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

SLTozer · 2025-04-23T13:44:53Z

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.

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.

llvmbot · 2025-04-23T13:45:24Z

@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-codegen

Author: Stephen Tozer (SLTozer)

Changes

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.

Full diff: https://github.com/llvm/llvm-project/pull/136867.diff

2 Files Affected:

(modified) clang/lib/CodeGen/CGCall.cpp (+11-1)
(added) clang/test/CodeGenCXX/fake-use-musttail.cpp (+72)

diff --git a/clang/lib/CodeGen/CGCall.cpp b/clang/lib/CodeGen/CGCall.cpp
index 8cb27420dd911..c8cddfc06fce7 100644
--- a/clang/lib/CodeGen/CGCall.cpp
+++ b/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();
diff --git a/clang/test/CodeGenCXX/fake-use-musttail.cpp b/clang/test/CodeGenCXX/fake-use-musttail.cpp
new file mode 100644
index 0000000000000..7d1420ce5fdb6
--- /dev/null
+++ b/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);
+}

gulfemsavrun · 2025-04-23T16:01:38Z

Thanks for the fix, and I verified that it resolves the issue that I reported in #118026 (comment).

wolfy1961 · 2025-04-23T17:54:34Z

LGTM

gulfemsavrun · 2025-04-25T00:55:44Z

Is there anything that is blocking this PR to be merged? If it needs more time to review, could you please revert the original change and reland with this fix? This is blocking us to roll Clang in our project.

rjmccall

Functionality seems fine. A couple minor requests, then LGTM.

rjmccall · 2025-04-25T07:06:05Z

clang/lib/CodeGen/CGCall.cpp

+      // 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.


The second half of this comment is odd. There isn't really a choice about this; the fake uses must be emitted prior to the tail call because there cannot be anything after it.

rjmccall · 2025-04-25T07:07:54Z

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

+// CHECK-NEXT:    [[TMP3:%.*]] = load ptr, ptr [[H_ADDR]], align 8
+// CHECK-NEXT:    [[TMP4:%.*]] = 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]+]]


If you're not testing the attributes, you can just leave this off; matches don't have to match the entire line.

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.

Relands this feature after several fixes: * Force fake uses to be emitted before musttail calls (#136867) * Added soften-float legalization for fake uses (#142714) * Treat fake uses as size-less instructions in a SystemZ assert (#144390) If further issues with fake uses are found then this may be reverted again, but all currently-known issues are resolved. This reverts commit 2dc6e98.

SLTozer added the clang:codegen IR generation bugs: mangling, exceptions, etc. label Apr 23, 2025

SLTozer requested review from asl, gulfemsavrun, rjmccall and wolfy1961 April 23, 2025 13:44

SLTozer self-assigned this Apr 23, 2025

llvmbot added the clang Clang issues not falling into any other category label Apr 23, 2025

SLTozer mentioned this pull request Apr 23, 2025

[Clang] Enable -fextend-variable-liveness at -Og #118026

Merged

SLTozer added 2 commits April 24, 2025 11:27

Simplify test

2765fb7

Use extern C to prevent name mangling

d0e4e68

rjmccall reviewed Apr 25, 2025

View reviewed changes

SLTozer added 2 commits April 25, 2025 09:51

Reduce comment, further simplify test

c8c371f

Set explicit triple

bc98cd0

SLTozer merged commit 2d00c73 into llvm:main Apr 25, 2025
11 checks passed

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

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

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

Uh oh!

SLTozer commented Apr 23, 2025

Uh oh!

llvmbot commented Apr 23, 2025 •

edited

Loading

Uh oh!

gulfemsavrun commented Apr 23, 2025

Uh oh!

wolfy1961 commented Apr 23, 2025

Uh oh!

gulfemsavrun commented Apr 25, 2025

Uh oh!

rjmccall left a comment

Uh oh!

rjmccall Apr 25, 2025

Uh oh!

rjmccall Apr 25, 2025

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

5 participants

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

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

Uh oh!

Conversation

SLTozer commented Apr 23, 2025

Uh oh!

llvmbot commented Apr 23, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

gulfemsavrun commented Apr 23, 2025

Uh oh!

wolfy1961 commented Apr 23, 2025

Uh oh!

gulfemsavrun commented Apr 25, 2025

Uh oh!

rjmccall left a comment

Choose a reason for hiding this comment

Uh oh!

rjmccall Apr 25, 2025

Choose a reason for hiding this comment

Uh oh!

rjmccall Apr 25, 2025

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

5 participants

llvmbot commented Apr 23, 2025 •

edited

Loading