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lld: fix Linx HI/LO relocation pairing #24

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lld: fix Linx HI/LO relocation pairing #24

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86 changes: 60 additions & 26 deletions lld/ELF/InputSection.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -765,33 +765,57 @@ static const Relocation *findLinxPCRelHiByOffset(const InputSection *isec,

static const Relocation *findLinxPCRelHiBySymbol(const InputSectionBase *sec,
uint64_t loOffset,
const Symbol *sym) {
const Symbol *sym,
int64_t addend) {
auto *isec = dyn_cast_or_null<InputSection>(sec);
if (!isec)
return nullptr;

auto it = llvm::lower_bound(
isec->relocs(), loOffset,
[](const Relocation &lhs, uint64_t rhs) { return lhs.offset < rhs; });

const Relocation *before = nullptr;
while (it != isec->relocs().begin()) {
--it;
if (it->sym == sym && isLinxPCRelHi(it->type))
return &*it;
if (it->sym == sym && it->addend == addend && isLinxPCRelHi(it->type)) {
before = &*it;
break;
}
}
return nullptr;

const Relocation *after = nullptr;
it = llvm::lower_bound(
isec->relocs(), loOffset,
[](const Relocation &lhs, uint64_t rhs) { return lhs.offset < rhs; });
for (; it != isec->relocs().end(); ++it) {
if (it->sym == sym && it->addend == addend && isLinxPCRelHi(it->type)) {
after = &*it;
break;
}
}
Comment on lines +778 to +796

@gemini-code-assist gemini-code-assist Bot Jun 3, 2026

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medium

We can avoid calling llvm::lower_bound twice by copying the iterator it for the backward search. This eliminates the redundant binary search overhead on the relocation list.

  const Relocation *before = nullptr;
  for (auto rollbackIt = it; rollbackIt != isec->relocs().begin();) {
    --rollbackIt;
    if (rollbackIt->sym == sym && rollbackIt->addend == addend && isLinxPCRelHi(rollbackIt->type)) {
      before = &*rollbackIt;
      break;
    }
  }

  const Relocation *after = nullptr;
  for (; it != isec->relocs().end(); ++it) {
    if (it->sym == sym && it->addend == addend && isLinxPCRelHi(it->type)) {
      after = &*it;
      break;
    }
  }


if (!before)
return after;
if (!after)
return before;
return loOffset - before->offset <= after->offset - loOffset ? before : after;
}

// Most R_LINX_LO12 relocations point at a local anchor symbol whose value
// matches the paired R_LINX_PCREL_HI20 relocation offset. Some startup objects
// instead target the final symbol directly (for example _DYNAMIC in static
// PIE), so the linker has to recover the HI relocation by symbol and then use
// the HI relocation's place when reconstructing the PC-relative target.
// PIE). Linx block layout can also place the HI in a later predecessor block,
// so the linker has to recover the HI relocation by exact symbol/addend in
// either direction before reconstructing the PC-relative target.
static const Relocation *getLinxPCRelHi20(Ctx &ctx,
const InputSectionBase *loSec,
const Relocation &loReloc) {
int64_t addend = loReloc.addend;
Symbol *sym = loReloc.sym;
std::string anchor = toStr(ctx, *sym);
bool unsupportedDefinedAnchor = false;
StringRef unsupportedSectionName;

if (const auto *d = dyn_cast<Defined>(sym)) {
if (d->section) {
Expand All @@ -803,31 +827,36 @@ static const Relocation *getLinxPCRelHi20(Ctx &ctx,
hiSec = eh->getParent();
}
if (!hiSec) {
Err(ctx) << loSec->getLocation(loReloc.offset)
<< ": R_LINX_LO12 relocation "
"points to unsupported anchor section '"
<< d->section->name << "' for symbol '" << sym->getName()
<< "'";
return nullptr;
unsupportedDefinedAnchor = true;
unsupportedSectionName = d->section->name;
} else {
anchor = sym->getName().empty() ? "offset 0x" + utohexstr(d->value)
: toStr(ctx, *sym) + "+0x" +
utohexstr(d->value);
if (const Relocation *hiRel = findLinxPCRelHiByOffset(hiSec, d->value)) {
if (addend != 0)
Warn(ctx) << loSec->getLocation(loReloc.offset)
<< ": non-zero addend in "
"R_LINX_LO12 relocation to "
<< anchor << " is ignored";
return hiRel;
}
}

anchor = sym->getName().empty() ? "offset 0x" + utohexstr(d->value)
: toStr(ctx, *sym) + "+0x" +
utohexstr(d->value);
if (addend != 0)
Warn(ctx) << loSec->getLocation(loReloc.offset)
<< ": non-zero addend in "
"R_LINX_LO12 relocation to "
<< anchor << " is ignored";
if (const Relocation *hiRel = findLinxPCRelHiByOffset(hiSec, d->value))
return hiRel;
}
}

if (const Relocation *hiRel =
findLinxPCRelHiBySymbol(loSec, loReloc.offset, sym))
findLinxPCRelHiBySymbol(loSec, loReloc.offset, sym, addend))
return hiRel;

if (unsupportedDefinedAnchor) {
Err(ctx) << loSec->getLocation(loReloc.offset)
<< ": R_LINX_LO12 relocation points to unsupported anchor section '"
<< unsupportedSectionName << "' for symbol '" << sym->getName()
<< "'";
return nullptr;
}

Err(ctx) << loSec->getLocation(loReloc.offset)
<< ": R_LINX_LO12 relocation points to " << anchor
<< " without an associated R_LINX_PCREL_HI20 relocation";
Expand Down Expand Up @@ -1014,8 +1043,13 @@ uint64_t InputSectionBase::getRelocTargetVA(Ctx &ctx, const Relocation &r,
return 0;
}
case RE_LINX_PC_INDIRECT: {
if (const Relocation *hiRel = getLinxPCRelHi20(ctx, this, r))
return getRelocTargetVA(ctx, *hiRel, getVA(hiRel->offset));
if (const Relocation *hiRel = getLinxPCRelHi20(ctx, this, r)) {
uint64_t p = getVA(hiRel->offset);
uint64_t val = hiRel->sym->isUndefWeak()
? p + hiRel->addend
: hiRel->sym->getVA(ctx, hiRel->addend);
return val;
}
return 0;
}
case RE_LOONGARCH_PC_INDIRECT: {
Expand Down