[llvm] [LCSSA] Avoid rewriting lifetime markers through PHIs (PR #210811)

[Aiden Grossman via llvm-commits]

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@@ -106,10 +106,22 @@ formLCSSAForInstructionsImpl(SmallVectorImpl<Instruction *> &Worklist,
     if (ExitBlocks.empty())
       continue;
 
+    SmallVector<Instruction *, 4> LifetimeMarkers;
+    bool DropLifetimeMarkers = false;
     for (Use &U : make_early_inc_range(I->uses())) {
       Instruction *User = cast<Instruction>(U.getUser());
       BasicBlock *UserBB = User->getParent();
 
+      // Lifetime markers must refer directly to an alloca. Rewriting their
+      // operands through LCSSA PHIs would produce invalid IR, so conservatively
+      // drop all lifetime markers when one crosses the loop boundary.
+      if (User->isLifetimeStartOrEnd()) {
+        LifetimeMarkers.push_back(User);
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boomanaiden154 wrote:

Why do we need to wait until we're outside of the loop to drop the user? We're already using `make_early_inc_range`.

https://github.com/llvm/llvm-project/pull/210811