2946cd7010
to reflect the new license. We understand that people may be surprised that we're moving the header entirely to discuss the new license. We checked this carefully with the Foundation's lawyer and we believe this is the correct approach. Essentially, all code in the project is now made available by the LLVM project under our new license, so you will see that the license headers include that license only. Some of our contributors have contributed code under our old license, and accordingly, we have retained a copy of our old license notice in the top-level files in each project and repository. llvm-svn: 351636
129 lines
4.6 KiB
C++
129 lines
4.6 KiB
C++
//===- lib/ReaderWriter/MachO/ShimPass.cpp -------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This linker pass updates branch-sites whose target is a different mode
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// (thumb vs arm).
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//
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// Arm code has two instruction encodings thumb and arm. When branching from
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// one code encoding to another, you need to use an instruction that switches
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// the instruction mode. Usually the transition only happens at call sites, and
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// the linker can transform a BL instruction in BLX (or vice versa). But if the
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// compiler did a tail call optimization and a function ends with a branch (not
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// branch and link), there is no pc-rel BX instruction.
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//
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// The ShimPass looks for pc-rel B instructions that will need to switch mode.
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// For those cases it synthesizes a shim which does the transition, then
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// modifies the original atom with the B instruction to target to the shim atom.
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//
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//===----------------------------------------------------------------------===//
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#include "ArchHandler.h"
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#include "File.h"
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#include "MachOPasses.h"
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#include "lld/Common/LLVM.h"
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#include "lld/Core/DefinedAtom.h"
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#include "lld/Core/File.h"
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#include "lld/Core/Reference.h"
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#include "lld/Core/Simple.h"
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#include "lld/ReaderWriter/MachOLinkingContext.h"
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#include "llvm/ADT/DenseMap.h"
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#include "llvm/ADT/STLExtras.h"
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namespace lld {
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namespace mach_o {
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class ShimPass : public Pass {
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public:
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ShimPass(const MachOLinkingContext &context)
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: _ctx(context), _archHandler(_ctx.archHandler()),
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_stubInfo(_archHandler.stubInfo()),
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_file(*_ctx.make_file<MachOFile>("<mach-o shim pass>")) {
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_file.setOrdinal(_ctx.getNextOrdinalAndIncrement());
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}
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llvm::Error perform(SimpleFile &mergedFile) override {
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// Scan all references in all atoms.
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for (const DefinedAtom *atom : mergedFile.defined()) {
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for (const Reference *ref : *atom) {
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// Look at non-call branches.
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if (!_archHandler.isNonCallBranch(*ref))
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continue;
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const Atom *target = ref->target();
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assert(target != nullptr);
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if (const lld::DefinedAtom *daTarget = dyn_cast<DefinedAtom>(target)) {
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bool atomIsThumb = _archHandler.isThumbFunction(*atom);
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bool targetIsThumb = _archHandler.isThumbFunction(*daTarget);
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if (atomIsThumb != targetIsThumb)
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updateBranchToUseShim(atomIsThumb, *daTarget, ref);
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}
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}
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}
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// Exit early if no shims needed.
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if (_targetToShim.empty())
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return llvm::Error::success();
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// Sort shim atoms so the layout order is stable.
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std::vector<const DefinedAtom *> shims;
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shims.reserve(_targetToShim.size());
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for (auto element : _targetToShim) {
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shims.push_back(element.second);
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}
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std::sort(shims.begin(), shims.end(),
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[](const DefinedAtom *l, const DefinedAtom *r) {
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return (l->name() < r->name());
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});
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// Add all shims to master file.
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for (const DefinedAtom *shim : shims)
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mergedFile.addAtom(*shim);
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return llvm::Error::success();
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}
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private:
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void updateBranchToUseShim(bool thumbToArm, const DefinedAtom& target,
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const Reference *ref) {
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// Make file-format specific stub and other support atoms.
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const DefinedAtom *shim = this->getShim(thumbToArm, target);
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assert(shim != nullptr);
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// Switch branch site to target shim atom.
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const_cast<Reference *>(ref)->setTarget(shim);
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}
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const DefinedAtom* getShim(bool thumbToArm, const DefinedAtom& target) {
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auto pos = _targetToShim.find(&target);
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if ( pos != _targetToShim.end() ) {
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// Reuse an existing shim.
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assert(pos->second != nullptr);
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return pos->second;
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} else {
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// There is no existing shim, so create a new one.
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const DefinedAtom *shim = _archHandler.createShim(_file, thumbToArm,
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target);
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_targetToShim[&target] = shim;
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return shim;
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}
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}
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const MachOLinkingContext &_ctx;
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mach_o::ArchHandler &_archHandler;
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const ArchHandler::StubInfo &_stubInfo;
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MachOFile &_file;
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llvm::DenseMap<const Atom*, const DefinedAtom*> _targetToShim;
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};
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void addShimPass(PassManager &pm, const MachOLinkingContext &ctx) {
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pm.add(llvm::make_unique<ShimPass>(ctx));
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}
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} // end namespace mach_o
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} // end namespace lld
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