[llvm-dev] Can DBG_VALUE instructions use undefined vregs?

[Amara Emerson via llvm-dev]

That’s what we were doing in the legalizer, but calling eraseFromParentAndMarkDBGValuesForRemoval() is much more expensive than a plain eraseFromParent(), so the motivating change was to use a heuristic to avoid calling it when it was very likely that it wasn’t going to have a debug user.

Whichever answer we get is fine, we can have a cleanup phase after legalization over all vregs in MachineRegisterInfo to mark ones without defs as undef, but I’d like to avoid doing that if we can so we don’t spend the extra compile time.

Amara

> On Oct 5, 2021, at 3:37 PM, Matthias Braun <matthiasb at fb.com> wrote:
> 
> I can't really be speak about debug instructions in particular. For machine instructions in general you just mark the use operand with "undef" and you are good...
> 
> - Matthias
> 
>> On Oct 5, 2021, at 3:29 PM, Amara Emerson via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> 
>> One GlobalISel compile time optimization patch (https://reviews.llvm.org/D109750) has generated some debate over whether it’s semantically allowed/not-an-error for a DBG_VALUE machine instruction to have a use of a vreg that doesn’t have a corresponding definition.
>> 
>> We talked internally with Adrian and Vedant and didn’t come to a strong conclusion either way. Does anyone have thoughts here?
>> 
>> Thanks,
>> Amara
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>