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

[Hal Finkel via llvm-dev]

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.

  -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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