[Clang][CodeGen] Emit fake uses before musttail calls (#136867)

Fixes the issue reported following the merge of #118026.

When a valid `musttail` call is made, the function it is made from must
return immediately after the call; if there are any cleanups left in the
function, then an error is triggered. This is not necessary for fake
uses however - it is perfectly valid to simply emit the fake use
"cleanup" code before the tail call, and indeed LLVM will automatically
move any fake uses following a tail call to come before the tail call.

Therefore, this patch specifically choose to handle fake use cleanups
when a musttail call is present by simply emitting them immediately
before the call.

Author: SLTozer

clang/lib/CodeGen/CGCall.cpp

@@ -6001,8 +6001,16 @@ RValue CodeGenFunction::EmitCall(const CGFunctionInfo &CallInfo,
     for (auto it = EHStack.find(CurrentCleanupScopeDepth); it != EHStack.end();
          ++it) {
       EHCleanupScope *Cleanup = dyn_cast<EHCleanupScope>(&*it);
-      if (!(Cleanup && Cleanup->getCleanup()->isRedundantBeforeReturn()))
+      // Fake uses can be safely emitted immediately prior to the tail call, so
+      // we choose to emit them just before the call here.
+      if (Cleanup && Cleanup->isFakeUse()) {
+        CGBuilderTy::InsertPointGuard IPG(Builder);
+        Builder.SetInsertPoint(CI);
+        Cleanup->getCleanup()->Emit(*this, EHScopeStack::Cleanup::Flags());
+      } else if (!(Cleanup &&
+                   Cleanup->getCleanup()->isRedundantBeforeReturn())) {
         CGM.ErrorUnsupported(MustTailCall, "tail call skipping over cleanups");
+      }
     }
     if (CI->getType()->isVoidTy())
       Builder.CreateRetVoid();

clang/test/CodeGenCXX/fake-use-musttail.cpp

@@ -0,0 +1,27 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown -emit-llvm -fextend-variable-liveness -o - %s | FileCheck %s
+
+/// Tests that when we have fake uses in a function ending in a musttail call,
+/// we emit the fake uses and their corresponding loads immediately prior to the
+/// tail call.
+
+extern "C" char *bar(int *);
+
+// CHECK-LABEL: define dso_local ptr @foo(
+// CHECK-SAME:    ptr noundef [[E:%.*]])
+// CHECK-NEXT:  [[ENTRY:.*:]]
+// CHECK-NEXT:    [[E_ADDR:%.*]] = alloca ptr, align 8
+// CHECK-NEXT:    store ptr [[E]], ptr [[E_ADDR]], align 8
+// CHECK-NEXT:    [[TMP0:%.*]] = load ptr, ptr [[E_ADDR]], align 8
+// CHECK-NEXT:    [[FAKE_USE:%.*]] = load ptr, ptr [[E_ADDR]]
+// CHECK-NEXT:    notail call void (...) @llvm.fake.use(ptr [[FAKE_USE]])
+// CHECK-NEXT:    [[CALL:%.*]] = musttail call ptr @bar(ptr noundef [[TMP0]])
+// CHECK-NEXT:    ret ptr [[CALL]]
+
+// CHECK:       [[BB1:.*:]]
+// CHECK-NEXT:    [[FAKE_USE1:%.*]] = load ptr, ptr [[E_ADDR]]
+// CHECK-NEXT:    notail call void (...) @llvm.fake.use(ptr [[FAKE_USE1]])
+// CHECK-NEXT:    ret ptr undef
+//
+extern "C" const char *foo(int *e) {
+  [[clang::musttail]] return bar(e);
+}