[LAA] Analyze pointers forked by a phi (#65834)

Given a function like the following: https://godbolt.org/z/T9c99fr88
```c
1161_noReadWrite(int *Preds) {
  for (int i = 0; i < LEN_1D-1; ++i) {
    if (Preds[i] != 0)
      b[i] = c[i] + 1;
    else
      a[i] = i * i;
  }
}
```

LLVM will optimize the IR to a single store by a phi instruction:

   ```llvm
      %1 = load ptr, ptr @a, align 64
      %2 = load ptr, ptr @b, align 64
      ...
    for.inc:
      %.sink = phi ptr [ %1, %if.then ], [ %2, %if.else ]
      %add.sink = phi double [ %add, %if.then ], [ %conv8, %if.else ]
%arrayidx7 = getelementptr inbounds double, ptr %.sink, i64 %indvars.iv
      store double %add.sink, ptr %arrayidx7, align 8
   ```

LAA is currently unable to analyze such IR, since ScalarEvolution will
return a SCEVUnknown for the forked pointer operand of the store.

This patch adds initial optional support for analyzing both
possibilities for the pointer and allowing LAA to generate runtime
checks for the bounds if required, refers to D108699, but here address
the phi node.

Fixes https://github.com/llvm/llvm-project/issues/64888

Reviewed By: huntergr-arm, fhahn
Differential Revision: https://reviews.llvm.org/D158965

1 files changed, 16 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/LoopAccessAnalysis.cpp b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
index 8a779ac..245cce2 100644
--- a/llvm/lib/Analysis/LoopAccessAnalysis.cpp
+++ b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
@@ -942,6 +942,22 @@ static void findForkedSCEVs(
       ScevList.emplace_back(Scev, !isGuaranteedNotToBeUndefOrPoison(Ptr));
     break;
   }
+  case Instruction::PHI: {
+    SmallVector<PointerIntPair<const SCEV *, 1, bool>, 2> ChildScevs;
+    // A phi means we've found a forked pointer, but we currently only
+    // support a single phi per pointer so if there's another behind this
+    // then we just bail out and return the generic SCEV.
+    if (I->getNumOperands() == 2) {
+      findForkedSCEVs(SE, L, I->getOperand(0), ChildScevs, Depth);
+      findForkedSCEVs(SE, L, I->getOperand(1), ChildScevs, Depth);
+    }
+    if (ChildScevs.size() == 2) {
+      ScevList.push_back(ChildScevs[0]);
+      ScevList.push_back(ChildScevs[1]);
+    } else
+      ScevList.emplace_back(Scev, !isGuaranteedNotToBeUndefOrPoison(Ptr));
+    break;
+  }
   case Instruction::Add:
   case Instruction::Sub: {
     SmallVector<PointerIntPair<const SCEV *, 1, bool>> LScevs;