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

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Merged
merged 5 commits into from
Apr 25, 2025
Merged

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

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90c09c8
[Clang] Emit Fake Uses before musttail calls
SLTozer Apr 23, 2025
2765fb7
Simplify test
SLTozer Apr 24, 2025
d0e4e68
Use extern C to prevent name mangling
SLTozer Apr 24, 2025
c8c371f
Reduce comment, further simplify test
SLTozer Apr 25, 2025
bc98cd0
Set explicit triple
SLTozer Apr 25, 2025
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10 changes: 9 additions & 1 deletion clang/lib/CodeGen/CGCall.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -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();
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27 changes: 27 additions & 0 deletions clang/test/CodeGenCXX/fake-use-musttail.cpp
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Original file line number Diff line number Diff line change
@@ -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);
}
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