108 lines
4.4 KiB
C++
108 lines
4.4 KiB
C++
//===- Passes/GOTPass.cpp - Adds GOT entries ------------------------------===//
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//
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// The LLVM Linker
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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///
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/// \file
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/// This linker pass transforms all GOT kind references to real references.
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/// That is, in assembly you can write something like:
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/// movq foo@GOTPCREL(%rip), %rax
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/// which means you want to load a pointer to "foo" out of the GOT (global
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/// Offsets Table). In the object file, the Atom containing this instruction
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/// has a Reference whose target is an Atom named "foo" and the Reference
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/// kind is a GOT load. The linker needs to instantiate a pointer sized
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/// GOT entry. This is done be creating a GOT Atom to represent that pointer
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/// sized data in this pass, and altering the Atom graph so the Reference now
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/// points to the GOT Atom entry (corresponding to "foo") and changing the
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/// Reference Kind to reflect it is now pointing to a GOT entry (rather
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/// then needing a GOT entry).
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///
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/// There is one optimization the linker can do here. If the target of the GOT
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/// is in the same linkage unit and does not need to be interposable, and
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/// the GOT use is just a load (not some other operation), this pass can
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/// transform that load into an LEA (add). This optimizes away one memory load
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/// which at runtime that could stall the pipeline. This optimization only
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/// works for architectures in which a (GOT) load instruction can be change to
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/// an LEA instruction that is the same size. The method isGOTAccess() should
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/// only return true for "canBypassGOT" if this optimization is supported.
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///
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//===----------------------------------------------------------------------===//
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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/LLVM.h"
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#include "lld/Core/Pass.h"
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#include "lld/Core/Reference.h"
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#include "llvm/ADT/DenseMap.h"
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namespace lld {
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static bool shouldReplaceTargetWithGOTAtom(const Atom *target,
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bool canBypassGOT) {
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// Accesses to shared library symbols must go through GOT.
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if (target->definition() == Atom::definitionSharedLibrary)
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return true;
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// Accesses to interposable symbols in same linkage unit must also go
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// through GOT.
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const DefinedAtom *defTarget = dyn_cast<DefinedAtom>(target);
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if (defTarget != nullptr &&
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defTarget->interposable() != DefinedAtom::interposeNo) {
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assert(defTarget->scope() != DefinedAtom::scopeTranslationUnit);
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return true;
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}
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// Target does not require indirection. So, if instruction allows GOT to be
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// by-passed, do that optimization and don't create GOT entry.
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return !canBypassGOT;
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}
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static const DefinedAtom *
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findGOTAtom(const Atom *target,
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llvm::DenseMap<const Atom *, const DefinedAtom *> &targetToGOT) {
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auto pos = targetToGOT.find(target);
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return (pos == targetToGOT.end()) ? nullptr : pos->second;
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}
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void GOTPass::perform(std::unique_ptr<MutableFile> &mergedFile) {
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// Use map so all pointers to same symbol use same GOT entry.
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llvm::DenseMap<const Atom*, const DefinedAtom*> targetToGOT;
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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 instructions accessing the GOT.
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bool canBypassGOT;
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if (!isGOTAccess(*ref, canBypassGOT))
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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 (!shouldReplaceTargetWithGOTAtom(target, canBypassGOT)) {
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// Update reference kind to reflect that target is a direct accesss.
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updateReferenceToGOT(ref, false);
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continue;
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}
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// Replace the target with a reference to a GOT entry.
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const DefinedAtom *gotEntry = findGOTAtom(target, targetToGOT);
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if (!gotEntry) {
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gotEntry = makeGOTEntry(*target);
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assert(gotEntry != nullptr);
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assert(gotEntry->contentType() == DefinedAtom::typeGOT);
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targetToGOT[target] = gotEntry;
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}
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const_cast<Reference *>(ref)->setTarget(gotEntry);
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// Update reference kind to reflect that target is now a GOT entry.
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updateReferenceToGOT(ref, true);
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}
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}
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// add all created GOT Atoms to master file
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for (auto &it : targetToGOT)
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mergedFile->addAtom(*it.second);
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}
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} // end namesapce lld
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