[LAA] Fix incorrect dependency classification. (#70819)

As shown in #70473, the following loop was not considered safe to
vectorize. When determining the memory access dependencies in
a loop which has negative iteration step, we invert the source and
sink of the dependence. Perhaps we should just invert the operands
to getMinusSCEV(). This way the dependency is not regarded to be
true, since the users of the `IsWrite` variables, which correspond to
each of the memory accesses, rely on program order and therefore
should not be swapped.

void vectorizable_Read_Write(int *A) {
  for (unsigned i = 1022; i >= 0; i--)
    A[i+1] = A[i] + 1;
}

1 files changed, 3 insertions, 6 deletions

diff --git a/llvm/lib/Analysis/LoopAccessAnalysis.cpp b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
index 91f5eab0..a8dbb66 100644
--- a/llvm/lib/Analysis/LoopAccessAnalysis.cpp
+++ b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
@@ -1916,15 +1916,12 @@ getDependenceDistanceStrideAndSize(
   const SCEV *Src = PSE.getSCEV(APtr);
   const SCEV *Sink = PSE.getSCEV(BPtr);
 
-  // If the induction step is negative we have to invert source and sink of
-  // the dependence.
+  // If the induction step is negative we have to invert source and sink of the
+  // dependence when measuring the distance between them. We should not swap
+  // AIsWrite with BIsWrite, as their uses expect them in program order.
   if (StrideAPtr < 0) {
-    std::swap(APtr, BPtr);
-    std::swap(ATy, BTy);
     std::swap(Src, Sink);
-    std::swap(AIsWrite, BIsWrite);
     std::swap(AInst, BInst);
-    std::swap(StrideAPtr, StrideBPtr);
   }
 
   const SCEV *Dist = SE.getMinusSCEV(Sink, Src);