[PATCH] D106099: [DependenceAnalysis] Guard analysis using getPointerBase()

[Eli Friedman via Phabricator via llvm-commits]

efriedma added inline comments.


================
Comment at: llvm/lib/Analysis/DependenceAnalysis.cpp:3556
   LLVM_DEBUG(dbgs() << "    DstSCEV = " << *DstSCEV << "\n");
+  if (SE->getPointerBase(SrcSCEV) != SE->getPointerBase(DstSCEV)) {
+    // If two pointers have different bases, trying to analyze indexes won't
----------------
artemrad wrote:
> This seems like a subset of cases we should be filtering out. Please also add checks for the access width being unaligned with the store/load size. In this case the base pointers are different but the accesses can still overlap. This case will not be caught by DependenceAnalysis. 
> 
> 
> ```
> define void @z0(i32* %A, i64 %n) nounwind uwtable ssp {
> entry:
>   %arrayidx = getelementptr inbounds i32, i32* %A, i64 %n
>   %arrayidx_cast = bitcast i32* %arrayidx to i64*
>   store i64 0, i64* %arrayidx_cast, align 4
> 
> ; CHECK: da analyze - confused!
> ; CHECK: da analyze - confused!
> ; CHECK: da analyze - confused!
> ; CHECK: da analyze - none
> ; CHECK: da analyze - confused!
> ; CHECK: da analyze - confused!
> 
>   %add1 = add i64 %n, 1
>   %arrayidx2 = getelementptr inbounds i32, i32* %A, i64 %add1
>   %0 = load i32, i32* %arrayidx2, align 4
>  
>   ; Even if the load and store types are the same, if their
>   ; alignments are smaller than the store size and they are
>   ; accesses through something other than a straightforward
>   ; gep, they can still overlap, as this access shows
>   %arrayidx2_cast = bitcast i32* %arrayidx2 to i64*
>   %1 = load i64, i64* %arrayidx2_cast, align 4
>   ret void
> }
> ```
> 
> 
> 
> 
Well, there are a few things we can do in that case:

1. Bail out if we don't have two naturally aligned accesses of the same size.
2. Perform multiple dependency checks, and merge the results.  For example, if you have a size 4 access, and a size 8 access, you can treat the size 8 access as two size 4 accesses, and merge the results accordingly.
3. Teach the dependence checks to use ranges, instead of plain indexes.

I'm afraid (1) will hurt performance. And I don't really want to sign up to do (2) or (3).


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D106099/new/

https://reviews.llvm.org/D106099