[llvm-dev] Expose aliasing information in getModRefInfo (or viceversa?)

[Alina Sbirlea via llvm-dev]

On Tue, Oct 10, 2017 at 1:05 PM, Hal Finkel <hfinkel at anl.gov> wrote:

>
> On 10/10/2017 02:49 PM, Alina Sbirlea wrote:
>
> Sigh
>> I should have taken the time to give a better example.
>> The must-alias part is irrelevant to an example (it only requires
>> read-onlyness)
>>
>> You said "LICM doesn't move calls, so we'd never really care about
>> must-alias for promotion". I was just pointing out other things move calls
>> any may want to know.
>>
>> If you want an example where the must-alias part would matter:
>>
>> *a = something
>> foo(a)
>> b = *a
>>
>> If foo mustalias a (and only a) not only can you move foo with a, you can
>> actually clone foo here, change it to be pass-by-value, and promote the
>> argument inside of it (if you wanted to).
>>
>> So you can use this info to, for example, do interprocedural promotion.
>>
>>
>>> Are we instead looking to set a MRI_Must bit, disjunct of MRI_Mod, and
>>> test for MRI_Ref&MRI_Must or MRI_Mod&MRI_Must?
>>>
>>
>> Yes.
>>
>
> I didn't mean to pick on the example, sorry if that's how it came through.
>
> Since the consensus is to expose the Must info in ModRefInfo, I'm trying
> to figure out how to add it in a way that makes sense to me.
> The way I see ModRefInfo is designed right now is to lower the lattice to
> NoModRef as fast as possible (start with ModRef as top, get to NoModRef as
> bottom). The implementation is based on having either Mod or Ref and
> masking out everything else.
> Introducing a Must bit, means setting it occasionally (since May is
> conservative) and then preserving it, so the opposite: start lattice at
> bottom, set to top.
>
> What I was trying, that *somewhat* made sense:
> enum ModRefInfo {
>   MRI_NoModRef = 0,
>   MRI_Ref = 1,
>   MRI_Mod = 2,
>   MRI_ModRef = MRI_Ref | MRI_Mod,
>   MRI_Must = 4,
>   MRI_MustRef = MRI_Ref | MRI_Must,
>   MRI_MustMod = MRI_Mod | MRI_Must,
>   MRI_MustModRef = MRI_ModRef | MRI_Must
> };
> // + shift values in FunctionModRefLocation to 8, 16, 32.
>
> Recursive masking of MRI_Ref/MRI_Mod would get replaced by
> MRI_MustRef/MRI_MustMod.
> But the top of the lattice is still MRI_ModRef.
> While the implementation details *may* be ok to resolve, there are calls
> checking for equality to MRI_Ref or MRI_Mod (not &), so adding the
> occasional Must bit seems bad.
>
>
> I don't see this as a major problem. Please feel free to fix these places
> by replacing the equality checks with mask checks.
>

Ok, thank you!

>
>
>  -Hal
>
> So I guess my question is, what's the right approach here? I feel like I'm
> not on the right path.
>
>
>> In getModRefInfo(CS, Loc), the MRI_Must bit would then be set if
>>> doesAccessArgPointees and ArgAlias == MustAlias for all Args, which seems
>>> correct.
>>>
>>
>>
>> alias == MustAlias for Loc, not for all args.
>> (IE It it returns a singular result about Loc, not a result about all
>> args)
>>
>> To get the set answer for all args, we'd have to query further.
>>
>
> Yes, that's what I meant. In getModRefInfo(CS, Loc) there is a loop over
> all args, it checks alias() for each one.
>
>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
>
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