[llvm-dev] Liveness of virtual registers

Matthias Braun via llvm-dev llvm-dev at lists.llvm.org
Thu Jul 28 14:10:15 PDT 2016

> On Jul 28, 2016, at 2:01 PM, Matthias Braun via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> 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...)
I re-checked the RegisterPressure.cpp code as well and it does not rely on dead-flags either (if there is no dead flag it queries the liveinterval for it). So it seems you are right with the statement of dead flags being optional.

- Matthias

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