lld/lib/Passes/GOTPass.cpp

//===- Passes/GOTPass.cpp - Adds GOT entries ------------------------------===//
//
//                             The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
///
/// \file
/// This linker pass transforms all GOT kind references to real references.
/// That is, in assembly you can write something like:
///     movq foo@GOTPCREL(%rip), %rax
/// which means you want to load a pointer to "foo" out of the GOT (global
/// Offsets Table). In the object file, the Atom containing this instruction
/// has a Reference whose target is an Atom named "foo" and the Reference
/// kind is a GOT load.  The linker needs to instantiate a pointer sized
/// GOT entry.  This is done be creating a GOT Atom to represent that pointer
/// sized data in this pass, and altering the Atom graph so the Reference now
/// points to the GOT Atom entry (corresponding to "foo") and changing the
/// Reference Kind to reflect it is now pointing to a GOT entry (rather
/// then needing a GOT entry).
///
/// There is one optimization the linker can do here.  If the target of the GOT
/// is in the same linkage unit and does not need to be interposable, and
/// the GOT use is just a load (not some other operation), this pass can
/// transform that load into an LEA (add).  This optimizes away one memory load
/// which at runtime that could stall the pipeline.  This optimization only
/// works for architectures in which a (GOT) load instruction can be change to
/// an LEA instruction that is the same size.  The method isGOTAccess() should
/// only return true for "canBypassGOT" if this optimization is supported.
///
//===----------------------------------------------------------------------===//

#include "lld/Core/DefinedAtom.h"
#include "lld/Core/File.h"
#include "lld/Core/LLVM.h"
#include "lld/Core/Pass.h"
#include "lld/Core/Reference.h"
#include "llvm/ADT/DenseMap.h"

namespace lld {
void GOTPass::perform(MutableFile &mergedFile) {
  // Use map so all pointers to same symbol use same GOT entry.
  llvm::DenseMap<const Atom*, const DefinedAtom*> targetToGOT;

  // Scan all references in all atoms.
  for(const DefinedAtom *atom : mergedFile.defined()) {
    for (const Reference *ref : *atom) {
      // Look at instructions accessing the GOT.
      bool canBypassGOT;
      if (isGOTAccess(ref->kind(), canBypassGOT)) {
        const Atom* target = ref->target();
        assert(target != nullptr);
        const DefinedAtom* defTarget = dyn_cast<DefinedAtom>(target);
        bool replaceTargetWithGOTAtom = false;
        if (target->definition() == Atom::definitionSharedLibrary) {
          // Accesses to shared library symbols must go through GOT.
          replaceTargetWithGOTAtom = true;
        } else if ((defTarget != nullptr) &&
                   (defTarget->interposable() != DefinedAtom::interposeNo)) {
          // Accesses to interposable symbols in same linkage unit
          // must also go through GOT.
          assert(defTarget->scope() != DefinedAtom::scopeTranslationUnit);
          replaceTargetWithGOTAtom = true;
        } else {
          // Target does not require indirection.  So, if instruction allows
          // GOT to be by-passed, do that optimization and don't create
          // GOT entry.
          replaceTargetWithGOTAtom = !canBypassGOT;
        }
        if (replaceTargetWithGOTAtom) {
          // Replace the target with a reference to a GOT entry.
          const DefinedAtom* gotEntry = nullptr;
          auto pos = targetToGOT.find(target);
          if (pos == targetToGOT.end()) {
            // This is no existing GOT entry.  Create a new one.
            gotEntry = makeGOTEntry(*target);
            assert(gotEntry != nullptr);
            assert(gotEntry->contentType() == DefinedAtom::typeGOT);
            targetToGOT[target] = gotEntry;
          } else {
            // Reuse an existing GOT entry.
            gotEntry = pos->second;
            assert(gotEntry != nullptr);
          }
          // Switch reference to GOT atom.
          const_cast<Reference*>(ref)->setTarget(gotEntry);
        }
        // Update reference kind to reflect
        // that target is now a GOT entry or a direct accesss.
        updateReferenceToGOT(ref, replaceTargetWithGOTAtom);
      }
    }
  }

  // add all created GOT Atoms to master file
  for (auto &it : targetToGOT) {
    mergedFile.addAtom(*it.second);
  }
}
}