[llvm-dev] Spare Register at one Machine Instruction

[Nemanja Ivanovic via llvm-dev]

Thanks for clarifying Matthias.

On Mon, Jan 23, 2017 at 8:49 PM, Matthias Braun <mbraun at apple.com> wrote:

> Sounds like Hu found a solution, so this is just for the record:
>
> On Jan 21, 2017, at 2:23 AM, Nemanja Ivanovic <nemanja.i.ibm at gmail.com>
> wrote:
>
> I just realized that I forgot to mention where you can get the info about
> a physical register unused in the function (if that's what you're after):
> MachineRegisterInfo::isConstantPhysReg()
>
> isConstantPhysReg() is not really a test for checking whether a register
> is completely unused in a function. That is done with MachineRegisterInfo::
> isPhysRegUsed().
>
>
> On Sat, Jan 21, 2017 at 11:17 AM, Nemanja Ivanovic <
> nemanja.i.ibm at gmail.com> wrote:
>
>> I'm not sure exactly what you're after.
>> I was under the impression that you want to know which register is live
>> at a specific point (an instruction). If that's the case, how do one of the
>> two suggested solutions not suffice?
>> If a register is live-in to a block and not killed before your
>> instruction or it has a def and no kill within the block, it is live.
>> Otherwise it is dead and available. Isn't it?
>> Or are you interested in whether a physical register is unused in the
>> entire function?
>>
>> On Sat, Jan 21, 2017 at 10:21 AM, Hong Hu <huhong789 at gmail.com> wrote:
>>
>>> Hi Nemanja and Matthias,
>>>
>>> Thanks for the reply.
>>>
>>> I checked both Register Scavenger and LivePhysReg. It seems that both
>>> work on the BasicBlock level, right?
>>>
>>> Is it possible to perform such analysis in a whole function? For
>>> example, considering the calling convention (where the rsi, rdi will be in
>>> use from the beginning) and the return convention (where rax will be in use
>>> in the last bb).
>>>
>>> Regards,
>>> Hu Hong
>>>
>>> On 20 January 2017 at 03:55, Matthias Braun <mbraun at apple.com> wrote:
>>>
>>>> There is also the LivePhysReg facility that I would recomment if you
>>>> just want to query for a free register and do not need the full feature set
>>>> of the RegisterScavenger.
>>>>
>>>> - Matthias
>>>>
>>>> On Jan 19, 2017, at 5:50 AM, Nemanja Ivanovic via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>> I believe what you're after is the register scavenger.
>>>> It's in: include/llvm/CodeGen/RegisterScavenging.h
>>>> Implementation: lib/CodeGen/RegisterScavenging.cpp
>>>>
>>>> On Thu, Jan 19, 2017 at 1:36 PM, Hong Hu via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> Hi All,
>>>>>
>>>>> Given a machine instruction, is it possible to tell which register(s)
>>>>> is still not in use?
>>>>>
>>>>> For example, given one instruction A, if the one follows it (say B)
>>>>> defines register rax, then I can tell rax should spare at instruction A.
>>>>>
>>>>> The purpose is to use the spare register to replace registers used by
>>>>> A, for instrumentation purpose.
>>>>>
>>>>> Regards,
>>>>> Hu Hong
>>>>>
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
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>>>>
>>>>
>>>>
>>>
>>
>
>
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