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

[Alina Sbirlea via llvm-dev]

Hi,

This came up in https://reviews.llvm.org/D38569, and I'd like some input on
what's the best way to get alias and mod-ref info without having two alias
calls.

A couple of ideas:
(a) Extend the getModRefInfo interface (+getModRefBehavior,
+gerArgModRefInfo) to return a pair {ModRefInfo, AliasResult}.

The AliasResult can be optional based on an argument
     e.g.:
    struct MRI_AR { ModRefInfo MRI, AliasResult AR };
    MRI_AR getModRefInfoAlias (LoadInst *LI, MemoryLocation Loc, bool
SetAliasResultField);

Add wrapper APIs to preserve current calls.
    e.g.:
     ModRefInfo getModRefInfo (LoadInst *LI, MemoryLocation Loc) {
          return getModRefInfoAlias (LI, Loc, false).MRI;
      }

(b) From talking offline with George, introducing a MRI_MustMod in
ModRefInfo.


Open question: How to handle callsites.


In terms of whether this is worth doing, as a preliminary timing test I
timed the llvm bootstrap build with 1 vs 2 alias calls in D38569:
instructionClobbersQuery:296, and got the following:
2 alias calls:
real    62m52.627s
user    2769m46.964s
sys     17m48.072s
1 alias call:
real    62m56.659s
user    2766m40.452s
sys     17m46.312s

Thoughts?


Thanks,
Alina
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