[llvm-dev] llvm.read_register for %RIP on x86_64

[Kostya Serebryany via llvm-dev]

FTR:
I've made a simple experiment with inline asm, it's as simple as
InlineAsm::get(FunctionType::get(IntptrTy, false),
                             StringRef("lea (%rip),$0"), StringRef("=r"),
                             /*hasSideEffects=*/false);

It generates a pretty efficient-looking code:
  42194c: 48 8d 0d 00 00 00 00 lea    0x0(%rip),%rcx        # 421953
<_Z3fooPi+0x13>
  421953: 48 89 0c c5 60 3a 08 mov    %rcx,0x1083a60(,%rax,8)
(first instruction gets the PC, the other stores it somewhere)

However, this introduces a significant slowdown in my case, over 20%
(I need to execute this roughly for 50% of basic blocks)

So, I'll be looking for some other solutions that don't require reading the
PC this often. :(

--kcc


On Tue, Jan 31, 2017 at 3:13 PM, Kostya Serebryany <kcc at google.com> wrote:

>
>
> On Tue, Jan 31, 2017 at 3:11 PM, Renato Golin <renato.golin at linaro.org>
> wrote:
>
>> On 31 Jan 2017 8:58 p.m., "Kostya Serebryany" <kcc at google.com> wrote:
>>
>> hmm. I am not sure I understood you. The last two paragraphs seem to
>> contradict each other.
>> So, you recommend to extend read_register to read the PC, or
>> "read_register is locked at the stack pointer as a design decision"?
>>
>>
>> Both. :-)
>>
>> There was a design decision to only support SP because we had no clear
>> case for anything other than the stack pointer.
>>
>> If you have one for the PC, it would be a much better technical decision
>> to reuse the machinery that already exists, is documented and tested, than
>> come up with an independent implementation.
>>
>> The discussion whether to support it in clang or not is orthogonal. But
>> once decided that we should support reading the PC, then read_register is
>> the obvious place.
>>
>> If the clang developers refuse the idea, then inline assembly is the only
>> option that would work across compilers.
>>
>> Makes sense?
>>
>
> Yep, will give it a try.
>
>
>>
>> Cheers,
>> Renato
>>
>
>
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