From 4b2acde21a9372fdbdb86155a6dda04c5f79f567 Mon Sep 17 00:00:00 2001 From: Nico Weber Date: Fri, 2 May 2014 18:35:25 +0000 Subject: Teach GlobalDCE how to remove empty global_ctor entries. MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit This moves most of GlobalOpt's constructor optimization code out of GlobalOpt into Transforms/Utils/CDtorUtils.{h,cpp}. The public interface is a single function OptimizeGlobalCtorsList() that takes a predicate returning which constructors to remove. GlobalOpt calls this with a function that statically evaluates all constructors, just like it did before. This part of the change is behavior-preserving. Also add a call to this from GlobalDCE with a filter that removes global constructors that contain a "ret" instruction and nothing else – this fixes PR19590. llvm-svn: 207856 --- llvm/lib/Transforms/Utils/CtorUtils.cpp | 181 ++++++++++++++++++++++++++++++++ 1 file changed, 181 insertions(+) create mode 100644 llvm/lib/Transforms/Utils/CtorUtils.cpp (limited to 'llvm/lib/Transforms/Utils/CtorUtils.cpp') diff --git a/llvm/lib/Transforms/Utils/CtorUtils.cpp b/llvm/lib/Transforms/Utils/CtorUtils.cpp new file mode 100644 index 00000000000..0082df01214 --- /dev/null +++ b/llvm/lib/Transforms/Utils/CtorUtils.cpp @@ -0,0 +1,181 @@ +//===- CtorUtils.cpp - Helpers for working with global_ctors ----*- C++ -*-===// +// +// The LLVM Compiler Infrastructure +// +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. +// +//===----------------------------------------------------------------------===// +// +// This file defines functions that are used to process llvm.global_ctors. +// +//===----------------------------------------------------------------------===// + +#include "llvm/Transforms/Utils/CtorUtils.h" +#include "llvm/IR/Constants.h" +#include "llvm/IR/Function.h" +#include "llvm/IR/GlobalVariable.h" +#include "llvm/IR/Instructions.h" +#include "llvm/IR/Module.h" +#include "llvm/Support/Debug.h" + +#define DEBUG_TYPE "ctor_utils" + +namespace llvm { + +namespace { +/// Given a specified llvm.global_ctors list, install the +/// specified array. +void installGlobalCtors(GlobalVariable *GCL, + const std::vector &Ctors) { + // If we made a change, reassemble the initializer list. + Constant *CSVals[2]; + CSVals[0] = ConstantInt::get(Type::getInt32Ty(GCL->getContext()), 65535); + CSVals[1] = nullptr; + + StructType *StructTy = + cast(GCL->getType()->getElementType()->getArrayElementType()); + + // Create the new init list. + std::vector CAList; + for (unsigned i = 0, e = Ctors.size(); i != e; ++i) { + if (Ctors[i]) { + CSVals[1] = Ctors[i]; + } else { + Type *FTy = FunctionType::get(Type::getVoidTy(GCL->getContext()), false); + PointerType *PFTy = PointerType::getUnqual(FTy); + CSVals[1] = Constant::getNullValue(PFTy); + CSVals[0] = + ConstantInt::get(Type::getInt32Ty(GCL->getContext()), 0x7fffffff); + } + CAList.push_back(ConstantStruct::get(StructTy, CSVals)); + } + + // Create the array initializer. + Constant *CA = + ConstantArray::get(ArrayType::get(StructTy, CAList.size()), CAList); + + // If we didn't change the number of elements, don't create a new GV. + if (CA->getType() == GCL->getInitializer()->getType()) { + GCL->setInitializer(CA); + return; + } + + // Create the new global and insert it next to the existing list. + GlobalVariable *NGV = + new GlobalVariable(CA->getType(), GCL->isConstant(), GCL->getLinkage(), + CA, "", GCL->getThreadLocalMode()); + GCL->getParent()->getGlobalList().insert(GCL, NGV); + NGV->takeName(GCL); + + // Nuke the old list, replacing any uses with the new one. + if (!GCL->use_empty()) { + Constant *V = NGV; + if (V->getType() != GCL->getType()) + V = ConstantExpr::getBitCast(V, GCL->getType()); + GCL->replaceAllUsesWith(V); + } + GCL->eraseFromParent(); +} + +/// Given a llvm.global_ctors list that we can understand, +/// return a list of the functions and null terminator as a vector. +std::vector parseGlobalCtors(GlobalVariable *GV) { + if (GV->getInitializer()->isNullValue()) + return std::vector(); + ConstantArray *CA = cast(GV->getInitializer()); + std::vector Result; + Result.reserve(CA->getNumOperands()); + for (User::op_iterator i = CA->op_begin(), e = CA->op_end(); i != e; ++i) { + ConstantStruct *CS = cast(*i); + Result.push_back(dyn_cast(CS->getOperand(1))); + } + return Result; +} + +/// Find the llvm.global_ctors list, verifying that all initializers have an +/// init priority of 65535. +GlobalVariable *findGlobalCtors(Module &M) { + GlobalVariable *GV = M.getGlobalVariable("llvm.global_ctors"); + if (!GV) + return nullptr; + + // Verify that the initializer is simple enough for us to handle. We are + // only allowed to optimize the initializer if it is unique. + if (!GV->hasUniqueInitializer()) + return nullptr; + + if (isa(GV->getInitializer())) + return GV; + ConstantArray *CA = cast(GV->getInitializer()); + + for (User::op_iterator i = CA->op_begin(), e = CA->op_end(); i != e; ++i) { + if (isa(*i)) + continue; + ConstantStruct *CS = cast(*i); + if (isa(CS->getOperand(1))) + continue; + + // Must have a function or null ptr. + if (!isa(CS->getOperand(1))) + return nullptr; + + // Init priority must be standard. + ConstantInt *CI = cast(CS->getOperand(0)); + if (CI->getZExtValue() != 65535) + return nullptr; + } + + return GV; +} +} // namespace + +/// Call "ShouldRemove" for every entry in M's global_ctor list and remove the +/// entries for which it returns true. Return true if anything changed. +bool optimizeGlobalCtorsList(Module &M, ShouldRemoveCtor ShouldRemove, + void *Context) { + GlobalVariable *GlobalCtors = findGlobalCtors(M); + if (!GlobalCtors) + return false; + + std::vector Ctors = parseGlobalCtors(GlobalCtors); + if (Ctors.empty()) + return false; + + bool MadeChange = false; + + // Loop over global ctors, optimizing them when we can. + for (unsigned i = 0; i != Ctors.size(); ++i) { + Function *F = Ctors[i]; + // Found a null terminator in the middle of the list, prune off the rest of + // the list. + if (!F) { + if (i != Ctors.size() - 1) { + Ctors.resize(i + 1); + MadeChange = true; + } + break; + } + DEBUG(dbgs() << "Optimizing Global Constructor: " << *F << "\n"); + + // We cannot simplify external ctor functions. + if (F->empty()) + continue; + + // If we can evaluate the ctor at compile time, do. + if (ShouldRemove(Context, F)) { + Ctors.erase(Ctors.begin() + i); + MadeChange = true; + --i; + continue; + } + } + + if (!MadeChange) + return false; + + installGlobalCtors(GlobalCtors, Ctors); + return true; +} + +} // End llvm namespace -- cgit v1.2.3