[Clang] Emit Fake Uses before musttail calls

Fixes the issue reported following the merge of llvm#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 +6001,18 @@ 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; we
+      // choose to emit the fake use before the call rather than after, to avoid
+      // forcing variable values from every call on the "stack" to be preserved
+      // simultaneously.
+      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 +1,72 @@
+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --version 5
+// RUN: %clang_cc1 -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.
+
+struct Class1 {
+  Class1(int);
+};
+
+class Class2 {
+  static const char *foo(int *, const char *, int *, Class1, const int *,
+                         unsigned long);
+  template <class>
+  static char *bar(int *, const char *, int *, Class1, const int *, unsigned long);
+};
+
+// CHECK-LABEL: define dso_local noundef ptr @_ZN6Class23fooEPiPKcS0_6Class1PKim(
+// CHECK-SAME: ptr noundef [[E:%.*]], ptr noundef [[F:%.*]], ptr noundef [[G:%.*]], ptr noundef [[H:%.*]], i64 noundef [[I:%.*]]) #[[ATTR0:[0-9]+]] align 2 {
+// CHECK-NEXT:  [[ENTRY:.*:]]
+// CHECK-NEXT:    [[TMP0:%.*]] = alloca [[STRUCT_CLASS1:%.*]], align 1
+// CHECK-NEXT:    [[E_ADDR:%.*]] = alloca ptr, align 8
+// CHECK-NEXT:    [[F_ADDR:%.*]] = alloca ptr, align 8
+// CHECK-NEXT:    [[G_ADDR:%.*]] = alloca ptr, align 8
+// CHECK-NEXT:    [[H_ADDR:%.*]] = alloca ptr, align 8
+// CHECK-NEXT:    [[I_ADDR:%.*]] = alloca i64, align 8
+// CHECK-NEXT:    [[AGG_TMP:%.*]] = alloca [[STRUCT_CLASS1]], align 1
+// CHECK-NEXT:    store ptr [[E]], ptr [[E_ADDR]], align 8
+// CHECK-NEXT:    store ptr [[F]], ptr [[F_ADDR]], align 8
+// CHECK-NEXT:    store ptr [[G]], ptr [[G_ADDR]], align 8
+// CHECK-NEXT:    store ptr [[H]], ptr [[H_ADDR]], align 8
+// CHECK-NEXT:    store i64 [[I]], ptr [[I_ADDR]], align 8
+// CHECK-NEXT:    [[TMP1:%.*]] = load ptr, ptr [[E_ADDR]], align 8
+// CHECK-NEXT:    [[TMP2:%.*]] = load ptr, ptr [[F_ADDR]], align 8
+// CHECK-NEXT:    [[TMP3:%.*]] = load ptr, ptr [[G_ADDR]], align 8
+// CHECK-NEXT:    call void @_ZN6Class1C1Ei(ptr noundef nonnull align 1 dereferenceable(1) [[AGG_TMP]], i32 noundef 0)
+// CHECK-NEXT:    [[TMP4:%.*]] = load ptr, ptr [[H_ADDR]], align 8
+// CHECK-NEXT:    [[TMP5:%.*]] = load i64, ptr [[I_ADDR]], align 8
+// CHECK-NEXT:    [[FAKE_USE:%.*]] = load i64, ptr [[I_ADDR]], align 8
+// CHECK-NEXT:    notail call void (...) @llvm.fake.use(i64 [[FAKE_USE]]) #[[ATTR3:[0-9]+]]
+// CHECK-NEXT:    [[FAKE_USE1:%.*]] = load ptr, ptr [[H_ADDR]], align 8
+// CHECK-NEXT:    notail call void (...) @llvm.fake.use(ptr [[FAKE_USE1]]) #[[ATTR3]]
+// CHECK-NEXT:    [[FAKE_USE2:%.*]] = load [[STRUCT_CLASS1]], ptr [[TMP0]], align 1
+// CHECK-NEXT:    notail call void (...) @llvm.fake.use([[STRUCT_CLASS1]] [[FAKE_USE2]]) #[[ATTR3]]
+// CHECK-NEXT:    [[FAKE_USE3:%.*]] = load ptr, ptr [[G_ADDR]], align 8
+// CHECK-NEXT:    notail call void (...) @llvm.fake.use(ptr [[FAKE_USE3]]) #[[ATTR3]]
+// CHECK-NEXT:    [[FAKE_USE4:%.*]] = load ptr, ptr [[F_ADDR]], align 8
+// CHECK-NEXT:    notail call void (...) @llvm.fake.use(ptr [[FAKE_USE4]]) #[[ATTR3]]
+// CHECK-NEXT:    [[FAKE_USE5:%.*]] = load ptr, ptr [[E_ADDR]], align 8
+// CHECK-NEXT:    notail call void (...) @llvm.fake.use(ptr [[FAKE_USE5]]) #[[ATTR3]]
+// CHECK-NEXT:    [[CALL:%.*]] = musttail call noundef ptr @_ZN6Class23barIiEEPcPiPKcS2_6Class1PKim(ptr noundef [[TMP1]], ptr noundef [[TMP2]], ptr noundef [[TMP3]], ptr noundef [[TMP4]], i64 noundef [[TMP5]])
+// CHECK-NEXT:    ret ptr [[CALL]]
+// CHECK:       [[BB6:.*:]]
+// CHECK-NEXT:    [[FAKE_USE6:%.*]] = load i64, ptr [[I_ADDR]], align 8
+// CHECK-NEXT:    notail call void (...) @llvm.fake.use(i64 [[FAKE_USE6]]) #[[ATTR3]]
+// CHECK-NEXT:    [[FAKE_USE7:%.*]] = load ptr, ptr [[H_ADDR]], align 8
+// CHECK-NEXT:    notail call void (...) @llvm.fake.use(ptr [[FAKE_USE7]]) #[[ATTR3]]
+// CHECK-NEXT:    [[FAKE_USE8:%.*]] = load [[STRUCT_CLASS1]], ptr [[TMP0]], align 1
+// CHECK-NEXT:    notail call void (...) @llvm.fake.use([[STRUCT_CLASS1]] [[FAKE_USE8]]) #[[ATTR3]]
+// CHECK-NEXT:    [[FAKE_USE9:%.*]] = load ptr, ptr [[G_ADDR]], align 8
+// CHECK-NEXT:    notail call void (...) @llvm.fake.use(ptr [[FAKE_USE9]]) #[[ATTR3]]
+// CHECK-NEXT:    [[FAKE_USE10:%.*]] = load ptr, ptr [[F_ADDR]], align 8
+// CHECK-NEXT:    notail call void (...) @llvm.fake.use(ptr [[FAKE_USE10]]) #[[ATTR3]]
+// CHECK-NEXT:    [[FAKE_USE11:%.*]] = load ptr, ptr [[E_ADDR]], align 8
+// CHECK-NEXT:    notail call void (...) @llvm.fake.use(ptr [[FAKE_USE11]]) #[[ATTR3]]
+// CHECK-NEXT:    ret ptr undef
+//
+const char *Class2::foo(int *e, const char *f, int *g, Class1, const int *h,
+                        unsigned long i) {
+  [[clang::musttail]] return bar<int>(e, f, g, int(), h, i);
+}