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[JITLink][ELF] PCRel32GOTLoad relocations are resolved like regular PCRel32 ones 

The difference is that the former are indirect and go to the GOT while the latter go to the target directly. This info can be used to relax indirect ones that don't need the GOT (because the target is in range). We check for this optimization beforehand. For formal correctness and to avoid confusion, we should only change the relocation kind if we actually apply the relaxation.
This commit is contained in:
Stefan Gränitz 2020-10-23 21:52:12 +02:00
parent b481e00bf4
commit 138b9f1928
1 changed files with 12 additions and 13 deletions
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25
llvm/lib/ExecutionEngine/JITLink/ELF_x86_64.cpp
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@ -125,13 +125,14 @@ static Error optimizeELF_x86_64_GOTAndStubs(LinkGraph &G) {
for (auto *B : G.blocks())
for (auto &E : B->edges())
if (E.getKind() == PCRel32GOTLoad) {
// Replace GOT load with LEA only for MOVQ instructions.
constexpr uint8_t MOVQRIPRel[] = {0x48, 0x8b};
if (strncmp(B->getContent().data() + E.getOffset() - 3,
reinterpret_cast<const char *>(MOVQRIPRel), 2) != 0)
continue;
assert(E.getOffset() >= 3 && "GOT edge occurs too early in block");
// Switch the edge kind to PCRel32: Whether we change the edge target
// or not this will be the desired kind.
E.setKind(PCRel32);
// Optimize GOT references.
auto &GOTBlock = E.getTarget().getBlock();
assert(GOTBlock.getSize() == G.getPointerSize() &&
"GOT entry block should be pointer sized");
@ -142,16 +143,13 @@ static Error optimizeELF_x86_64_GOTAndStubs(LinkGraph &G) {
JITTargetAddress EdgeAddr = B->getAddress() + E.getOffset();
JITTargetAddress TargetAddr = GOTTarget.getAddress();
// Check that this is a recognized MOV instruction.
// FIXME: Can we assume this?
constexpr uint8_t MOVQRIPRel[] = {0x48, 0x8b};
if (strncmp(B->getContent().data() + E.getOffset() - 3,
reinterpret_cast<const char *>(MOVQRIPRel), 2) != 0)
continue;
int64_t Displacement = TargetAddr - EdgeAddr + 4;
if (Displacement >= std::numeric_limits<int32_t>::min() &&
Displacement <= std::numeric_limits<int32_t>::max()) {
// Change the edge kind as we don't go through GOT anymore. This is
// for formal correctness only. Technically, the two relocation kinds
// are resolved the same way.
E.setKind(PCRel32);
E.setTarget(GOTTarget);
auto *BlockData = reinterpret_cast<uint8_t *>(
const_cast<char *>(B->getContent().data()));
@ -636,7 +634,8 @@ private:
char *FixupPtr = BlockWorkingMem + E.getOffset();
JITTargetAddress FixupAddress = B.getAddress() + E.getOffset();
switch (E.getKind()) {
case ELFX86RelocationKind::PCRel32: {
case ELFX86RelocationKind::PCRel32:
case ELFX86RelocationKind::PCRel32GOTLoad: {
int64_t Value = E.getTarget().getAddress() + E.getAddend() - FixupAddress;
endian::write32le(FixupPtr, Value);
break;