[llvm] [DeadArgElim] fix verifier failure when changing musttail's function signature (PR #127366)

[Reid Kleckner via llvm-commits]

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@@ -522,29 +517,25 @@ void DeadArgumentEliminationPass::surveyFunction(const Function &F) {
   // MaybeLive. Initialized to a list of RetCount empty lists.
   RetUses MaybeLiveRetUses(RetCount);
 
-  bool HasMustTailCalls = false;
   for (const BasicBlock &BB : F) {
-    // If we have any returns of `musttail` results - the signature can't
-    // change
-    if (const auto *TC = BB.getTerminatingMustTailCall()) {
-      HasMustTailCalls = true;
-      // In addition, if the called function is not locally defined (or unknown,
-      // if this is an indirect call), we can't change the callsite and thus
-      // can't change this function's signature either.
-      if (!isMustTailCalleeAnalyzable(*TC)) {
-        markLive(F);
+    if (BB.getTerminatingMustTailCall()) {
+      LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - " << F.getName()
+                        << " has musttail calls\n");
+      // If the calling convention is not swifttailcc or tailcc, the caller and
+      // callee of musttail must have exactly the same signature. Otherwise we
+      // only needs to guarantee they have the same return type.
+      if (F.getCallingConv() != CallingConv::SwiftTail ||
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rnk wrote:

Please create a helper method that takes `F` and freezes the whole prototype or just the return type based on the calling convention, since you do this in two places and the logic looks the same.

https://github.com/llvm/llvm-project/pull/127366