[llvm] [DropUnnecessaryAssumes] Make the ephemeral value check more precise (PR #160700)

[Ramkumar Ramachandra via llvm-commits]

================
@@ -17,13 +18,48 @@ using namespace llvm;
 using namespace llvm::PatternMatch;
 
 static bool affectedValuesAreEphemeral(ArrayRef<Value *> Affected) {
-  // If all the affected uses have only one use (part of the assume), then
-  // the assume does not provide useful information. Note that additional
-  // users may appear as a result of inlining and CSE, so we should only
-  // make this assumption late in the optimization pipeline.
-  // TODO: Handle dead cyclic usages.
-  // TODO: Handle multiple dead assumes on the same value.
-  return all_of(Affected, match_fn(m_OneUse(m_Value())));
+  // Check whether all the uses are ephemeral, i.e. recursively only used
+  // by assumes. In that case, the assume does not provide useful information.
+  // Note that additional users may appear as a result of inlining and CSE,
+  // so we should only make this assumption late in the optimization pipeline.
+  SmallSetVector<Instruction *, 32> Worklist;
+  auto AddUsers = [&](Value *V) {
+    for (User *U : V->users()) {
+      // Bail out if we need to inspect too many users.
+      if (Worklist.size() >= 32)
+        return false;
+      Worklist.insert(cast<Instruction>(U));
+    }
+    return true;
+  };
+
+  for (Value *V : Affected) {
+    // Do not handle assumes on globals for now. The use list for them may
+    // contain uses in other functions.
+    if (!isa<Instruction, Argument>(V))
+      return false;
+
+    if (!AddUsers(V))
+      return false;
+  }
+
+  for (unsigned Idx = 0; Idx < Worklist.size(); ++Idx) {
+    Instruction *I = Worklist[Idx];
----------------
artagnon wrote:

Ah yes, I missed this detail: quite clever, thanks!

https://github.com/llvm/llvm-project/pull/160700