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[clang] Limit lifetimes of temporaries to the full expression
We have several issues describing suboptimal stack usage related to the lifetimes of temporary objects, such as #68747, #43598, and #109204. Previously, https://reviews.llvm.org/D74094 tried to address this. In that review, a few issues were brought up, particularly a concern about the lifetimes of the temporaries needing to be extended to end of the full expression. While there are arguably more optimal lifetime bounds we could enforce, for now we can conservatively make them extend to the end of the full expression, and later refine the optimization to use tighter bounds (or perhaps a better mechanism in the middle end?). Fixes #68747 Co-authored-by: Nick Desaulniers <[email protected]> Co-authored-by: Erik Pilkington <[email protected]>
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‎clang/docs/ReleaseNotes.rst‎

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@@ -86,6 +86,15 @@ Potentially Breaking Changes
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options-related code has been moved out of the Driver into a separate library.
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- The ``clangFrontend`` library no longer depends on ``clangDriver``, which may
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break downstream projects that relied on this transitive dependency.
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- Clang is now more precise with regards to the lifetime of temporary objects
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such as when aggregates are passed by value to a function, resulting in
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better sharing of stack slots and reduced stack usage. This change can lead
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to use-after-scope related issues in code that unintentionally relied on the
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previous behavior. If recompiling with ``-fsanitize=address`` shows a
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use-after-scope warning, then this is likely the case, and the report printed
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should be able to help users pinpoint where the use-after-scope is occurring.
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Users can use ``-Xclang -sloppy-temporary-lifetimes`` to retain the old
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behavior until they are able to find and resolve issues in their code.
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C/C++ Language Potentially Breaking Changes
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-------------------------------------------

‎clang/include/clang/Basic/CodeGenOptions.def‎

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@@ -475,6 +475,10 @@ ENUM_CODEGENOPT(ZeroCallUsedRegs, ZeroCallUsedRegsKind,
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/// non-deleting destructors. (No effect on Microsoft ABI.)
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CODEGENOPT(CtorDtorReturnThis, 1, 0, Benign)
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/// Set via -Xclang -sloppy-temporary-lifetimes to disable emission of lifetime
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/// marker intrinsic calls.
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CODEGENOPT(NoLifetimeMarkersForTemporaries, 1, 0, Benign)
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/// Enables emitting Import Call sections on supported targets that can be used
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/// by the Windows kernel to enable import call optimization.
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CODEGENOPT(ImportCallOptimization, 1, 0, Benign)

‎clang/include/clang/Options/Options.td‎

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@@ -8151,6 +8151,11 @@ def import_call_optimization : Flag<["-"], "import-call-optimization">,
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def replaceable_function: Joined<["-"], "loader-replaceable-function=">,
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MarshallingInfoStringVector<CodeGenOpts<"LoaderReplaceableFunctionNames">>;
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def sloppy_temporary_lifetimes
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: Flag<["-"], "sloppy-temporary-lifetimes">,
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HelpText<"Don't emit lifetime markers for temporary objects">,
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MarshallingInfoFlag<CodeGenOpts<"NoLifetimeMarkersForTemporaries">>;
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} // let Visibility = [CC1Option]
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//===----------------------------------------------------------------------===//

‎clang/lib/CodeGen/CGCall.cpp‎

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@@ -43,6 +43,7 @@
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#include "llvm/IR/IntrinsicInst.h"
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#include "llvm/IR/Intrinsics.h"
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#include "llvm/IR/Type.h"
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#include "llvm/Support/TypeSize.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include <optional>
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using namespace clang;
@@ -4960,7 +4961,23 @@ void CodeGenFunction::EmitCallArg(CallArgList &args, const Expr *E,
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return;
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}
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args.add(EmitAnyExprToTemp(E), type);
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AggValueSlot ArgSlot = AggValueSlot::ignored();
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// If the callee returns a reference, skip this stack saving optimization;
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// we don't want to prematurely end the lifetime of the temporary. It may be
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// possible to still perform this optimization if the return type is a
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// reference to a different type than the parameter.
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if (hasAggregateEvaluationKind(E->getType())) {
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RawAddress ArgSlotAlloca = Address::invalid();
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ArgSlot = CreateAggTemp(E->getType(), "agg.tmp", &ArgSlotAlloca);
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// Emit a lifetime start/end for this temporary at the end of the full
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// expression.
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if (!CGM.getCodeGenOpts().NoLifetimeMarkersForTemporaries &&
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EmitLifetimeStart(ArgSlotAlloca.getPointer()))
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pushFullExprCleanup<CallLifetimeEnd>(NormalAndEHCleanup, ArgSlotAlloca);
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}
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args.add(EmitAnyExpr(E, ArgSlot), type);
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}
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QualType CodeGenFunction::getVarArgType(const Expr *Arg) {

‎clang/test/CodeGen/lifetime-call-temp.c‎

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// RUN: %clang -cc1 -triple x86_64-apple-macos -O1 -disable-llvm-passes %s \
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// RUN: -emit-llvm -o - | FileCheck %s --implicit-check-not=llvm.lifetime
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// RUN: %clang -cc1 -xc++ -std=c++17 -triple x86_64-apple-macos -O1 \
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// RUN: -disable-llvm-passes %s -emit-llvm -o - -Wno-return-type-c-linkage | \
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// RUN: FileCheck %s --implicit-check-not=llvm.lifetime --check-prefixes=CHECK,CXX
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// RUN: %clang -cc1 -xobjective-c -triple x86_64-apple-macos -O1 \
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// RUN: -disable-llvm-passes %s -emit-llvm -o - | \
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// RUN: FileCheck %s --implicit-check-not=llvm.lifetime --check-prefixes=CHECK,OBJC
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// RUN: %clang -cc1 -triple x86_64-apple-macos -O1 -disable-llvm-passes %s \
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// RUN: -emit-llvm -o - -sloppy-temporary-lifetimes | \
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// RUN: FileCheck %s --implicit-check-not=llvm.lifetime --check-prefixes=SLOPPY
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typedef struct { int x[100]; } aggregate;
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#ifdef __cplusplus
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extern "C" {
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#endif
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void takes_aggregate(aggregate);
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aggregate gives_aggregate();
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// CHECK-LABEL: define void @t1
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void t1() {
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takes_aggregate(gives_aggregate());
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// CHECK: [[AGGTMP:%.*]] = alloca %struct.aggregate, align 8
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// CHECK: call void @llvm.lifetime.start.p0(ptr [[AGGTMP]])
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// CHECK: call void{{.*}} @gives_aggregate(ptr{{.*}}sret(%struct.aggregate) align 4 [[AGGTMP]])
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// CHECK: call void @takes_aggregate(ptr noundef byval(%struct.aggregate) align 8 [[AGGTMP]])
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// CHECK: call void @llvm.lifetime.end.p0(ptr [[AGGTMP]])
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// SLOPPY: [[AGGTMP:%.*]] = alloca %struct.aggregate, align 8
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// SLOPPY-NEXT: call void (ptr, ...) @gives_aggregate(ptr{{.*}}sret(%struct.aggregate) align 4 [[AGGTMP]])
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// SLOPPY-NEXT: call void @takes_aggregate(ptr noundef byval(%struct.aggregate) align 8 [[AGGTMP]])
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}
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// CHECK: declare {{.*}}llvm.lifetime.start
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// CHECK: declare {{.*}}llvm.lifetime.end
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#ifdef __cplusplus
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// CXX: define void @t2
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void t2() {
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struct S {
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S(aggregate) {}
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};
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S{gives_aggregate()};
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// CXX: [[AGG:%.*]] = alloca %struct.aggregate
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// CXX: call void @llvm.lifetime.start.p0(ptr [[AGG]]
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// CXX: call void @gives_aggregate(ptr{{.*}}sret(%struct.aggregate) align 4 [[AGG]])
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// CXX: call void @_ZZ2t2EN1SC1E9aggregate(ptr {{.*}}, ptr {{.*}} byval(%struct.aggregate) align 8 [[AGG]])
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// CXX: call void @llvm.lifetime.end.p0(ptr [[AGG]]
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}
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struct Dtor {
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~Dtor();
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};
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void takes_dtor(Dtor);
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Dtor gives_dtor();
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// CXX: define void @t3
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void t3() {
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takes_dtor(gives_dtor());
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// CXX: [[AGG:%.*]] = alloca %struct.Dtor
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// CXX: call void @llvm.lifetime.start.p0(ptr [[AGG]])
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// CXX: call void @gives_dtor(ptr{{.*}}sret(%struct.Dtor) align 1 [[AGG]])
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// CXX: call void @takes_dtor(ptr noundef [[AGG]])
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// CXX: call void @_ZN4DtorD1Ev(ptr {{.*}} [[AGG]])
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// CXX: call void @llvm.lifetime.end.p0(ptr [[AGG]])
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// CXX: ret void
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}
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#endif
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#ifdef __OBJC__
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@interface X
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-m:(aggregate)x;
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@end
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// OBJC: define void @t4
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void t4(X *x) {
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[x m: gives_aggregate()];
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// OBJC: [[AGG:%.*]] = alloca %struct.aggregate
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// OBJC: call void @llvm.lifetime.start.p0(ptr [[AGG]]
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// OBJC: call void{{.*}} @gives_aggregate(ptr{{.*}}sret(%struct.aggregate) align 4 [[AGGTMP]])
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// OBJC: call {{.*}}@objc_msgSend
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// OBJC: call void @llvm.lifetime.end.p0(ptr [[AGG]]
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}
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#endif
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#ifdef __cplusplus
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}
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#endif

‎clang/test/CodeGen/stack-usage-lifetimes.c‎

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// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -O1 -emit-codegen-only -Rpass-analysis=prologepilog %s -verify=x86-precise
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// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -O1 -emit-codegen-only -Rpass-analysis=prologepilog -sloppy-temporary-lifetimes %s -verify=x86-sloppy
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// RUN: %clang_cc1 -triple aarch64-unknown-linux-gnu -O1 -emit-codegen-only -Rpass-analysis=prologepilog %s -verify=aarch64-precise
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// RUN: %clang_cc1 -triple aarch64-unknown-linux-gnu -O1 -emit-codegen-only -Rpass-analysis=prologepilog -sloppy-temporary-lifetimes %s -verify=aarch64-sloppy
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// RUN: %clang_cc1 -triple riscv64-unknown-linux-gnu -O1 -emit-codegen-only -Rpass-analysis=prologepilog %s -verify=riscv-precise
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// RUN: %clang_cc1 -triple riscv64-unknown-linux-gnu -O1 -emit-codegen-only -Rpass-analysis=prologepilog -sloppy-temporary-lifetimes %s -verify=riscv-sloppy
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// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -O1 -emit-codegen-only -Rpass-analysis=prologepilog %s -verify=x86-precise -xc++
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// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -O1 -emit-codegen-only -Rpass-analysis=prologepilog -sloppy-temporary-lifetimes %s -verify=x86-sloppy -xc++
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// RUN: %clang_cc1 -triple aarch64-unknown-linux-gnu -O1 -emit-codegen-only -Rpass-analysis=prologepilog %s -verify=aarch64-precise -xc++
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// RUN: %clang_cc1 -triple aarch64-unknown-linux-gnu -O1 -emit-codegen-only -Rpass-analysis=prologepilog -sloppy-temporary-lifetimes %s -verify=aarch64-sloppy -xc++
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// RUN: %clang_cc1 -triple riscv64-unknown-linux-gnu -O1 -emit-codegen-only -Rpass-analysis=prologepilog %s -verify=riscv-precise -xc++
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// RUN: %clang_cc1 -triple riscv64-unknown-linux-gnu -O1 -emit-codegen-only -Rpass-analysis=prologepilog -sloppy-temporary-lifetimes %s -verify=riscv-sloppy -xc++
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typedef struct { char x[32]; } A;
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typedef struct { char *w, *x, *y, *z; } B;
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void useA(A);
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void useB(B);
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A genA(void);
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B genB(void);
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void t1(int c) {
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// x86-precise-remark@-1 {{40 stack bytes}}
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// x86-sloppy-remark@-2 {{72 stack bytes}}
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// aarch64-precise-remark@-3 {{48 stack bytes}}
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// aarch64-sloppy-remark@-4 {{80 stack bytes}}
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// riscv-precise-remark@-5 {{48 stack bytes}}
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// riscv-sloppy-remark@-6 {{80 stack bytes}}
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if (c)
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useA(genA());
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else
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useA(genA());
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}
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void t2(void) {
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// x86-precise-remark@-1 {{72 stack bytes}}
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// x86-sloppy-remark@-2 {{72 stack bytes}}
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// aarch64-precise-remark@-3 {{80 stack bytes}}
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// aarch64-sloppy-remark@-4 {{80 stack bytes}}
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// riscv-precise-remark@-5 {{80 stack bytes}}
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// riscv-sloppy-remark@-6 {{80 stack bytes}}
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useA(genA());
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useA(genA());
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}
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void t3(void) {
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// x86-precise-remark@-1 {{72 stack bytes}}
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// x86-sloppy-remark@-2 {{72 stack bytes}}
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// aarch64-precise-remark@-3 {{80 stack bytes}}
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// aarch64-sloppy-remark@-4 {{80 stack bytes}}
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// riscv-precise-remark@-5 {{80 stack bytes}}
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// riscv-sloppy-remark@-6 {{80 stack bytes}}
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useB(genB());
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useB(genB());
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}
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#ifdef __cplusplus
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struct C {
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char x[24];
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char *ptr;
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~C() {};
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};
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void useC(C);
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C genC(void);
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// This case works in C++, since its AST is structured slightly differently
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// than it is in C (CompundStmt/ExprWithCleanup/CallExpr vs CompundStmt/CallExpr).
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void t4() {
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// x86-precise-remark@-1 {{40 stack bytes}}
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// x86-sloppy-remark@-2 {{72 stack bytes}}
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// aarch64-precise-remark@-3 {{48 stack bytes}}
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// aarch64-sloppy-remark@-4 {{80 stack bytes}}
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// riscv-precise-remark@-5 {{48 stack bytes}}
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// riscv-sloppy-remark@-6 {{80 stack bytes}}
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useC(genC());
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useC(genC());
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}
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#endif

‎clang/test/CodeGenCXX/amdgcn-call-with-aggarg.cc‎

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// RUN: %clang_cc1 -triple amdgcn-amd-amdhsa -emit-llvm -O3 -disable-llvm-passes -o - %s | FileCheck %s
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struct A {
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float x, y, z, w;
5+
};
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void foo(A a);
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// CHECK-LABEL: @_Z4testv
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// CHECK: [[A:%.*]] = alloca [[STRUCT_A:%.*]], align 4, addrspace(5)
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// CHECK-NEXT: [[AGG_TMP:%.*]] = alloca [[STRUCT_A]], align 4, addrspace(5)
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// CHECK-NEXT: [[A_ASCAST:%.*]] = addrspacecast ptr addrspace(5) [[A]] to ptr
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// CHECK-NEXT: [[AGG_TMP_ASCAST:%.*]] = addrspacecast ptr addrspace(5) [[AGG_TMP]] to ptr
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// CHECK-NEXT: call void @llvm.lifetime.start.p5(i64 16, ptr addrspace(5) [[A]]) #[[ATTR4:[0-9]+]]
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// CHECK-NEXT: call void @llvm.lifetime.start.p5(i64 16, ptr addrspace(5) [[AGG_TMP]]) #[[ATTR4]]
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void test() {
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A a;
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foo(a);
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}

‎clang/test/CodeGenCXX/stack-reuse-miscompile.cpp‎

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@@ -26,6 +26,8 @@ const char * f(S s)
2626
// CHECK: [[T2:%.*]] = alloca %class.T, align 4
2727
// CHECK: [[T3:%.*]] = alloca %class.T, align 4
2828
//
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// CHECK: [[AGG:%.*]] = alloca %class.S, align 4
30+
//
2931
// FIXME: We could defer starting the lifetime of the return object of concat
3032
// until the call.
3133
// CHECK: call void @llvm.lifetime.start.p0(ptr [[T1]])
@@ -34,10 +36,12 @@ const char * f(S s)
3436
// CHECK: [[T4:%.*]] = call noundef ptr @_ZN1TC1EPKc(ptr {{[^,]*}} [[T2]], ptr noundef @.str)
3537
//
3638
// CHECK: call void @llvm.lifetime.start.p0(ptr [[T3]])
39+
// CHECK: call void @llvm.lifetime.start.p0(ptr [[AGG]])
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// CHECK: [[T5:%.*]] = call noundef ptr @_ZN1TC1E1S(ptr {{[^,]*}} [[T3]], [2 x i32] %{{.*}})
3841
//
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// CHECK: call void @_ZNK1T6concatERKS_(ptr dead_on_unwind writable sret(%class.T) align 4 [[T1]], ptr {{[^,]*}} [[T2]], ptr noundef nonnull align 4 dereferenceable(16) [[T3]])
4043
// CHECK: [[T6:%.*]] = call noundef ptr @_ZNK1T3strEv(ptr {{[^,]*}} [[T1]])
44+
// CHECK: call void @llvm.lifetime.end.p0(ptr [[AGG]])
4145
//
4246
// CHECK: call void @llvm.lifetime.end.p0(
4347
// CHECK: call void @llvm.lifetime.end.p0(

‎clang/test/CodeGenCoroutines/pr59181.cpp‎

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@@ -49,6 +49,7 @@ void foo() {
4949
}
5050

5151
// CHECK: cleanup.cont:{{.*}}
52+
// CHECK-NEXT: call void @llvm.lifetime.start.p0(ptr [[AGG:%agg.tmp]])
5253
// CHECK-NEXT: load i8
5354
// CHECK-NEXT: trunc
5455
// CHECK-NEXT: store i1 false
@@ -57,3 +58,6 @@ void foo() {
5758
// CHECK-NOT: call void @llvm.lifetime
5859
// CHECK: call void @llvm.coro.await.suspend.void(
5960
// CHECK-NEXT: %{{[0-9]+}} = call i8 @llvm.coro.suspend(
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62+
// CHECK-LABEL: cond.end:
63+
// check call @llvm.lifetime.end.p0(ptr [[AGG]])

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