .L.str.2 > cmove rcx, rax > test edi, edi > mov eax, offset .L.str > cmovne rax, rcx > ret > https://godbolt.org/g/tuAVT7 <https://godbolt.org/g/tuAVT7>_______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180509/8eaed29c/attachment.html>
Dávid Bolvanský via llvm-dev
2018-May-09 18:33 UTC
[llvm-dev] Ignored branch predictor hints
I did https://bugs.llvm.org/show_bug.cgi?id=37368 2018-05-09 20:33 GMT+02:00 Dávid Bolvanský <david.bolvansky at gmail.com>:> I did > > https://bugs.llvm.org/show_bug.cgi?id=37368 > > 2018-05-09 20:29 GMT+02:00 David Zarzycki <dave at znu.io>: > >> I’d wager that the if-else chain is being converted to a "switch >> statement” during an optimization pass and the __builtin_expect() hint is >> lost. Can you file a bug? https://bugs.llvm.org >> >> >> On May 9, 2018, at 1:57 PM, Dávid Bolvanský via llvm-dev < >> llvm-dev at lists.llvm.org> wrote: >> >> Hello, >> >> #define likely(x) __builtin_expect((x),1) >> >> // switch like >> char * b(int e) { >> if (likely(e == 0)) >> return "0"; >> else if (e == 1) >> return "1"; >> else return "f"; >> } >> >> GCC correctly prefers the first case: >> >> b(int): >> mov eax, OFFSET FLAT:.LC0 >> test edi, edi >> jne .L7 >> ret >> >> But Clang seems to ignore _builtin_expect hints in this case. >> >> b(int): # @b(int) >> cmp edi, 1 >> mov eax, offset .L.str.1 >> mov ecx, offset .L.str.2 >> cmove rcx, rax >> test edi, edi >> mov eax, offset .L.str >> cmovne rax, rcx >> ret >> >> https://godbolt.org/g/tuAVT7 >> >> _______________________________________________ >> LLVM Developers mailing list >> llvm-dev at lists.llvm.org >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >> >> >> >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180509/012baba4/attachment.html>
David Zarzycki via llvm-dev
2018-May-09 18:40 UTC
[llvm-dev] Ignored branch predictor hints
Hi Dávid,
Looks like you can defeat the switch conversion by adding a dummy asm(“”):
#define likely(x) __builtin_expect((x),1)
// switch like
char * b(int e) {
if (likely(e == 0))
return "0";
asm("");
if (e == 1)
return "1";
else return "f";
}
Dave
> On May 9, 2018, at 2:33 PM, Dávid Bolvanský via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
>
> I did
> https://bugs.llvm.org/show_bug.cgi?id=37368
<https://bugs.llvm.org/show_bug.cgi?id=37368>
>
> 2018-05-09 20:33 GMT+02:00 Dávid Bolvanský <david.bolvansky at gmail.com
<mailto:david.bolvansky at gmail.com>>:
> I did
>
> https://bugs.llvm.org/show_bug.cgi?id=37368
<https://bugs.llvm.org/show_bug.cgi?id=37368>
>
> 2018-05-09 20:29 GMT+02:00 David Zarzycki <dave at znu.io
<mailto:dave at znu.io>>:
> I’d wager that the if-else chain is being converted to a "switch
statement” during an optimization pass and the __builtin_expect() hint is lost.
Can you file a bug? https://bugs.llvm.org <https://bugs.llvm.org/>
>
>
>> On May 9, 2018, at 1:57 PM, Dávid Bolvanský via llvm-dev <llvm-dev
at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>
>> Hello,
>> #define likely(x) __builtin_expect((x),1)
>>
>> // switch like
>> char * b(int e) {
>> if (likely(e == 0))
>> return "0";
>> else if (e == 1)
>> return "1";
>> else return "f";
>> }
>> GCC correctly prefers the first case:
>> b(int):
>> mov eax, OFFSET FLAT:.LC0
>> test edi, edi
>> jne .L7
>> ret
>>
>> But Clang seems to ignore _builtin_expect hints in this case.
>> b(int): # @b(int)
>> cmp edi, 1
>> mov eax, offset .L.str.1
>> mov ecx, offset .L.str.2
>> cmove rcx, rax
>> test edi, edi
>> mov eax, offset .L.str
>> cmovne rax, rcx
>> ret
>> https://godbolt.org/g/tuAVT7
<https://godbolt.org/g/tuAVT7>_______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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