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[Clang] Allow vanilla C function symbol name to be used in __attribute__((alias)) when -funique-internal-linkage-names is specified #145652

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[Clang] Allow vanilla C function symbol name to be used in __attribute__((alias)) when -funique-internal-linkage-names is specified #145652

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9a29dd6
[Clang] Allow vanilla C function symbol name to be used in __attribut…
HighW4y2H3ll Jun 25, 2025
aedff8d
Check Itanium mangling only
HighW4y2H3ll Jun 25, 2025
80e7bda
Fix frontend crash with extern_weak when using __attribute__((weak, a…
HighW4y2H3ll Jul 3, 2025
1e116a9
Fix missed aliasee function emit when function alias is used after th…
HighW4y2H3ll Jul 6, 2025
3beeb6a
Make the original function GlobalAlias to avoid broken references in …
HighW4y2H3ll Jul 7, 2025
0003a3f
Create GlobalAlias for AlwaysInline internal functions
HighW4y2H3ll Jul 8, 2025
cab9bf6
Check static __always_inline/inline functions to avoid inline assembl…
HighW4y2H3ll Jul 9, 2025
18e9543
Fixup deferred inline function check and inline assembly inlining
HighW4y2H3ll Jul 10, 2025
a81a5c7
Refactor && Fix a bug when traversing the callees
HighW4y2H3ll Jul 11, 2025
cc5b165
Cleanups
HighW4y2H3ll Jul 14, 2025
7b30a80
fix unittest
HighW4y2H3ll Jul 16, 2025
75ae162
format
HighW4y2H3ll Jul 16, 2025
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48 changes: 44 additions & 4 deletions clang/lib/CodeGen/CodeGenModule.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -588,8 +588,9 @@ static const llvm::GlobalValue *getAliasedGlobal(const llvm::GlobalValue *GV) {
}

static bool checkAliasedGlobal(
const ASTContext &Context, DiagnosticsEngine &Diags, SourceLocation Location,
bool IsIFunc, const llvm::GlobalValue *Alias, const llvm::GlobalValue *&GV,
const CodeGenModule *CGM, const ASTContext &Context,
DiagnosticsEngine &Diags, SourceLocation Location, bool IsIFunc,
const llvm::GlobalValue *Alias, const llvm::GlobalValue *&GV,
const llvm::MapVector<GlobalDecl, StringRef> &MangledDeclNames,
SourceRange AliasRange) {
GV = getAliasedGlobal(Alias);
Expand All @@ -598,6 +599,23 @@ static bool checkAliasedGlobal(
return false;
}

// Only resolve unique internal linkage symbols for C code
if (!CGM->getLangOpts().CPlusPlus) {
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@efriedma-quic efriedma-quic Jun 25, 2025

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Is this C-only? Consider something like:

long bar(const struct A*p) __attribute__((__alias__("foo")));
namespace {
extern "C" long foo(const struct A*p) {return 1;}
}

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@HighW4y2H3ll HighW4y2H3ll Jun 26, 2025

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Yup, we are trying to build a codebase written entirely in C... I'm not too sure how namespace and extern "C" would translate in C tho...

for (const auto &[Decl, Name] : MangledDeclNames) {
if (const auto *ND = dyn_cast<NamedDecl>(Decl.getDecl())) {
IdentifierInfo *II = ND->getIdentifier();
if (II && II->getName() == GV->getName() &&
Name.contains(llvm::FunctionSamples::UniqSuffix)) {
GlobalDecl GD;
if (CGM->lookupRepresentativeDecl(Name, GD)) {
GV = CGM->getModule().getNamedValue(Name);
break;
}
}
}
}
}

if (GV->hasCommonLinkage()) {
const llvm::Triple &Triple = Context.getTargetInfo().getTriple();
if (Triple.getObjectFormat() == llvm::Triple::XCOFF) {
Expand Down Expand Up @@ -687,8 +705,8 @@ void CodeGenModule::checkAliases() {
StringRef MangledName = getMangledName(GD);
llvm::GlobalValue *Alias = GetGlobalValue(MangledName);
const llvm::GlobalValue *GV = nullptr;
if (!checkAliasedGlobal(getContext(), Diags, Location, IsIFunc, Alias, GV,
MangledDeclNames, Range)) {
if (!checkAliasedGlobal(this, getContext(), Diags, Location, IsIFunc, Alias,
GV, MangledDeclNames, Range)) {
Error = true;
continue;
}
Expand Down Expand Up @@ -4038,6 +4056,7 @@ void CodeGenModule::EmitGlobal(GlobalDecl GD) {
CXXGlobalInits.push_back(nullptr);
}

const auto *ND = dyn_cast<NamedDecl>(GD.getDecl());
StringRef MangledName = getMangledName(GD);
if (GetGlobalValue(MangledName) != nullptr) {
// The value has already been used and should therefore be emitted.
Expand All @@ -4046,6 +4065,12 @@ void CodeGenModule::EmitGlobal(GlobalDecl GD) {
// The value must be emitted, but cannot be emitted eagerly.
assert(!MayBeEmittedEagerly(Global));
addDeferredDeclToEmit(GD);
} else if (!getLangOpts().CPlusPlus && ND &&
GetGlobalValue(ND->getName()) != nullptr &&
MangledName.contains(llvm::FunctionSamples::UniqSuffix)) {
// Emit static C function that is mangled with
// -funique-internal-linkage-names.
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@efriedma-quic efriedma-quic Jun 25, 2025

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I'm not really happy with this. The code for deferred declarations and diagnosing name conflicts is complicated enough without adding the possibility that the name of a declaration can change later.

Can we just suppress mangling in cases where we reference a symbol without knowing if it's internal, then it turns out to be internal later?

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@HighW4y2H3ll HighW4y2H3ll Jun 26, 2025

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The problem is we need -funique-internal-linkage-names to keep all the static function symbols unique... It is to make sure that we have unique symbols names in the DWARF info after linking. Besides.... it's also a bit weird to specify something like __attribute__((__alias__(_ZL3fooPK1A.__uniq.43626431813877880438002914689114607187))) in the C Code.. It's only a problem with attribute alias tho... Do you think we should add more unit tests for this change?

addDeferredDeclToEmit(GD);
} else {
// Otherwise, remember that we saw a deferred decl with this name. The
// first use of the mangled name will cause it to move into
Expand Down Expand Up @@ -6189,6 +6214,21 @@ void CodeGenModule::EmitGlobalFunctionDefinition(GlobalDecl GD,
/*DontDefer=*/true,
ForDefinition));

if (!getLangOpts().CPlusPlus &&
getCXXABI().getMangleContext().shouldMangleDeclName(D)) {
// -funique-internal-linkage-names may change the symbol name of C function.
// Replace all uses of old symbol with the emitted global value.
if (IdentifierInfo *II = D->getIdentifier()) {
if (II->getName() != GV->getName() &&
GV->getName().contains(llvm::FunctionSamples::UniqSuffix)) {
if (llvm::GlobalValue *GVDef =
getModule().getNamedValue(D->getName())) {
GVDef->replaceAllUsesWith(GV);
}
}
}
}

// Already emitted.
if (!GV->isDeclaration())
return;
Expand Down
10 changes: 10 additions & 0 deletions clang/test/CodeGen/unique-internal-linkage-names-alias.c
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Original file line number Diff line number Diff line change
@@ -0,0 +1,10 @@
// RUN: %clang_cc1 %s -emit-llvm -funique-internal-linkage-names -o - | FileCheck %s

struct A;
static long foo(const struct A*p);

long bar(const struct A*p);
long bar(const struct A*p) __attribute__((__alias__("foo")));

// CHECK: define internal i64 @_ZL3fooPK1A.__uniq.[[ATTR:[0-9]+]](ptr noundef %p) #1 {
static long foo(const struct A*p) {return 1;}
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