[llvm-dev] Print Register Liveness Information

[Quentin Colombet via llvm-dev]

> On Feb 17, 2017, at 6:30 AM, Anitha B Gollamudi <anitha.boyapati at gmail.com> wrote:
> 
> On 16 February 2017 at 17:48, Quentin Colombet <qcolombet at apple.com <mailto:qcolombet at apple.com>> wrote:
>> 
>> Good point!
>> Additionally this might gets invalidated by some passes (look for MachineRegisterInfo::invalidateLiveness)
> 
> Thanks for the pointers! What is the rational behind invalidating
> register liveness info? Does it consume lot of memory during compilation?

There are mainly two reasons:
1. If you don’t need it later, we don’t bother maintaining it to save compile time
2. Some transformations may do complex transformations (e.g., on the CFG) and keeping the information up-to-date is too error-prone and we just bail

#2 is usually less common but it can happen.

> 
> -Anitha
> 
> 
>> 
>> On Feb 16, 2017, at 2:32 PM, Matthias Braun <mbraun at apple.com> wrote:
>> 
>> Note that this only works for targets that say TargetRegisterInfo::trackLivenessAfterRegAlloc()== true.
>> 
>> On Feb 16, 2017, at 1:21 PM, Quentin Colombet via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> 
>> Hi Anitha,
>> 
>> The liveness information is usually dropped after regalloc.
>> Therefore the short answer is no, this information is not available before producing assembly.
>> 
>> However, we have the live-ins and you can request the kill flag to be set. From both this information, you can recompute the liveness.
>> I’d suggest you use the LivePhysReg helper class and use its result to annotate your assembly (emitComment or something along those lines).
>> 
>> Cheers,
>> -Quentin
>> 
>> On Feb 16, 2017, at 12:46 PM, Anitha B Gollamudi via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> 
>> Hi,
>> 
>> Is there a way to print the register liveness information into the assembly file or as some intermediate listing?
>> To be more precise, I am looking at figuring out the following information in an assembly file/listing.
>> 
>> Say, for simplicity, our register set is only { rsp, rbp, eax }
>> 
>> 
>> subq    $16, %rsp     // rsp is killed here, eax and rbp are live
>> movl    $0, -4(%rbp)  // rbp is killed here, rsp and eax are live
>> xorl    %eax, %eax    // eax is killed here, rsp and rbp are live
>> movl    $1, %eax   // eax is killed here, rsp and rbp are live
>> 
>> Is there a way LLVM stores this information somewhere before producing assembly?
>> I haven't  dug into the source code yet, but intuitively the information must be there and hence maybe some pass dumps that info?
>> 
>> Thanks!
>> 
>> Anitha
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>> 
> 
> 
> 
> -- 
> Anitha

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