[PATCH] D91027: [BasicAA] Generalize base offset modulus handling

[Nikita Popov via Phabricator via llvm-commits]

nikic added inline comments.


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Comment at: llvm/lib/Analysis/BasicAliasAnalysis.cpp:1399
+        (GCD - ModOffset).uge(V1Size.getValue()))
       return NoAlias;
 
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jdoerfert wrote:
> Maybe a comment above this to explain, I don't get the reasoning TBH.
> 
> Here is my idea of what is happening but it is not complete:
> ```
> // X[0:N] is a vector of N+1 unknowns
> GEP1 = GEP1BaseOffset + X[0:N] * GEP1.scale[0:N]
> GCD = gcd(GEP1.scale[0], ..., GEP1.scale[N])
> ModOffset = GEP1BaseOffset % GCD
> // Every value of GEP1 looks like: GEP1 := Y * GCD + ModOffset
> // and ModOffset < GCD.
> Diff = GCD - ModOffset
> if (ModOffset >=u V2Size  // I guess V2 fits in the size between Y * GCD and GEP1
>  &&      Diff >=u V1Size  // this I don't understand.
>   return NoAlias;
> ```
Does the new comment make sense to you? We have the second access at [0..V2Size) with the remaining [V2Size..GCD) not being accessed. We don't alias if the first access fits in there, i.e. ModOffset >= V2Size and ModOffset+V1Size <= GCD.


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  https://reviews.llvm.org/D91027/new/

https://reviews.llvm.org/D91027