[llvm-dev] Spare Register at one Machine Instruction

[Nemanja Ivanovic via llvm-dev]

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()

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
>>>>
>>>>
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>>>
>>>
>>
>
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