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