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

[Alina Sbirlea via llvm-dev]

>
> 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.
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.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20171010/300a72a2/attachment.html>