//
// The tail-call optimzation may result in a function ending in a jump (b)
// to another functions. At compile time the compiler does not know
// if the target of the jump will be in the same mode (arm vs thumb).
// The arm/thumb instruction set has a way to change modes in a bl(x)
// insruction, but no instruction to change mode in a jump (b) instruction.
// In those rare cases, the linker needs to insert a shim of code to
// make the mode switch.
//
void doPass(const Options& opts, ld::Internal& state)
{
// only make branch shims in final linked images
if ( opts.outputKind() == Options::kObjectFile )
return;
// only ARM need branch islands
if ( opts.architecture() != CPU_TYPE_ARM )
return;
const bool makingKextBundle = (opts.outputKind() == Options::kKextBundle);
// scan all sections
for (std::vector<ld::Internal::FinalSection*>::iterator sit=state.sections.begin(); sit != state.sections.end(); ++sit) {
ld::Internal::FinalSection* sect = *sit;
std::map<const Atom*, const Atom*> atomToThumbMap;
std::map<const Atom*, const Atom*> thumbToAtomMap;
std::vector<const Atom*> shims;
// scan section for branch instructions that need to switch mode
for (std::vector<const ld::Atom*>::iterator ait=sect->atoms.begin(); ait != sect->atoms.end(); ++ait) {
const ld::Atom* atom = *ait;
const ld::Atom* target = NULL;
bool targetIsProxy;
for (ld::Fixup::iterator fit = atom->fixupsBegin(), end=atom->fixupsEnd(); fit != end; ++fit) {
switch ( fit->kind ) {
case ld::Fixup::kindStoreTargetAddressThumbBranch22:
extractTarget(fit, state, &target);
targetIsProxy = (target->definition() == ld::Atom::definitionProxy);
if ( ! target->isThumb() ) {
const uint8_t* fixUpLocation = atom->rawContentPointer();
// <rdar://problem/9544194> don't try to scan atom for branches if atom unwilling to supply raw content
if ( fixUpLocation == NULL )
break;
fixUpLocation += fit->offsetInAtom;
uint32_t instruction = *((uint32_t*)fixUpLocation);
bool is_b = ((instruction & 0xD000F800) == 0x9000F000);
// need shim for branch from thumb to arm, or for call to function outside kext
if ( is_b || (targetIsProxy && makingKextBundle) ) {
if ( _s_log ) fprintf(stderr, "need to add thumb->arm instr=0x%08X shim to %s for %s\n", instruction, target->name(), atom->name());
const Atom* shim = NULL;
std::map<const Atom*, const Atom*>::iterator pos = thumbToAtomMap.find(target);
if ( pos == thumbToAtomMap.end() ) {
if ( opts.archSupportsThumb2() ) {
// <rdar://problem/9116044> make long-branch style shims for arm kexts
if ( makingKextBundle && opts.allowTextRelocs() )
shim = new NoPICThumb2ToArmShimAtom(target, *sect);
else
shim = new Thumb2ToArmShimAtom(target, *sect);
}
else {
shim = new Thumb1ToArmShimAtom(target, *sect);
}
shims.push_back(shim);
thumbToAtomMap[target] = shim;
}
else {
shim = pos->second;
}
fit->binding = ld::Fixup::bindingDirectlyBound;
fit->u.target = shim;
}
}
break;
case ld::Fixup::kindStoreTargetAddressARMBranch24:
extractTarget(fit, state, &target);
targetIsProxy = (target->definition() == ld::Atom::definitionProxy);
if ( target->isThumb() || (targetIsProxy && makingKextBundle) ) {
const uint8_t* fixUpLocation = atom->rawContentPointer();
// <rdar://problem/9544194> don't try to scan atom for branches if atom unwilling to supply raw content
if ( fixUpLocation == NULL )
break;
fixUpLocation += fit->offsetInAtom;
uint32_t instruction = *((uint32_t*)fixUpLocation);
bool is_b = ((instruction & 0x0F000000) == 0x0A000000) && ((instruction & 0xF0000000) != 0xF0000000);
// need shim for branch from arm to thumb, or for call to function outside kext
if ( is_b || (targetIsProxy && makingKextBundle) ) {
if ( _s_log ) fprintf(stderr, "need to add arm->thumb instr=0x%08X shim to %s for %s\n", instruction, target->name(), atom->name());
const Atom* shim = NULL;
std::map<const Atom*, const Atom*>::iterator pos = atomToThumbMap.find(target);
if ( pos == atomToThumbMap.end() ) {
// <rdar://problem/9116044> make long-branch style shims for arm kexts
if ( makingKextBundle && opts.allowTextRelocs() )
shim = new NoPICARMtoThumbShimAtom(target, *sect);
else
shim = new ARMtoThumbShimAtom(target, *sect);
shims.push_back(shim);
atomToThumbMap[target] = shim;
}
else {
shim = pos->second;
}
fit->binding = ld::Fixup::bindingDirectlyBound;
fit->u.target = shim;
}
}
break;
//case ld::Fixup::kindStoreARMBranch24:
//case ld::Fixup::kindStoreThumbBranch22:
// Note: these fixups will only be seen if the the b/bl is to a symbol plus addend
// for now we don't handle making shims. If a shim is needed there will
// be an error later.
// break;
default:
break;
}
}
}
// append all new shims to end of __text
sect->atoms.insert(sect->atoms.end(), shims.begin(), shims.end());
}
}
