OSchip
/
llvm-project
mirror of https://github.com/THU-DSP-LAB/llvm-project.git
13
0
Fork 6
Code Issues Packages Projects Releases Wiki Activity

[Passes][GOT] Fix style. 

llvm-svn: 171530
This commit is contained in:
Michael J. Spencer 2013-01-04 21:18:13 +00:00
parent 85d998a109
commit b8c853e5eb
1 changed files with 40 additions and 48 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

88
lld/lib/Passes/GOTPass.cpp
View File

@ -6,29 +6,30 @@
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// 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.
//
///
/// \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"
@ -39,72 +40,63 @@
#include "llvm/ADT/DenseMap.h"
namespace lld {
void GOTPass::perform(File& 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 ( this->isGOTAccess(ref->kind(), 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 ) {
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
} 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 {
} 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 ) {
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() ) {
if (pos == targetToGOT.end()) {
// This is no existing GOT entry. Create a new one.
gotEntry = this->makeGOTEntry(*target);
gotEntry = makeGOTEntry(*target);
assert(gotEntry != nullptr);
assert(gotEntry->contentType() == DefinedAtom::typeGOT);
targetToGOT[target] = gotEntry;
}
else {
} else {
// Reuse an existing GOT entry.
gotEntry = pos->second;
assert(gotEntry != nullptr);
}
// Switch reference to GOT atom.
(const_cast<Reference*>(ref))->setTarget(gotEntry);
const_cast<Reference*>(ref)->setTarget(gotEntry);
}
// Update reference kind to reflect
// that target is now a GOT entry or a direct accesss.
this->updateReferenceToGOT(ref, replaceTargetWithGOTAtom);
updateReferenceToGOT(ref, replaceTargetWithGOTAtom);
}
}
}
// add all created GOT Atoms to master file
for (auto it=targetToGOT.begin(), end=targetToGOT.end(); it != end; ++it) {
mergedFile.addAtom(*it->second);
for (auto &it : targetToGOT) {
mergedFile.addAtom(*it.second);
}
}
}