[llvm-branch-commits] [llvm] [CFI] Create an external linkage alias instead of promoting internals (PR #203171)
Mircea Trofin via llvm-branch-commits
llvm-branch-commits at lists.llvm.org
Mon Jun 22 18:24:03 PDT 2026
https://github.com/mtrofin updated https://github.com/llvm/llvm-project/pull/203171
>From c1b8c2a384b8dd06e5d7ad983b060b85518127a4 Mon Sep 17 00:00:00 2001
From: Mircea Trofin <mtrofin at google.com>
Date: Wed, 17 Jun 2026 11:06:09 -0700
Subject: [PATCH] [CFI] Create an external linkage alias instead of promoting
internals
---
llvm/lib/Transforms/IPO/LowerTypeTests.cpp | 16 ++++++
.../Transforms/IPO/ThinLTOBitcodeWriter.cpp | 53 +++++++------------
.../lib/Transforms/IPO/WholeProgramDevirt.cpp | 25 +++++++--
.../ThinLTO/X86/devirt_function_alias2.ll | 10 ++--
.../cfi-icall-static-inline-asm.ll | 4 +-
.../Transforms/ThinLTOBitcodeWriter/comdat.ll | 17 +++---
.../ThinLTOBitcodeWriter/split-internal1.ll | 3 +-
.../ThinLTOBitcodeWriter/split-internal2.ll | 5 +-
.../split-vfunc-internal.ll | 6 ++-
9 files changed, 83 insertions(+), 56 deletions(-)
diff --git a/llvm/lib/Transforms/IPO/LowerTypeTests.cpp b/llvm/lib/Transforms/IPO/LowerTypeTests.cpp
index 7c045c90ead5b..d399b6f9d1090 100644
--- a/llvm/lib/Transforms/IPO/LowerTypeTests.cpp
+++ b/llvm/lib/Transforms/IPO/LowerTypeTests.cpp
@@ -2138,6 +2138,22 @@ bool LowerTypeTestsModule::lower() {
report_fatal_error(
"unexpected call to llvm.icall.branch.funnel during import phase");
+ for (auto &A : llvm::make_early_inc_range(M.aliases())) {
+ if (A.hasLocalLinkage())
+ continue;
+ if (ImportSummary->cfiFunctionDefs().contains(A.getName()) ||
+ ImportSummary->cfiFunctionDecls().contains(A.getName())) {
+ if (auto *F = dyn_cast<Function>(A.getAliasee()->stripPointerCasts())) {
+ A.replaceAllUsesWith(F);
+ std::string Name = std::string(A.getName());
+ A.eraseFromParent();
+ F->setName(Name);
+ F->setLinkage(GlobalValue::ExternalLinkage);
+ F->setVisibility(GlobalValue::HiddenVisibility);
+ }
+ }
+ }
+
SmallVector<Function *, 8> Defs;
SmallVector<Function *, 8> Decls;
for (auto &F : M) {
diff --git a/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp b/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
index 40200a6554249..1f4dff0aae472 100644
--- a/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
+++ b/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
@@ -30,23 +30,10 @@ using namespace llvm;
namespace {
-// Determine if a promotion alias should be created for a symbol name.
-static bool allowPromotionAlias(const std::string &Name) {
- // Promotion aliases are used only in inline assembly. It's safe to
- // simply skip unusual names. Subset of MCAsmInfo::isAcceptableChar()
- // and MCAsmInfoXCOFF::isAcceptableChar().
- for (const char &C : Name) {
- if (isAlnum(C) || C == '_' || C == '.')
- continue;
- return false;
- }
- return true;
-}
-
// Promote each local-linkage entity defined by ExportM and used by ImportM by
// changing visibility and appending the given ModuleId.
void promoteInternals(Module &ExportM, Module &ImportM, StringRef ModuleId,
- const SetVector<GlobalValue *> &PromoteExtra) {
+ SetVector<GlobalValue *> *PromoteExtra = nullptr) {
DenseMap<const Comdat *, Comdat *> RenamedComdats;
for (auto &ExportGV : ExportM.global_values()) {
if (!ExportGV.hasLocalLinkage())
@@ -54,7 +41,8 @@ void promoteInternals(Module &ExportM, Module &ImportM, StringRef ModuleId,
auto Name = ExportGV.getName();
GlobalValue *ImportGV = nullptr;
- if (!PromoteExtra.count(&ExportGV)) {
+ const bool MustPromote = PromoteExtra && PromoteExtra->count(&ExportGV);
+ if (!MustPromote) {
ImportGV = ImportM.getNamedValue(Name);
if (!ImportGV)
continue;
@@ -72,24 +60,24 @@ void promoteInternals(Module &ExportM, Module &ImportM, StringRef ModuleId,
if (C->getName() == Name)
RenamedComdats.try_emplace(C, ExportM.getOrInsertComdat(NewName));
- ExportGV.setName(NewName);
- ExportGV.setLinkage(GlobalValue::ExternalLinkage);
- ExportGV.setVisibility(GlobalValue::HiddenVisibility);
- // TODO: remove this reassign and instead create an alias.
- ExportGV.reassignGUID();
+ auto *ExternalAlias = GlobalAlias::create(
+ ExportGV.getType(), ExportGV.getAddressSpace(),
+ GlobalValue::ExternalLinkage, NewName, &ExportGV, &ExportM);
+ ExternalAlias->setVisibility(GlobalValue::HiddenVisibility);
+ ExportGV.replaceUsesWithIf(ExternalAlias, [ExternalAlias](Use &U) {
+ return U.getUser() != ExternalAlias;
+ });
+
+ if (MustPromote) {
+ PromoteExtra->remove(&ExportGV);
+ PromoteExtra->insert(ExternalAlias);
+ }
+
if (ImportGV) {
ImportGV->setName(NewName);
ImportGV->setVisibility(GlobalValue::HiddenVisibility);
ImportGV->reassignGUID();
}
-
- if (isa<Function>(&ExportGV) && allowPromotionAlias(OldName)) {
- // Create a local alias with the original name to avoid breaking
- // references from inline assembly.
- std::string Alias =
- ".lto_set_conditional " + OldName + "," + NewName + "\n";
- ExportM.appendModuleInlineAsm(Alias);
- }
}
if (!RenamedComdats.empty())
@@ -191,9 +179,8 @@ void simplifyExternals(Module &M) {
F.getName().starts_with("llvm."))
continue;
- Function *NewF =
- Function::Create(EmptyFT, GlobalValue::ExternalLinkage,
- F.getAddressSpace(), "", &M);
+ Function *NewF = Function::Create(EmptyFT, GlobalValue::ExternalLinkage,
+ F.getAddressSpace(), "", &M);
NewF->copyAttributesFrom(&F);
// Only copy function attribtues.
NewF->setAttributes(AttributeList::get(M.getContext(),
@@ -419,8 +406,8 @@ void splitAndWriteThinLTOBitcode(
// match values from its first argument (the "exporting module") in
// CfiFunctions. So we only need CfiFunctions for the second promotion (M ->
// MergedM)
- promoteInternals(*MergedM, M, ModuleId, {});
- promoteInternals(M, *MergedM, ModuleId, CfiFunctions);
+ promoteInternals(*MergedM, M, ModuleId, nullptr);
+ promoteInternals(M, *MergedM, ModuleId, &CfiFunctions);
auto &Ctx = MergedM->getContext();
SmallVector<MDNode *, 8> CfiFunctionMDs;
diff --git a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
index f876d4f02bb15..2c199896e2fe3 100644
--- a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
+++ b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
@@ -1147,6 +1147,8 @@ bool DevirtModule::tryFindVirtualCallTargets(
// target.
auto *GV = dyn_cast<GlobalValue>(C);
assert(GV);
+ if (auto *GA = dyn_cast<GlobalAlias>(GV))
+ GV = GA->getAliaseeObject();
TargetsForSlot.push_back({GV, &TM});
}
@@ -1203,7 +1205,17 @@ bool DevirtIndex::tryFindVirtualCallTargets(
if (mustBeUnreachableFunction(VTP.FuncVI))
continue;
- TargetsForSlot.push_back(VTP.FuncVI);
+ ValueInfo TheFn = VTP.FuncVI;
+ // Internal linkage functions get an external linkage alias to them.
+ if (!TheFn.getSummaryList().empty())
+ if (auto *AS =
+ dyn_cast<AliasSummary>(TheFn.getSummaryList()[0].get())) {
+ assert(TheFn.getSummaryList().size() == 1 &&
+ "Aliases to internal linkage targets of indirect calls are "
+ "expected to point to exactly one definition.");
+ TheFn = AS->getAliaseeVI();
+ }
+ TargetsForSlot.push_back(TheFn);
}
}
@@ -2261,10 +2273,13 @@ void DevirtModule::importResolution(VTableSlot Slot, VTableSlotInfo &SlotInfo) {
assert(!Res.SingleImplName.empty());
// The type of the function in the declaration is irrelevant because every
// call site will cast it to the correct type.
- Constant *SingleImpl =
- cast<Constant>(M.getOrInsertFunction(Res.SingleImplName,
- Type::getVoidTy(M.getContext()))
- .getCallee());
+ Value *SingleImplVal =
+ M.getOrInsertFunction(Res.SingleImplName,
+ Type::getVoidTy(M.getContext()))
+ .getCallee();
+ if (auto *A = dyn_cast<GlobalAlias>(SingleImplVal->stripPointerCasts()))
+ SingleImplVal = A->getAliaseeObject();
+ Constant *SingleImpl = cast<Constant>(SingleImplVal);
// This is the import phase so we should not be exporting anything.
bool IsExported = false;
diff --git a/llvm/test/ThinLTO/X86/devirt_function_alias2.ll b/llvm/test/ThinLTO/X86/devirt_function_alias2.ll
index 3b3208e300aa8..e4f93208fba54 100644
--- a/llvm/test/ThinLTO/X86/devirt_function_alias2.ll
+++ b/llvm/test/ThinLTO/X86/devirt_function_alias2.ll
@@ -24,8 +24,8 @@
; RUN: 2>&1 | FileCheck %s --check-prefix=REMARK --check-prefix=PRINT
; RUN: llvm-dis %t2.1.4.opt.bc -o - | FileCheck %s --check-prefix=CHECK-IR1
-; PRINT-DAG: Devirtualized call to {{.*}} (_ZN1D1mEiAlias)
-; REMARK-DAG: single-impl: devirtualized a call to _ZN1D1mEiAlias
+; PRINT-DAG: Devirtualized call to {{.*}} (_ZN1D1mEi)
+; REMARK-DAG: single-impl: devirtualized a call to _ZN1D1mEi
;; Test hybrid Thin/Regular LTO
@@ -63,13 +63,15 @@ target triple = "x86_64-grtev4-linux-gnu"
@_ZTV1D = constant { [3 x ptr] } { [3 x ptr] [ptr null, ptr undef, ptr @_ZN1D1mEiAlias] }, !type !3
-define i32 @_ZN1D1mEi(ptr %this, i32 %a) {
+define i32 @_ZN1D1mEi(ptr %this, i32 %a) #0 {
ret i32 0;
}
@_ZN1D1mEiAlias = unnamed_addr alias i32 (ptr, i32), ptr @_ZN1D1mEi
-; CHECK-IR1-LABEL: define i32 @test
+attributes #0 = { noinline optnone }
+
+; CHECK-IR1-LABEL: define {{.*}}i32 @test
define i32 @test(ptr %obj2, i32 %a) {
entry:
%vtable2 = load ptr, ptr %obj2
diff --git a/llvm/test/Transforms/ThinLTOBitcodeWriter/cfi-icall-static-inline-asm.ll b/llvm/test/Transforms/ThinLTOBitcodeWriter/cfi-icall-static-inline-asm.ll
index d8ebae17d4693..9d2ee425031ab 100644
--- a/llvm/test/Transforms/ThinLTOBitcodeWriter/cfi-icall-static-inline-asm.ll
+++ b/llvm/test/Transforms/ThinLTOBitcodeWriter/cfi-icall-static-inline-asm.ll
@@ -3,7 +3,7 @@
target triple = "x86_64-unknown-linux-gnu"
-; CHECK: module asm ".lto_set_conditional a,a.[[HASH:[0-9a-f]+]]"
+; CHECK: @a.[[HASH:[0-9a-f]+]] = hidden alias
define void @b() {
%f = alloca ptr, align 8
@@ -14,7 +14,7 @@ define void @b() {
ret void
}
-; CHECK: define{{.*}} @a.[[HASH]](){{.*}} !type
+; CHECK: define internal void @a() {{.*}}!type
define internal void @a() !type !0 {
ret void
}
diff --git a/llvm/test/Transforms/ThinLTOBitcodeWriter/comdat.ll b/llvm/test/Transforms/ThinLTOBitcodeWriter/comdat.ll
index f9545e2010c10..48d371531ed7b 100644
--- a/llvm/test/Transforms/ThinLTOBitcodeWriter/comdat.ll
+++ b/llvm/test/Transforms/ThinLTOBitcodeWriter/comdat.ll
@@ -25,9 +25,8 @@ $nt = comdat any
; MERGED-SAME: comdat(${{"?lwt[^ ]+}})
@lwt_aliasee = private unnamed_addr global [1 x ptr] [ptr null], comdat($lwt), !type !0
-; MERGED: {{@"?lwt_nl[^ ]+}} = hidden unnamed_addr global
-; MERGED-SAME: comdat(${{"?lwt[^ ]+}})
-; THIN: {{@"?lwt_nl[^ ]+}} = external hidden
+; MERGED: @lwt_nl = internal unnamed_addr global i32 0, comdat(${{"?lwt[^ ]+}})
+; THIN: {{@"?lwt_nl\.[^ ]+}} = external hidden unnamed_addr global i32
@lwt_nl = internal unnamed_addr global i32 0, comdat($lwt)
; MERGED: @nlwt_aliasee = private unnamed_addr global
@@ -47,12 +46,16 @@ $nt = comdat any
; THIN-SAME: comdat($nt)
@nt_nl = internal unnamed_addr global i32 0, comdat($nt)
-; MERGED: {{@"?lwt[^ ]+}} = hidden unnamed_addr alias
-; THIN: {{@"?lwt[^ ]+}} = external hidden
+; MERGED: @lwt = internal unnamed_addr alias [1 x ptr], ptr @lwt_aliasee
+; MERGED: @nlwt_nl = internal unnamed_addr alias [1 x ptr], ptr @nlwt_aliasee
+; MERGED: {{@"?lwt_nl\.[^ ]+}} = hidden alias ptr, ptr @lwt_nl
+; MERGED: {{@"?lwt\.[^ ]+}} = hidden alias ptr, ptr @lwt
+; MERGED: {{@"?nlwt_nl\.[^ ]+}} = hidden alias ptr, ptr @nlwt_nl
+
+; THIN: {{@"?lwt\.[^ ]+}} = external hidden global [1 x ptr]
@lwt = internal unnamed_addr alias [1 x ptr], ptr @lwt_aliasee
-; MERGED: {{@"?nlwt_nl[^ ]+}} = hidden unnamed_addr alias
-; THIN: {{@"?nlwt_nl[^ ]+}} = external hidden
+; THIN: {{@"?nlwt_nl\.[^ ]+}} = external hidden global [1 x ptr]
@nlwt_nl = internal unnamed_addr alias [1 x ptr], ptr @nlwt_aliasee
; The functions below exist just to make sure the globals are used.
diff --git a/llvm/test/Transforms/ThinLTOBitcodeWriter/split-internal1.ll b/llvm/test/Transforms/ThinLTOBitcodeWriter/split-internal1.ll
index 1a6cbb7e7e84b..2c84ab86cac63 100644
--- a/llvm/test/Transforms/ThinLTOBitcodeWriter/split-internal1.ll
+++ b/llvm/test/Transforms/ThinLTOBitcodeWriter/split-internal1.ll
@@ -13,7 +13,8 @@
; BCA1-NOT: <GLOBALVAL_SUMMARY_BLOCK
; M0: @g.581d7631532fa146ba4061179da39272 = external hidden global i8
-; M1: @g.581d7631532fa146ba4061179da39272 = hidden global i8 42, !type !0
+; M1: @g = internal global i8 42, !type !0
+; M1: @g.581d7631532fa146ba4061179da39272 = hidden alias ptr, ptr @g
@g = internal global i8 42, !type !0
; M0: define ptr @f()
diff --git a/llvm/test/Transforms/ThinLTOBitcodeWriter/split-internal2.ll b/llvm/test/Transforms/ThinLTOBitcodeWriter/split-internal2.ll
index 1a7b1aba66576..d9d9f516015d9 100644
--- a/llvm/test/Transforms/ThinLTOBitcodeWriter/split-internal2.ll
+++ b/llvm/test/Transforms/ThinLTOBitcodeWriter/split-internal2.ll
@@ -20,8 +20,9 @@ target triple = "x86_64-unknown-linux-gnu"
; M1: @g = global ptr @f.13757e0fb71915e385efa4dc9d1e08fd, !type !0
@g = global ptr @f, !type !0
-; M0: define hidden void @f.13757e0fb71915e385efa4dc9d1e08fd()
-; M1: declare !guid !4 hidden void @f.13757e0fb71915e385efa4dc9d1e08fd()
+; M0: @f.13757e0fb71915e385efa4dc9d1e08fd = hidden alias ptr, ptr @f
+; M0: define internal void @f()
+; M1: declare !guid !{{[0-9]+}} hidden void @f.13757e0fb71915e385efa4dc9d1e08fd()
define internal void @f() {
call void @f2()
ret void
diff --git a/llvm/test/Transforms/ThinLTOBitcodeWriter/split-vfunc-internal.ll b/llvm/test/Transforms/ThinLTOBitcodeWriter/split-vfunc-internal.ll
index 75932fd84d83e..1d17699be1c3c 100644
--- a/llvm/test/Transforms/ThinLTOBitcodeWriter/split-vfunc-internal.ll
+++ b/llvm/test/Transforms/ThinLTOBitcodeWriter/split-vfunc-internal.ll
@@ -10,12 +10,14 @@ define ptr @source() {
}
; M0: @g.84f59439b469192440047efc8de357fb = external hidden constant [1 x ptr]
-; M1: @g.84f59439b469192440047efc8de357fb = hidden constant [1 x ptr] [ptr @ok.84f59439b469192440047efc8de357fb]
+; M1: @g = internal constant [1 x ptr] [ptr @ok.84f59439b469192440047efc8de357fb]
+; M1: @g.84f59439b469192440047efc8de357fb = hidden alias ptr, ptr @g
@g = internal constant [1 x ptr] [
ptr @ok
], !type !0
-; M0: define hidden i64 @ok.84f59439b469192440047efc8de357fb
+; M0: @ok.84f59439b469192440047efc8de357fb = hidden alias ptr, ptr @ok
+; M0: define internal i64 @ok(ptr %this)
; M1: define available_externally hidden i64 @ok.84f59439b469192440047efc8de357fb
define internal i64 @ok(ptr %this) {
ret i64 42
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