[llvm] [SimplifyCFG] Simplify switch instruction that has duplicate arms (PR #114262)

[Nikita Popov via llvm-commits]

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@@ -7436,6 +7437,185 @@ static bool simplifySwitchOfCmpIntrinsic(SwitchInst *SI, IRBuilderBase &Builder,
   return true;
 }
 
+/// Checking whether two cases of SI are equal depends on the contents of the
+/// BasicBlock and the incoming values of their successor PHINodes.
+/// PHINode::getIncomingValueForBlock is O(|Preds|), so we'd like to avoid
+/// calling this function on each BasicBlock every time isEqual is called,
+/// especially since the same BasicBlock may be passed as an argument multiple
+/// times. To do this, we can precompute a map of PHINode -> Pred BasicBlock ->
+/// IncomingValue and add it in the Wrapper so isEqual can do O(1) checking
+/// of the incoming values.
+struct SwitchSuccWrapper {
+  // Keep so we can use SwitchInst::setSuccessor to do the replacement. It won't
+  // be important to equality though.
+  unsigned SuccNum;
+  BasicBlock *Dest;
+  DenseMap<PHINode *, DenseMap<BasicBlock *, Value *>> *PhiPredIVs;
+};
+
+namespace llvm {
+template <> struct DenseMapInfo<const SwitchSuccWrapper *> {
+  static const SwitchSuccWrapper *getEmptyKey() {
+    return static_cast<SwitchSuccWrapper *>(
+        DenseMapInfo<void *>::getEmptyKey());
+  }
+  static const SwitchSuccWrapper *getTombstoneKey() {
+    return static_cast<SwitchSuccWrapper *>(
+        DenseMapInfo<void *>::getTombstoneKey());
+  }
+  static unsigned getHashValue(const SwitchSuccWrapper *SSW) {
+    BasicBlock *Succ = SSW->Dest;
+    BranchInst *BI = cast<BranchInst>(Succ->getTerminator());
+    assert(BI->isUnconditional() &&
+           "Only supporting unconditional branches for now");
+    assert(BI->getNumSuccessors() == 1 &&
+           "Expected unconditional branches to have one successor");
+    assert(Succ->size() == 1 && "Expected just a single branch in the BB");
+
+    // Since we assume the BB is just a single BranchInst with a single
+    // succsessor, we hash as the BB and the incoming Values of its successor
+    // PHIs. Initially, we tried to just use the successor BB as the hash, but
+    // this had poor performance. We find that the extra computation of getting
+    // the incoming PHI values here leads to better performance on overall Set
+    // performance. We also tried to build a map from BB -> Succs.IncomingValues
----------------
nikic wrote:

```suggestion
    // PHIs. Initially, we tried to just use the successor BB as the hash, but
    // including the incoming PHI values leads to better performance.
    // We also tried to build a map from BB -> Succs.IncomingValues
```
There was a lot of performance in here.

https://github.com/llvm/llvm-project/pull/114262