lld/MachO/MarkLive.cpp - llvm-project - Git at Google

 //===- MarkLive.cpp -------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//

 #include "MarkLive.h"
 #include "Config.h"
 #include "OutputSegment.h"
 #include "SymbolTable.h"
 #include "Symbols.h"
 #include "UnwindInfoSection.h"

 #include "lld/Common/ErrorHandler.h"
 #include "llvm/Support/TimeProfiler.h"

 namespace lld::macho {

 using namespace llvm;
 using namespace llvm::MachO;

 struct WhyLiveEntry {
   InputSection *isec;
   // Keep track of the entry that caused us to mark `isec` as live.
   const WhyLiveEntry *prev;

   WhyLiveEntry(InputSection *isec, const WhyLiveEntry *prev)
       : isec(isec), prev(prev) {}
 };

 // Type-erased interface to MarkLiveImpl. Used for adding roots to the liveness
 // graph.
 class MarkLive {
 public:
   virtual void enqueue(InputSection *isec, uint64_t off) = 0;
   virtual void addSym(Symbol *s) = 0;
   virtual void markTransitively() = 0;
   virtual ~MarkLive() = default;
 };

 template <bool RecordWhyLive> class MarkLiveImpl : public MarkLive {
 public:
   // -why_live is a rarely used option, so we don't want support for that flag
   // to slow down the main -dead_strip code path. As such, we employ templates
   // to avoid the usage of WhyLiveEntry in the main code path. This saves us
   // from needless allocations and pointer indirections.
   using WorklistEntry =
       std::conditional_t<RecordWhyLive, WhyLiveEntry, InputSection>;

   void enqueue(InputSection *isec, uint64_t off) override {
     enqueue(isec, off, nullptr);
   }
   void addSym(Symbol *s) override { addSym(s, nullptr); }
   void markTransitively() override;

 private:
   void enqueue(InputSection *isec, uint64_t off, const WorklistEntry *prev);
   void addSym(Symbol *s, const WorklistEntry *prev);
   const InputSection *getInputSection(const WorklistEntry *) const;
   WorklistEntry *makeEntry(InputSection *, const WorklistEntry *prev) const;

   // We build up a worklist of sections which have been marked as live. We
   // only push into the worklist when we discover an unmarked section, and we
   // mark as we push, so sections never appear twice in the list. Literal
   // sections cannot contain references to other sections, so we only store
   // ConcatInputSections in our worklist.
   SmallVector<WorklistEntry *, 256> worklist;
 };

 template <bool RecordWhyLive>
 void MarkLiveImpl<RecordWhyLive>::enqueue(
     InputSection *isec, uint64_t off,
     const typename MarkLiveImpl<RecordWhyLive>::WorklistEntry *prev) {
   if (isec->isLive(off))
     return;
   isec->markLive(off);
   if (auto s = dyn_cast<ConcatInputSection>(isec)) {
     assert(!s->isCoalescedWeak());
     worklist.push_back(makeEntry(s, prev));
   }
 }

 static void printWhyLive(const Symbol *s, const WhyLiveEntry *prev) {
   std::string out = toString(*s) + " from " + toString(s->getFile());
   int indent = 2;
   for (const WhyLiveEntry *entry = prev; entry;
        entry = entry->prev, indent += 2) {
     const TinyPtrVector<Defined *> &symbols = entry->isec->symbols;
     // With .subsections_with_symbols set, most isecs will have exactly one
     // entry in their symbols vector, so we just print the first one.
     if (!symbols.empty())
       out += "\n" + std::string(indent, ' ') + toString(*symbols.front()) +
              " from " + toString(symbols.front()->getFile());
   }
   message(out);
 }

 template <bool RecordWhyLive>
 void MarkLiveImpl<RecordWhyLive>::addSym(
     Symbol *s,
     const typename MarkLiveImpl<RecordWhyLive>::WorklistEntry *prev) {
   if (s->used)
     return;
   s->used = true;
   if constexpr (RecordWhyLive)
     if (!config->whyLive.empty() && config->whyLive.match(s->getName()))
       printWhyLive(s, prev);
   if (auto *d = dyn_cast<Defined>(s)) {
     if (d->isec())
       enqueue(d->isec(), d->value, prev);
     if (d->unwindEntry())
       enqueue(d->unwindEntry(), 0, prev);
   }
 }

 template <bool RecordWhyLive>
 const InputSection *MarkLiveImpl<RecordWhyLive>::getInputSection(
     const MarkLiveImpl<RecordWhyLive>::WorklistEntry *entry) const {
   if constexpr (RecordWhyLive)
     return entry->isec;
   else
     return entry;
 }

 template <bool RecordWhyLive>
 typename MarkLiveImpl<RecordWhyLive>::WorklistEntry *
 MarkLiveImpl<RecordWhyLive>::makeEntry(
     InputSection *isec,
     const MarkLiveImpl<RecordWhyLive>::WorklistEntry *prev) const {
   if constexpr (RecordWhyLive) {
     if (!isec) {
       assert(!prev);
       return nullptr;
     }
     return make<WhyLiveEntry>(isec, prev);
   } else {
     return isec;
   }
 }

 template <bool RecordWhyLive>
 void MarkLiveImpl<RecordWhyLive>::markTransitively() {
   do {
     // Mark things reachable from GC roots as live.
     while (!worklist.empty()) {
       WorklistEntry *entry = worklist.pop_back_val();
       // Entries that get placed onto the worklist always contain
       // ConcatInputSections. `WhyLiveEntry::prev` may point to entries that
       // contain other types of InputSections (due to S_ATTR_LIVE_SUPPORT), but
       // those entries should never be pushed onto the worklist.
       auto *isec = cast<ConcatInputSection>(getInputSection(entry));
       assert(isec->live && "We mark as live when pushing onto the worklist!");

       // Mark all symbols listed in the relocation table for this section.
       for (const Reloc &r : isec->relocs) {
         if (auto *s = r.referent.dyn_cast<Symbol *>())
           addSym(s, entry);
         else
           enqueue(cast<InputSection *>(r.referent), r.addend, entry);
       }
       for (Defined *d : getInputSection(entry)->symbols)
         addSym(d, entry);
     }

     // S_ATTR_LIVE_SUPPORT sections are live if they point _to_ a live
     // section. Process them in a second pass.
     for (ConcatInputSection *isec : inputSections) {
       // FIXME: Check if copying all S_ATTR_LIVE_SUPPORT sections into a
       // separate vector and only walking that here is faster.
       if (!(isec->getFlags() & S_ATTR_LIVE_SUPPORT) || isec->live)
         continue;

       for (const Reloc &r : isec->relocs) {
         if (auto *s = r.referent.dyn_cast<Symbol *>()) {
           if (s->isLive()) {
             InputSection *referentIsec = nullptr;
             if (auto *d = dyn_cast<Defined>(s))
               referentIsec = d->isec();
             enqueue(isec, 0, makeEntry(referentIsec, nullptr));
           }
         } else {
           auto *referentIsec = cast<InputSection *>(r.referent);
           if (referentIsec->isLive(r.addend))
             enqueue(isec, 0, makeEntry(referentIsec, nullptr));
         }
       }
     }

     // S_ATTR_LIVE_SUPPORT could have marked additional sections live,
     // which in turn could mark additional S_ATTR_LIVE_SUPPORT sections live.
     // Iterate. In practice, the second iteration won't mark additional
     // S_ATTR_LIVE_SUPPORT sections live.
   } while (!worklist.empty());
 }

 // Set live bit on for each reachable chunk. Unmarked (unreachable)
 // InputSections will be ignored by Writer, so they will be excluded
 // from the final output.
 void markLive() {
   TimeTraceScope timeScope("markLive");
   MarkLive *marker;
   if (config->whyLive.empty())
     marker = make<MarkLiveImpl<false>>();
   else
     marker = make<MarkLiveImpl<true>>();
   // Add GC roots.
   if (config->entry)
     marker->addSym(config->entry);
   for (Symbol *sym : symtab->getSymbols()) {
     if (auto *defined = dyn_cast<Defined>(sym)) {
       // -exported_symbol(s_list)
       if (!config->exportedSymbols.empty() &&
           config->exportedSymbols.match(defined->getName())) {
         // NOTE: Even though exporting private externs is an ill-defined
         // operation, we are purposely not checking for privateExtern in
         // order to follow ld64's behavior of treating all exported private
         // extern symbols as live, irrespective of whether they are autohide.
         marker->addSym(defined);
         continue;
       }

       // public symbols explicitly marked .no_dead_strip
       if (defined->referencedDynamically || defined->noDeadStrip) {
         marker->addSym(defined);
         continue;
       }

       // FIXME: When we implement these flags, make symbols from them GC
       // roots:
       // * -reexported_symbol(s_list)
       // * -alias_list
       // * -init

       // In dylibs and bundles and in executables with -export_dynamic,
       // all external functions are GC roots.
       bool externsAreRoots =
           config->outputType != MH_EXECUTE || config->exportDynamic;
       if (externsAreRoots && !defined->privateExtern) {
         marker->addSym(defined);
         continue;
       }
     }
   }
   // -u symbols
   for (Symbol *sym : config->explicitUndefineds)
     marker->addSym(sym);
   // local symbols explicitly marked .no_dead_strip
   for (const InputFile *file : inputFiles)
     if (auto *objFile = dyn_cast<ObjFile>(file))
       for (Symbol *sym : objFile->symbols)
         if (auto *defined = dyn_cast_or_null<Defined>(sym))
           if (!defined->isExternal() && defined->noDeadStrip)
             marker->addSym(defined);
   if (auto *stubBinder =
           dyn_cast_or_null<DylibSymbol>(symtab->find("dyld_stub_binder")))
     marker->addSym(stubBinder);
   for (ConcatInputSection *isec : inputSections) {
     // Sections marked no_dead_strip
     if (isec->getFlags() & S_ATTR_NO_DEAD_STRIP) {
       marker->enqueue(isec, 0);
       continue;
     }

     // mod_init_funcs, mod_term_funcs sections
     if (sectionType(isec->getFlags()) == S_MOD_INIT_FUNC_POINTERS ||
         sectionType(isec->getFlags()) == S_MOD_TERM_FUNC_POINTERS) {
       assert(!config->emitInitOffsets ||
              sectionType(isec->getFlags()) != S_MOD_INIT_FUNC_POINTERS);
       marker->enqueue(isec, 0);
       continue;
     }
   }

   for (ConcatInputSection *isec : in.initOffsets->inputs())
     marker->enqueue(isec, 0);

   marker->markTransitively();
 }

 } // namespace lld::macho
	//===- MarkLive.cpp -------------------------------------------------------===//
	//
	// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
	// See https://llvm.org/LICENSE.txt for license information.
	// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
	//
	//===----------------------------------------------------------------------===//

	#include "MarkLive.h"
	#include "Config.h"
	#include "OutputSegment.h"
	#include "SymbolTable.h"
	#include "Symbols.h"
	#include "UnwindInfoSection.h"

	#include "lld/Common/ErrorHandler.h"
	#include "llvm/Support/TimeProfiler.h"

	namespace lld::macho {

	using namespace llvm;
	using namespace llvm::MachO;

	struct WhyLiveEntry {
	InputSection *isec;
	// Keep track of the entry that caused us to mark `isec` as live.
	const WhyLiveEntry *prev;

	WhyLiveEntry(InputSection isec, const WhyLiveEntry prev)
	: isec(isec), prev(prev) {}
	};

	// Type-erased interface to MarkLiveImpl. Used for adding roots to the liveness
	// graph.
	class MarkLive {
	public:
	virtual void enqueue(InputSection *isec, uint64_t off) = 0;
	virtual void addSym(Symbol *s) = 0;
	virtual void markTransitively() = 0;
	virtual ~MarkLive() = default;
	};

	template <bool RecordWhyLive> class MarkLiveImpl : public MarkLive {
	public:
	// -why_live is a rarely used option, so we don't want support for that flag
	// to slow down the main -dead_strip code path. As such, we employ templates
	// to avoid the usage of WhyLiveEntry in the main code path. This saves us
	// from needless allocations and pointer indirections.
	using WorklistEntry =
	std::conditional_t<RecordWhyLive, WhyLiveEntry, InputSection>;

	void enqueue(InputSection *isec, uint64_t off) override {
	enqueue(isec, off, nullptr);
	}
	void addSym(Symbol *s) override { addSym(s, nullptr); }
	void markTransitively() override;

	private:
	void enqueue(InputSection isec, uint64_t off, const WorklistEntry prev);
	void addSym(Symbol s, const WorklistEntry prev);
	const InputSection getInputSection(const WorklistEntry ) const;
	WorklistEntry makeEntry(InputSection , const WorklistEntry *prev) const;

	// We build up a worklist of sections which have been marked as live. We
	// only push into the worklist when we discover an unmarked section, and we
	// mark as we push, so sections never appear twice in the list. Literal
	// sections cannot contain references to other sections, so we only store
	// ConcatInputSections in our worklist.
	SmallVector<WorklistEntry *, 256> worklist;
	};

	template <bool RecordWhyLive>
	void MarkLiveImpl<RecordWhyLive>::enqueue(
	InputSection *isec, uint64_t off,
	const typename MarkLiveImpl<RecordWhyLive>::WorklistEntry *prev) {
	if (isec->isLive(off))
	return;
	isec->markLive(off);
	if (auto s = dyn_cast<ConcatInputSection>(isec)) {
	assert(!s->isCoalescedWeak());
	worklist.push_back(makeEntry(s, prev));
	}
	}

	static void printWhyLive(const Symbol s, const WhyLiveEntry prev) {
	std::string out = toString(*s) + " from " + toString(s->getFile());
	int indent = 2;
	for (const WhyLiveEntry *entry = prev; entry;
	entry = entry->prev, indent += 2) {
	const TinyPtrVector<Defined *> &symbols = entry->isec->symbols;
	// With .subsections_with_symbols set, most isecs will have exactly one
	// entry in their symbols vector, so we just print the first one.
	if (!symbols.empty())
	out += "\n" + std::string(indent, ' ') + toString(*symbols.front()) +
	" from " + toString(symbols.front()->getFile());
	}
	message(out);
	}

	template <bool RecordWhyLive>
	void MarkLiveImpl<RecordWhyLive>::addSym(
	Symbol *s,
	const typename MarkLiveImpl<RecordWhyLive>::WorklistEntry *prev) {
	if (s->used)
	return;
	s->used = true;
	if constexpr (RecordWhyLive)
	if (!config->whyLive.empty() && config->whyLive.match(s->getName()))
	printWhyLive(s, prev);
	if (auto *d = dyn_cast<Defined>(s)) {
	if (d->isec())
	enqueue(d->isec(), d->value, prev);
	if (d->unwindEntry())
	enqueue(d->unwindEntry(), 0, prev);
	}
	}

	template <bool RecordWhyLive>
	const InputSection *MarkLiveImpl<RecordWhyLive>::getInputSection(
	const MarkLiveImpl<RecordWhyLive>::WorklistEntry *entry) const {
	if constexpr (RecordWhyLive)
	return entry->isec;
	else
	return entry;
	}

	template <bool RecordWhyLive>
	typename MarkLiveImpl<RecordWhyLive>::WorklistEntry *
	MarkLiveImpl<RecordWhyLive>::makeEntry(
	InputSection *isec,
	const MarkLiveImpl<RecordWhyLive>::WorklistEntry *prev) const {
	if constexpr (RecordWhyLive) {
	if (!isec) {
	assert(!prev);
	return nullptr;
	}
	return make<WhyLiveEntry>(isec, prev);
	} else {
	return isec;
	}
	}

	template <bool RecordWhyLive>
	void MarkLiveImpl<RecordWhyLive>::markTransitively() {
	do {
	// Mark things reachable from GC roots as live.
	while (!worklist.empty()) {
	WorklistEntry *entry = worklist.pop_back_val();
	// Entries that get placed onto the worklist always contain
	// ConcatInputSections. `WhyLiveEntry::prev` may point to entries that
	// contain other types of InputSections (due to S_ATTR_LIVE_SUPPORT), but
	// those entries should never be pushed onto the worklist.
	auto *isec = cast<ConcatInputSection>(getInputSection(entry));
	assert(isec->live && "We mark as live when pushing onto the worklist!");

	// Mark all symbols listed in the relocation table for this section.
	for (const Reloc &r : isec->relocs) {
	if (auto s = r.referent.dyn_cast<Symbol >())
	addSym(s, entry);
	else
	enqueue(cast<InputSection *>(r.referent), r.addend, entry);
	}
	for (Defined *d : getInputSection(entry)->symbols)
	addSym(d, entry);
	}

	// S_ATTR_LIVE_SUPPORT sections are live if they point _to_ a live
	// section. Process them in a second pass.
	for (ConcatInputSection *isec : inputSections) {
	// FIXME: Check if copying all S_ATTR_LIVE_SUPPORT sections into a
	// separate vector and only walking that here is faster.
	if (!(isec->getFlags() & S_ATTR_LIVE_SUPPORT) \|\| isec->live)
	continue;

	for (const Reloc &r : isec->relocs) {
	if (auto s = r.referent.dyn_cast<Symbol >()) {
	if (s->isLive()) {
	InputSection *referentIsec = nullptr;
	if (auto *d = dyn_cast<Defined>(s))
	referentIsec = d->isec();
	enqueue(isec, 0, makeEntry(referentIsec, nullptr));
	}
	} else {
	auto referentIsec = cast<InputSection >(r.referent);
	if (referentIsec->isLive(r.addend))
	enqueue(isec, 0, makeEntry(referentIsec, nullptr));
	}
	}
	}

	// S_ATTR_LIVE_SUPPORT could have marked additional sections live,
	// which in turn could mark additional S_ATTR_LIVE_SUPPORT sections live.
	// Iterate. In practice, the second iteration won't mark additional
	// S_ATTR_LIVE_SUPPORT sections live.
	} while (!worklist.empty());
	}

	// Set live bit on for each reachable chunk. Unmarked (unreachable)
	// InputSections will be ignored by Writer, so they will be excluded
	// from the final output.
	void markLive() {
	TimeTraceScope timeScope("markLive");
	MarkLive *marker;
	if (config->whyLive.empty())
	marker = make<MarkLiveImpl<false>>();
	else
	marker = make<MarkLiveImpl<true>>();
	// Add GC roots.
	if (config->entry)
	marker->addSym(config->entry);
	for (Symbol *sym : symtab->getSymbols()) {
	if (auto *defined = dyn_cast<Defined>(sym)) {
	// -exported_symbol(s_list)
	if (!config->exportedSymbols.empty() &&
	config->exportedSymbols.match(defined->getName())) {
	// NOTE: Even though exporting private externs is an ill-defined
	// operation, we are purposely not checking for privateExtern in
	// order to follow ld64's behavior of treating all exported private
	// extern symbols as live, irrespective of whether they are autohide.
	marker->addSym(defined);
	continue;
	}

	// public symbols explicitly marked .no_dead_strip
	if (defined->referencedDynamically \|\| defined->noDeadStrip) {
	marker->addSym(defined);
	continue;
	}

	// FIXME: When we implement these flags, make symbols from them GC
	// roots:
	// * -reexported_symbol(s_list)
	// * -alias_list
	// * -init

	// In dylibs and bundles and in executables with -export_dynamic,
	// all external functions are GC roots.
	bool externsAreRoots =
	config->outputType != MH_EXECUTE \|\| config->exportDynamic;
	if (externsAreRoots && !defined->privateExtern) {
	marker->addSym(defined);
	continue;
	}
	}
	}
	// -u symbols
	for (Symbol *sym : config->explicitUndefineds)
	marker->addSym(sym);
	// local symbols explicitly marked .no_dead_strip
	for (const InputFile *file : inputFiles)
	if (auto *objFile = dyn_cast<ObjFile>(file))
	for (Symbol *sym : objFile->symbols)
	if (auto *defined = dyn_cast_or_null<Defined>(sym))
	if (!defined->isExternal() && defined->noDeadStrip)
	marker->addSym(defined);
	if (auto *stubBinder =
	dyn_cast_or_null<DylibSymbol>(symtab->find("dyld_stub_binder")))
	marker->addSym(stubBinder);
	for (ConcatInputSection *isec : inputSections) {
	// Sections marked no_dead_strip
	if (isec->getFlags() & S_ATTR_NO_DEAD_STRIP) {
	marker->enqueue(isec, 0);
	continue;
	}

	// mod_init_funcs, mod_term_funcs sections
	if (sectionType(isec->getFlags()) == S_MOD_INIT_FUNC_POINTERS \|\|
	sectionType(isec->getFlags()) == S_MOD_TERM_FUNC_POINTERS) {
	assert(!config->emitInitOffsets \|\|
	sectionType(isec->getFlags()) != S_MOD_INIT_FUNC_POINTERS);
	marker->enqueue(isec, 0);
	continue;
	}
	}

	for (ConcatInputSection *isec : in.initOffsets->inputs())
	marker->enqueue(isec, 0);

	marker->markTransitively();
	}

	} // namespace lld::macho