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//===- ElimAvailExtern.cpp - DCE unreachable internal functions -----------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This transform is designed to eliminate available external global
// definitions from the program, turning them into declarations.
#include "llvm/Transforms/IPO/ElimAvailExtern.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/Module.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Transforms/IPO.h"
#include "llvm/Transforms/Utils/GlobalStatus.h"
#include "llvm/Transforms/Utils/ModuleUtils.h"
using namespace llvm;
#define DEBUG_TYPE "elim-avail-extern"
cl::opt<bool> ConvertToLocal(
"avail-extern-to-local", cl::Hidden,
cl::desc("Convert available_externally into locals, renaming them "
"to avoid link-time clashes."));
STATISTIC(NumRemovals, "Number of functions removed");
STATISTIC(NumConversions, "Number of functions converted");
STATISTIC(NumVariables, "Number of global variables removed");
void deleteFunction(Function &F) {
// This will set the linkage to external
/// Create a copy of the thinlto import, mark it local, and redirect direct
/// calls to the copy. Only direct calls are replaced, so that e.g. indirect
/// call function pointer tests would use the global identity of the function.
/// Currently, Value Profiling ("VP") MD_prof data isn't updated to refer to the
/// clone's GUID (which will be different, because the name and linkage is
/// different), under the assumption that the last consumer of this data is
/// upstream the pipeline (e.g. ICP).
static void convertToLocalCopy(Module &M, Function &F) {
// If we can't find a single use that's a call, just delete the function.
if (F.uses().end() == llvm::find_if(F.uses(), [&](Use &U) {
return isa<CallBase>(U.getUser());
return deleteFunction(F);
auto OrigName = F.getName().str();
// Build a new name. We still need the old name (see below).
// We could just rely on internal linking allowing 2 modules have internal
// functions with the same name, but that just creates more trouble than
// necessary e.g. distinguishing profiles or debugging. Instead, we append the
// module identifier.
auto NewName = OrigName + ".__uniq" + getUniqueModuleId(&M);
if (auto *SP = F.getSubprogram())
SP->replaceLinkageName(MDString::get(F.getParent()->getContext(), NewName));
// Now make a declaration for the old name. We'll use it if there are non-call
// uses. For those, it would be incorrect to replace them with the local copy:
// for example, one such use could be taking the address of the function and
// passing it to an external function, which, in turn, might compare the
// function pointer to the original (non-local) function pointer, e.g. as part
// of indirect call promotion.
auto *Decl =
Function::Create(F.getFunctionType(), GlobalValue::ExternalLinkage,
F.getAddressSpace(), OrigName, F.getParent());
[&](Use &U) { return !isa<CallBase>(U.getUser()); });
static bool eliminateAvailableExternally(Module &M) {
bool Changed = false;
// Drop initializers of available externally global variables.
for (GlobalVariable &GV : M.globals()) {
if (!GV.hasAvailableExternallyLinkage())
if (GV.hasInitializer()) {
Constant *Init = GV.getInitializer();
if (isSafeToDestroyConstant(Init))
Changed = true;
// Drop the bodies of available externally functions.
for (Function &F : llvm::make_early_inc_range(M)) {
if (F.isDeclaration() || !F.hasAvailableExternallyLinkage())
if (ConvertToLocal)
convertToLocalCopy(M, F);
Changed = true;
return Changed;
EliminateAvailableExternallyPass::run(Module &M, ModuleAnalysisManager &) {
if (!eliminateAvailableExternally(M))
return PreservedAnalyses::all();
return PreservedAnalyses::none();