[PATCH] D68128: [InstCombine] Fold PHIs with equal incoming pointers

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp:1144-1155
+  for (Use &Incoming : PN.incoming_values()) {
+    if (!isa<Instruction>(Incoming))
+      return nullptr;
+    int64_t CurrentOffset;
+    bool IsCurrentInBounds = true;
+    Value *CurrentBase = GetPointerBaseWithConstantOffset(
+        Incoming, CurrentOffset, DL, /* AllowNonInbounds */ true,
----------------
lebedev.ri wrote:
> This still looks like a hoisting problem to me, honestly.
> 
> In particular, why do we want *all* the incoming values to be identical?
> I.e. why can't we track *which* values are identical,
> and if there are two-or-more of of particular value hoist+common them?
To be more specific
```
struct ZZZ {
  Value *Base;
  int64_t Offset;
  bool IsInBounds;
}

DenseMap<ZZZ, SmallVector<Value *, 4>> QQQ;

for (Use &Incoming : PN.incoming_values()) {
  QQQ[GetPointerBaseWithConstantOffset(Incoming)].append(Incoming);
}

for(??? I : QQQ) {
  if(I.second.size() < 2)
    continue;
  <hoist>
  replaceInstUsesWith()
}
```



CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D68128/new/

https://reviews.llvm.org/D68128