llvm dev - Jul 2016 - Liveness of virtual registers

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
> 
> 
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On 7/28/2016 3:49 PM, Matthias Braun wrote:
>
> The isDead and isUndef flags however are required to be correct.
Undef yes, but what relies on isDead being accurate (before live 
interval computation)?

-Krzysztof

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

> On Jul 28, 2016, at 1:52 PM, Krzysztof Parzyszek <kparzysz at
codeaurora.org> wrote:
> 
> On 7/28/2016 3:49 PM, Matthias Braun wrote:
>> 
>> The isDead and isUndef flags however are required to be correct.
> 
> Undef yes, but what relies on isDead being accurate (before live interval
computation)?
I only remember Andy/Quentin making those conservative correctness claims about
the kill flags but not the dead/undef flags. I am also pretty sure the
RegisterPressure.cpp / -verify-misched fail on missing dead flags (although
-verify-misched fails in several instances anyway at the moment...)

On the other hand I just checked the MachineVerifier which indeed only checks
for sonervatively correct dead flags as you say and I can't think of other
places being problematic with missing dead flags.
I wonder if that is by accident or by design.

- Matthias