[lld] r193017 - [PECOFF] Only COMDAT symbols are allowed to be dead-stripped.
Shankar Easwaran
shankare at codeaurora.org
Wed Oct 23 16:01:13 PDT 2013
Yet to be added. Not in there now :)
On 10/23/2013 5:13 PM, Rui Ueyama wrote:
> On Wed, Oct 23, 2013 at 2:49 PM, Shankar Easwaran
> <shankare at codeaurora.org>wrote:
>
>> It would remove only if .init_array is not added to the root set. By
>> default the reader should make sure certain sections like
>> .ctors/.dtors/.init_array ... gets added to the rootset.
>>
> Where is the code? I looked for addDeadStripRoot and deadStripNever but
> couldn't find it.
>
>
>> Thanks
>>
>> Shankar Easwaran
>>
>>
>> On 10/23/2013 4:45 PM, Rui Ueyama wrote:
>>
>> I think GNU ld keeps .init_array sections using KEEP linker script macro
>> command. Because LLD currently does not the command, I believe it would
>> remove .init_array sections, which is wrong.
>> https://sourceware.org/binutils/docs/ld/Input-Section-Keep.html#Input-Section-Keep
>>
>> In COFF, looks like there's no generic mechanism to tell the linker which
>> sections are safe to be GC'ed, and the linker is allowed to remove only
>> COMDAT symbols. That's different from ELF.
>>
>>
>> On Wed, Oct 23, 2013 at 2:14 PM, Reid Kleckner <rnk at google.com> <rnk at google.com> wrote:
>>
>>
>> ELF uses exactly the same technique for .init_array, and you can use
>> .init_array.priority to get adjust the order of your initializers. Are you
>> handling it the same way that ELF does, or is this something else?
>>
>> I think link.exe will only dead strip if you use /Gy, which has the effect
>> of putting all functions in COMDATs. It looks like it will not dead strip
>> data like these function pointers.
>>
>>
>> On Fri, Oct 18, 2013 at 4:54 PM, Rui Ueyama <ruiu at google.com> <ruiu at google.com> wrote:
>>
>>
>> Author: ruiu
>> Date: Fri Oct 18 18:54:55 2013
>> New Revision: 193017
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=193017&view=rev
>> Log:
>> [PECOFF] Only COMDAT symbols are allowed to be dead-stripped.
>>
>> We should dead-strip atoms only if they are created for COMDAT symbols.
>> If we
>> remove non-COMDAT atoms from a binary, it will no longer be guaranteed
>> that
>> the binary will work correctly.
>>
>> In COFF, you can manipulate the order of section contents in the resulting
>> binary by section name. For example, if you have four sections
>> .data$unique_prefix_{a,b,c,d}, it's guaranteed that the contents of A, B,
>> C,
>> and D will be consecutive in the resulting .data section in that order.
>> Thus, you can access B's and C's contents by incrementing a pointer
>> pointing
>> to A until it reached to D. That's why we cannot dead-strip B or C even if
>> no one is directly referencing to them.
>>
>> Some object files in the standard library actually use that technique.
>>
>> Modified:
>> lld/trunk/lib/ReaderWriter/PECOFF/Atoms.h
>> lld/trunk/lib/ReaderWriter/PECOFF/ReaderCOFF.cpp
>> lld/trunk/test/pecoff/grouped-sections.test
>>
>> Modified: lld/trunk/lib/ReaderWriter/PECOFF/Atoms.h
>> URL:http://llvm.org/viewvc/llvm-project/lld/trunk/lib/ReaderWriter/PECOFF/Atoms.h?rev=193017&r1=193016&r2=193017&view=diff
>>
>> ==============================================================================
>> --- lld/trunk/lib/ReaderWriter/PECOFF/Atoms.h (original)
>> +++ lld/trunk/lib/ReaderWriter/PECOFF/Atoms.h Fri Oct 18 18:54:55 2013
>> @@ -181,17 +181,24 @@ private:
>> class COFFDefinedAtom : public COFFDefinedFileAtom {
>> public:
>> COFFDefinedAtom(const File &file, StringRef name, StringRef
>> sectionName,
>> - Scope scope, ContentType type, ContentPermissions
>> perms,
>> - Merge merge, ArrayRef<uint8_t> data, uint64_t ordinal)
>> + Scope scope, ContentType type, bool isComdat,
>> + ContentPermissions perms, Merge merge,
>> ArrayRef<uint8_t> data,
>> + uint64_t ordinal)
>> : COFFDefinedFileAtom(file, name, sectionName, scope, type, perms,
>> ordinal),
>> - _merge(merge), _dataref(data) {}
>> + _isComdat(isComdat), _merge(merge), _dataref(data) {}
>>
>> virtual Merge merge() const { return _merge; }
>> virtual uint64_t size() const { return _dataref.size(); }
>> virtual ArrayRef<uint8_t> rawContent() const { return _dataref; }
>>
>> + virtual DeadStripKind deadStrip() const {
>> + // Only COMDAT symbols would be dead-stripped.
>> + return _isComdat ? deadStripNormal : deadStripNever;
>> + }
>> +
>> private:
>> + bool _isComdat;
>> Merge _merge;
>> ArrayRef<uint8_t> _dataref;
>> };
>>
>> Modified: lld/trunk/lib/ReaderWriter/PECOFF/ReaderCOFF.cpp
>> URL:http://llvm.org/viewvc/llvm-project/lld/trunk/lib/ReaderWriter/PECOFF/ReaderCOFF.cpp?rev=193017&r1=193016&r2=193017&view=diff
>>
>> ==============================================================================
>> --- lld/trunk/lib/ReaderWriter/PECOFF/ReaderCOFF.cpp (original)
>> +++ lld/trunk/lib/ReaderWriter/PECOFF/ReaderCOFF.cpp Fri Oct 18 18:54:55
>> 2013
>> @@ -397,6 +397,8 @@ private:
>> if (!(sec->Characteristics & llvm::COFF::IMAGE_SCN_LNK_COMDAT))
>> continue;
>>
>> + _comdatSections.insert(sec);
>> +
>> if (sym->NumberOfAuxSymbols == 0)
>> return llvm::object::object_error::parse_failed;
>> const coff_aux_section_definition *aux =
>> @@ -473,13 +475,14 @@ private:
>>
>> DefinedAtom::ContentType type = getContentType(section);
>> DefinedAtom::ContentPermissions perms = getPermissions(section);
>> + bool isComdat = (_comdatSections.count(section) == 1);
>>
>> // Create an atom for the entire section.
>> if (symbols.empty()) {
>> ArrayRef<uint8_t> data(secData.data(), secData.size());
>> auto *atom = new (_alloc)
>> COFFDefinedAtom(*this, "", sectionName,
>> Atom::scopeTranslationUnit,
>> - type, perms, _merge[section], data, 0);
>> + type, isComdat, perms, _merge[section], data,
>> 0);
>> atoms.push_back(atom);
>> _definedAtomLocations[section][0].push_back(atom);
>> return error_code::success();
>> @@ -490,9 +493,9 @@ private:
>> if (symbols[0]->Value != 0) {
>> uint64_t size = symbols[0]->Value;
>> ArrayRef<uint8_t> data(secData.data(), size);
>> - auto *atom = new (_alloc)
>> - COFFDefinedAtom(*this, "", sectionName,
>> Atom::scopeTranslationUnit,
>> - type, perms, _merge[section], data, ++ordinal);
>> + auto *atom = new (_alloc) COFFDefinedAtom(
>> + *this, "", sectionName, Atom::scopeTranslationUnit, type,
>> isComdat,
>> + perms, _merge[section], data, ++ordinal);
>> atoms.push_back(atom);
>> _definedAtomLocations[section][0].push_back(atom);
>> }
>> @@ -503,9 +506,9 @@ private:
>> const uint8_t *end = (si + 1 == se) ? secData.data() +
>> secData.size()
>> : secData.data() + (*(si +
>> 1))->Value;
>> ArrayRef<uint8_t> data(start, end);
>> - auto *atom = new (_alloc)
>> - COFFDefinedAtom(*this, _symbolName[*si], sectionName,
>> getScope(*si),
>> - type, perms, _merge[section], data, ++ordinal);
>> + auto *atom = new (_alloc) COFFDefinedAtom(
>> + *this, _symbolName[*si], sectionName, getScope(*si), type,
>> isComdat,
>> + perms, _merge[section], data, ++ordinal);
>> atoms.push_back(atom);
>> _symbolAtom[*si] = atom;
>> _definedAtomLocations[section][(*si)->Value].push_back(atom);
>> @@ -696,6 +699,9 @@ private:
>> // A map from section to its atoms.
>> std::map<const coff_section *, vector<COFFDefinedFileAtom *> >
>> _sectionAtoms;
>>
>> + // A set of COMDAT sections.
>> + std::set<const coff_section *> _comdatSections;
>> +
>> // A map to get whether the section allows its contents to be merged
>> or not.
>> std::map<const coff_section *, DefinedAtom::Merge> _merge;
>>
>>
>> Modified: lld/trunk/test/pecoff/grouped-sections.test
>> URL:http://llvm.org/viewvc/llvm-project/lld/trunk/test/pecoff/grouped-sections.test?rev=193017&r1=193016&r2=193017&view=diff
>>
>> ==============================================================================
>> --- lld/trunk/test/pecoff/grouped-sections.test (original)
>> +++ lld/trunk/test/pecoff/grouped-sections.test Fri Oct 18 18:54:55 2013
>> @@ -1,6 +1,6 @@
>> # RUN: yaml2obj %p/Inputs/grouped-sections.obj.yaml > %t.obj
>> #
>> -# RUN: lld -flavor link /out:%t1 /subsystem:console /force /opt:noref \
>> +# RUN: lld -flavor link /out:%t1 /subsystem:console /force \
>> # RUN: -- %t.obj && llvm-objdump -s %t1 | FileCheck %s
>> #
>> # The file "grouped-sections.obj" has three data sections in the
>> following
>>
>>
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>>
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