llvm dev - Jul 2016 - Liveness of virtual registers

Dear there,

I am trying to use liveness of a variable while writing a machinefunction
pass. But it seems that some virtual registers are not marked as dead while
they should be (never used by anything else). I get those info by callings
isDead or isDef function for that MachineOperand (vreg). Is there a way to
get more accurate liveness info?

Thanks,
Xiaochu
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On 7/28/2016 3:37 PM, Xiaochu Liu via llvm-dev wrote:
>
> I am trying to use liveness of a variable while writing a
> machinefunction pass. But it seems that some virtual registers are not
> marked as dead while they should be (never used by anything else). I get
> those info by callings isDead or isDef function for that MachineOperand
> (vreg). Is there a way to get more accurate liveness info?
The absence of these flags does not mean anything, but if they are 
present, then they must be correct.

You can always use functions use_empty and use_nodbg_empty, both in 
MachineRegisterInfo.  A precise liveness information is calculated by 
Live Interval Analysis.

-Krzysztof


-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, 
hosted by The Linux Foundation

The isKill must be correct if present, but a vreg may still be killed even if
the operand has no Kill flag.

The isDead and isUndef flags however are required to be correct. We need further
details to say anything about your problem, a LiveIntervals::dump() dump for
example would be helpful to assess whether the liveness data is correct in your
case.

- Matthias
> On Jul 28, 2016, at 1:42 PM, Krzysztof Parzyszek via llvm-dev <llvm-dev
at lists.llvm.org> wrote:
> 
> On 7/28/2016 3:37 PM, Xiaochu Liu via llvm-dev wrote:
>> 
>> I am trying to use liveness of a variable while writing a
>> machinefunction pass. But it seems that some virtual registers are not
>> marked as dead while they should be (never used by anything else). I
get
>> those info by callings isDead or isDef function for that MachineOperand
>> (vreg). Is there a way to get more accurate liveness info?
> 
> The absence of these flags does not mean anything, but if they are present,
then they must be correct.
> 
> You can always use functions use_empty and use_nodbg_empty, both in
MachineRegisterInfo.  A precise liveness information is calculated by Live
Interval Analysis.
> 
> -Krzysztof
> 
> 
> -- 
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted
by The Linux Foundation
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