llvmorg-github-actions[bot] wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang
Author: Nemi26
<details>
<summary>Changes</summary>
Under the Microsoft ABI, a lambda in a default argument of a non-member
function template can get the same mangled name as a lambda in the function's
body. When that happens, calls to one call operator go to the other. It's a
silent miscompile:
```cpp
template <typename F> struct Lazy { F f; int get() { int v = 0; f(v);
return v; } };
template <typename B> int shoot(B &&b) { Lazy<B &>
l{b}; return l.get(); }
template <int N, typename E = void (*)(int &)>
int track(E &&e = [](int &) {}) {
return shoot([&](int &v) { v = N; e(v); });
}
int main() { return track<1>() * 10 + track<2>(); } // 12; 10 for
x86_64-pc-windows-msvc
```
The default argument's lambda has no mangling number and no context
declaration, because `getCurrentMangleNumberContext` leaves default arguments
of non-member function templates unnumbered. Its instantiation still lives in
the function. The Microsoft mangler then names it `<lambda_N>`, taking N
from the translation unit's count of unnumbered lambdas, which starts at 0. The
function's own lambdas are numbered from 1 in the same scope. In
`track<2>`, both closure types become
`<lambda_1>@?0???$track@$01...`. Itanium is unaffected, because it spells
unnumbered lambdas `$_N`.
This patch finds the parameter whose default argument spans such a lambda and
mangles the lambda in the default-argument form `<lambda_K_N>`, which
lambdas numbered in a default argument (class members) already use. The fix
stays in the mangler. Numbering these lambdas in Sema would also change
Itanium's mangling.
Tested with the new `mangle-ms-lambda-template-default-arg.cpp`, which fails
without the patch. `clang/test/CodeGenCXX` and `clang/test/DebugInfo/CXX` pass,
except 8 tests that need tools my build doesn't have (`llvm-dis`, `llvm-link`,
`llvm-profdata`).
This patch was written with the assistance of an AI tool (Claude Code, Claude
Opus); I reviewed it.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
---
Full diff: https://github.com/llvm/llvm-project/pull/232294.diff
2 Files Affected:
- (modified) clang/lib/AST/MicrosoftMangle.cpp (+24-4)
- (added) clang/test/CodeGenCXX/mangle-ms-lambda-template-default-arg.cpp (+16)
``````````diff
diff --git a/clang/lib/AST/MicrosoftMangle.cpp
b/clang/lib/AST/MicrosoftMangle.cpp
index 74edee531e48a..60f3d804c2bc3 100644
--- a/clang/lib/AST/MicrosoftMangle.cpp
+++ b/clang/lib/AST/MicrosoftMangle.cpp
@@ -86,6 +86,28 @@ struct msvc_hashing_ostream : public
llvm::raw_svector_ostream {
}
};
+/// The parameter whose default argument holds the lambda, if any. An
unnumbered
+/// lambda has no context declaration, so it is found by its location.
+static const ParmVarDecl *getLambdaDefaultArgParm(const CXXRecordDecl *Lambda)
{
+ if (const auto *Parm =
+ dyn_cast_or_null<ParmVarDecl>(Lambda->getLambdaContextDecl()))
+ return Parm;
+ if (Lambda->getLambdaManglingNumber())
+ return nullptr;
+
+ const auto *Func = dyn_cast<FunctionDecl>(Lambda->getDeclContext());
+ if (!Func)
+ return nullptr;
+ const SourceManager &SM = Lambda->getASTContext().getSourceManager();
+ for (const ParmVarDecl *Parm : Func->parameters()) {
+ SourceRange Range = Parm->getDefaultArgRange();
+ if (Range.isValid() && SM.isPointWithin(Lambda->getLocation(),
+ Range.getBegin(), Range.getEnd()))
+ return Parm;
+ }
+ return nullptr;
+}
+
static const DeclContext *
getLambdaDefaultArgumentDeclContext(const Decl *D) {
if (const auto *RD = dyn_cast<CXXRecordDecl>(D))
@@ -250,10 +272,9 @@ class MicrosoftMangleContextImpl : public
MicrosoftMangleContext {
assert(Lambda->isLambda() && "RD must be a lambda!");
std::string Name("<lambda_");
- Decl *LambdaContextDecl = Lambda->getLambdaContextDecl();
unsigned LambdaManglingNumber = Lambda->getLambdaManglingNumber();
unsigned LambdaId;
- const ParmVarDecl *Parm = dyn_cast_or_null<ParmVarDecl>(LambdaContextDecl);
+ const ParmVarDecl *Parm = getLambdaDefaultArgParm(Lambda);
const FunctionDecl *Func =
Parm ? dyn_cast<FunctionDecl>(Parm->getDeclContext()) : nullptr;
@@ -1245,8 +1266,7 @@ void
MicrosoftCXXNameMangler::mangleUnqualifiedName(GlobalDecl GD,
Decl *LambdaContextDecl = Record->getLambdaContextDecl();
unsigned LambdaManglingNumber = Record->getLambdaManglingNumber();
unsigned LambdaId;
- const ParmVarDecl *Parm =
- dyn_cast_or_null<ParmVarDecl>(LambdaContextDecl);
+ const ParmVarDecl *Parm = getLambdaDefaultArgParm(Record);
const FunctionDecl *Func =
Parm ? dyn_cast<FunctionDecl>(Parm->getDeclContext()) : nullptr;
diff --git a/clang/test/CodeGenCXX/mangle-ms-lambda-template-default-arg.cpp
b/clang/test/CodeGenCXX/mangle-ms-lambda-template-default-arg.cpp
new file mode 100644
index 0000000000000..16759ccdca3c0
--- /dev/null
+++ b/clang/test/CodeGenCXX/mangle-ms-lambda-template-default-arg.cpp
@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -std=c++20 -emit-llvm %s -o -
-triple=x86_64-pc-windows-msvc | FileCheck %s
+
+// A default argument's lambda is named apart from the function's own lambdas.
+
+template <typename F> int call(F f) { return f(); }
+
+template <int N> int g(int (*f)() = [] { return 10; }) {
+ return f() + call([] { return N; });
+}
+
+int use() { return g<1>() + g<2>(); }
+
+// CHECK-DAG: define {{.*}}
@"??R<lambda_1>@?0???$g@$00@@YAHP6AHXZ@Z@QEBA?A?<auto>@@XZ"(
+// CHECK-DAG: define {{.*}}
@"??R<lambda_1>@?0???$g@$01@@YAHP6AHXZ@Z@QEBA?A?<auto>@@XZ"(
+// CHECK-DAG: define internal {{.*}}
@"??R<lambda_1_{{[0-9]+}}>@?0???$g@$00@@YAHP6AHXZ@Z@QEBA?A?<auto>@@XZ"(
+// CHECK-DAG: define internal {{.*}}
@"??R<lambda_1_{{[0-9]+}}>@?0???$g@$01@@YAHP6AHXZ@Z@QEBA?A?<auto>@@XZ"(
``````````
</details>
https://github.com/llvm/llvm-project/pull/232294
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