[lld] r238936 - COFF: Change OutputSections' type from vector<unique_ptr<T>> to vector<T*>.
David Blaikie
dblaikie at gmail.com
Thu Jun 4 14:41:08 PDT 2015
On Thu, Jun 4, 2015 at 2:37 PM, Sean Silva <chisophugis at gmail.com> wrote:
>
>
> On Thu, Jun 4, 2015 at 11:16 AM, David Blaikie <dblaikie at gmail.com> wrote:
>
>>
>>
>> On Wed, Jun 3, 2015 at 12:33 PM, Sean Silva <chisophugis at gmail.com>
>> wrote:
>>
>>>
>>>
>>> On Wed, Jun 3, 2015 at 10:27 AM, Aaron Ballman <aaron at aaronballman.com>
>>> wrote:
>>>
>>>> On Wed, Jun 3, 2015 at 1:15 PM, Rui Ueyama <ruiu at google.com> wrote:
>>>> > It has clear ownership semantics -- the writer always owns these
>>>> object.
>>>>
>>>> There are now naked pointers stored in a vector. I don't see that as
>>>> being clear ownership semantics. Nothing enforces that ownership. We
>>>> have this problem in a lot of places, but the push is usually to turn
>>>> those naked pointers into std::unique_ptr or some other smart pointer,
>>>> not the converse. That's why I was surprised at this commit and was
>>>> hoping there was some performance reason (or something else) to
>>>> justify it.
>>>>
>>>
>>> std::unique_ptr isn't the right choice when it isn't actually the thing
>>> keeping the memory alive, as is the case here.
>>>
>>
>> But the unique_ptrs were keeping the memory alive, until the
>> BumpPtrAllocator was added in this patch, no? We aren't usually adding
>> 'interesting' allocation techniques except where they're justified/required
>> for performance, in my experience.
>>
>> Or am I missing something subtle about how the allocation/ownership
>> worked prior to this patch?
>>
>
> Yeah, I think you're right. I must have understood the existing code.
> Actually the current state seems leaky since the destructor of
> `std::vector<Chunk *> Chunks;` inside OutputSection doesn't seem to be
> called.
>
*nod* I'm going to +1 Aaron's original feedback: this patch doesn't seem
like an obvious readability (or correctness) improvement. I think it's
reasonable to prefer unique_ptr over bump pointer allocator, all things
being equal (in the absence of evidence that the allocation is a hot
path/cost that needs to be reduced - and in that case, it requires some
care to check that the different semantics (not running dtors, mostly) are
applicable)
>
> -- Sean Silva
>
>
>>
>>>
>>> -- Sean Silva
>>>
>>>
>>>>
>>>> > I
>>>> > think it is more clear on that point than before because the ownership
>>>> > cannot be transferred by any means now.
>>>>
>>>> You can assign these pointers, let them escape the parent container,
>>>> and you can call delete on them. All of these are bad ideas that
>>>> people should not do, but nothing prevents that from happening any
>>>> longer.
>>>>
>>>> ~Aaron
>>>>
>>>> >
>>>> > On Wed, Jun 3, 2015 at 9:55 AM, Aaron Ballman <aaron at aaronballman.com
>>>> >
>>>> > wrote:
>>>> >>
>>>> >> On Wed, Jun 3, 2015 at 12:44 PM, Rui Ueyama <ruiu at google.com> wrote:
>>>> >> > Author: ruiu
>>>> >> > Date: Wed Jun 3 11:44:00 2015
>>>> >> > New Revision: 238936
>>>> >> >
>>>> >> > URL: http://llvm.org/viewvc/llvm-project?rev=238936&view=rev
>>>> <https://urldefense.proofpoint.com/v2/url?u=http-3A__llvm.org_viewvc_llvm-2Dproject-3Frev-3D238936-26view-3Drev&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=mQ4LZ2PUj9hpadE3cDHZnIdEwhEBrbAstXeMaFoB9tg&m=OulgiZemPQ2F5klDUiM5ii6E1-R2Skzn4fBikFgKkFw&s=cDW7ZXt9L2VDUMzIFcgEWcYS5dv3svFt_LxWrsovBcQ&e=>
>>>> >> > Log:
>>>> >> > COFF: Change OutputSections' type from vector<unique_ptr<T>> to
>>>> >> > vector<T*>.
>>>> >> >
>>>> >> > This is mainly for readability. OutputSection objects are still
>>>> owned
>>>> >> > by the writer using SpecificBumpPtrAllocator.
>>>> >>
>>>> >> I think clear ownership semantics are more readable than naked
>>>> >> pointers. If this is just for readability, it seems like a step in
>>>> the
>>>> >> wrong direction for maintenance. Am I simply missing something?
>>>> >>
>>>> >> ~Aaron
>>>> >>
>>>> >> >
>>>> >> > Modified:
>>>> >> > lld/trunk/COFF/Writer.cpp
>>>> >> > lld/trunk/COFF/Writer.h
>>>> >> >
>>>> >> > Modified: lld/trunk/COFF/Writer.cpp
>>>> >> > URL:
>>>> >> >
>>>> http://llvm.org/viewvc/llvm-project/lld/trunk/COFF/Writer.cpp?rev=238936&r1=238935&r2=238936&view=diff
>>>> <https://urldefense.proofpoint.com/v2/url?u=http-3A__llvm.org_viewvc_llvm-2Dproject_lld_trunk_COFF_Writer.cpp-3Frev-3D238936-26r1-3D238935-26r2-3D238936-26view-3Ddiff&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=mQ4LZ2PUj9hpadE3cDHZnIdEwhEBrbAstXeMaFoB9tg&m=OulgiZemPQ2F5klDUiM5ii6E1-R2Skzn4fBikFgKkFw&s=7_N7oOaPzOQDaUrLlcy3KRN6Xp3-2MoqLsCcvXqXGNA&e=>
>>>> >> >
>>>> >> >
>>>> ==============================================================================
>>>> >> > --- lld/trunk/COFF/Writer.cpp (original)
>>>> >> > +++ lld/trunk/COFF/Writer.cpp Wed Jun 3 11:44:00 2015
>>>> >> > @@ -117,14 +117,14 @@ void Writer::createSections() {
>>>> >> > StringRef SectionName = P.first;
>>>> >> > std::vector<Chunk *> &Chunks = P.second;
>>>> >> > std::stable_sort(Chunks.begin(), Chunks.end(), Comp);
>>>> >> > - auto Sec =
>>>> >> > - llvm::make_unique<OutputSection>(SectionName,
>>>> >> > OutputSections.size());
>>>> >> > + size_t SectIdx = OutputSections.size();
>>>> >> > + auto Sec = new (CAlloc.Allocate()) OutputSection(SectionName,
>>>> >> > SectIdx);
>>>> >> > for (Chunk *C : Chunks) {
>>>> >> > - C->setOutputSection(Sec.get());
>>>> >> > + C->setOutputSection(Sec);
>>>> >> > Sec->addChunk(C);
>>>> >> > Sec->addPermissions(C->getPermissions());
>>>> >> > }
>>>> >> > - OutputSections.push_back(std::move(Sec));
>>>> >> > + OutputSections.push_back(Sec);
>>>> >> > }
>>>> >> > }
>>>> >> >
>>>> >> > @@ -210,9 +210,7 @@ void Writer::createImportTables() {
>>>> >> > // The Windows loader doesn't seem to like empty sections,
>>>> >> > // so we remove them if any.
>>>> >> > void Writer::removeEmptySections() {
>>>> >> > - auto IsEmpty = [](const std::unique_ptr<OutputSection> &S) {
>>>> >> > - return S->getVirtualSize() == 0;
>>>> >> > - };
>>>> >> > + auto IsEmpty = [](OutputSection *S) { return
>>>> S->getVirtualSize() ==
>>>> >> > 0; };
>>>> >> > OutputSections.erase(
>>>> >> > std::remove_if(OutputSections.begin(), OutputSections.end(),
>>>> >> > IsEmpty),
>>>> >> > OutputSections.end());
>>>> >> > @@ -225,7 +223,7 @@ void Writer::assignAddresses() {
>>>> >> > HeaderSize + sizeof(coff_section) * OutputSections.size(),
>>>> >> > PageSize);
>>>> >> > uint64_t RVA = 0x1000; // The first page is kept unmapped.
>>>> >> > uint64_t FileOff = SizeOfHeaders;
>>>> >> > - for (std::unique_ptr<OutputSection> &Sec : OutputSections) {
>>>> >> > + for (OutputSection *Sec : OutputSections) {
>>>> >> > Sec->setRVA(RVA);
>>>> >> > Sec->setFileOffset(FileOff);
>>>> >> > RVA += RoundUpToAlignment(Sec->getVirtualSize(), PageSize);
>>>> >> > @@ -320,7 +318,7 @@ void Writer::writeHeader() {
>>>> >> > // Name field in the section table is 8 byte long. Longer names
>>>> need
>>>> >> > // to be written to the string table. First, construct string
>>>> table.
>>>> >> > std::vector<char> Strtab;
>>>> >> > - for (std::unique_ptr<OutputSection> &Sec : OutputSections) {
>>>> >> > + for (OutputSection *Sec : OutputSections) {
>>>> >> > StringRef Name = Sec->getName();
>>>> >> > if (Name.size() <= COFF::NameSize)
>>>> >> > continue;
>>>> >> > @@ -330,7 +328,7 @@ void Writer::writeHeader() {
>>>> >> > }
>>>> >> >
>>>> >> > // Write section table
>>>> >> > - for (std::unique_ptr<OutputSection> &Sec : OutputSections) {
>>>> >> > + for (OutputSection *Sec : OutputSections) {
>>>> >> > Sec->writeHeader(Buf);
>>>> >> > Buf += sizeof(coff_section);
>>>> >> > }
>>>> >> > @@ -365,7 +363,7 @@ std::error_code Writer::openFile(StringR
>>>> >> > // Write section contents to a mmap'ed file.
>>>> >> > void Writer::writeSections() {
>>>> >> > uint8_t *Buf = Buffer->getBufferStart();
>>>> >> > - for (std::unique_ptr<OutputSection> &Sec : OutputSections) {
>>>> >> > + for (OutputSection *Sec : OutputSections) {
>>>> >> > // Fill gaps between functions in .text with INT3 instructions
>>>> >> > // instead of leaving as NUL bytes (which can be interpreted
>>>> as
>>>> >> > // ADD instructions).
>>>> >> > @@ -377,15 +375,15 @@ void Writer::writeSections() {
>>>> >> > }
>>>> >> >
>>>> >> > OutputSection *Writer::findSection(StringRef Name) {
>>>> >> > - for (std::unique_ptr<OutputSection> &Sec : OutputSections)
>>>> >> > + for (OutputSection *Sec : OutputSections)
>>>> >> > if (Sec->getName() == Name)
>>>> >> > - return Sec.get();
>>>> >> > + return Sec;
>>>> >> > return nullptr;
>>>> >> > }
>>>> >> >
>>>> >> > uint32_t Writer::getSizeOfInitializedData() {
>>>> >> > uint32_t Res = 0;
>>>> >> > - for (std::unique_ptr<OutputSection> &S : OutputSections)
>>>> >> > + for (OutputSection *S : OutputSections)
>>>> >> > if (S->getPermissions() & IMAGE_SCN_CNT_INITIALIZED_DATA)
>>>> >> > Res += S->getRawSize();
>>>> >> > return Res;
>>>> >> > @@ -407,15 +405,16 @@ OutputSection *Writer::createSection(Str
>>>> >> > .Default(0);
>>>> >> > if (!Perm)
>>>> >> > llvm_unreachable("unknown section name");
>>>> >> > - auto Sec = new OutputSection(Name, OutputSections.size());
>>>> >> > + size_t SectIdx = OutputSections.size();
>>>> >> > + auto Sec = new (CAlloc.Allocate()) OutputSection(Name, SectIdx);
>>>> >> > Sec->addPermissions(Perm);
>>>> >> > - OutputSections.push_back(std::unique_ptr<OutputSection>(Sec));
>>>> >> > + OutputSections.push_back(Sec);
>>>> >> > return Sec;
>>>> >> > }
>>>> >> >
>>>> >> > void Writer::applyRelocations() {
>>>> >> > uint8_t *Buf = Buffer->getBufferStart();
>>>> >> > - for (std::unique_ptr<OutputSection> &Sec : OutputSections)
>>>> >> > + for (OutputSection *Sec : OutputSections)
>>>> >> > for (Chunk *C : Sec->getChunks())
>>>> >> > C->applyRelocations(Buf);
>>>> >> > }
>>>> >> >
>>>> >> > Modified: lld/trunk/COFF/Writer.h
>>>> >> > URL:
>>>> >> >
>>>> http://llvm.org/viewvc/llvm-project/lld/trunk/COFF/Writer.h?rev=238936&r1=238935&r2=238936&view=diff
>>>> <https://urldefense.proofpoint.com/v2/url?u=http-3A__llvm.org_viewvc_llvm-2Dproject_lld_trunk_COFF_Writer.h-3Frev-3D238936-26r1-3D238935-26r2-3D238936-26view-3Ddiff&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=mQ4LZ2PUj9hpadE3cDHZnIdEwhEBrbAstXeMaFoB9tg&m=OulgiZemPQ2F5klDUiM5ii6E1-R2Skzn4fBikFgKkFw&s=NvMaybesUToiXNxfE2TrkpG51eKPwxtEjUhpZWB4xrY&e=>
>>>> >> >
>>>> >> >
>>>> ==============================================================================
>>>> >> > --- lld/trunk/COFF/Writer.h (original)
>>>> >> > +++ lld/trunk/COFF/Writer.h Wed Jun 3 11:44:00 2015
>>>> >> > @@ -92,7 +92,8 @@ private:
>>>> >> >
>>>> >> > SymbolTable *Symtab;
>>>> >> > std::unique_ptr<llvm::FileOutputBuffer> Buffer;
>>>> >> > - std::vector<std::unique_ptr<OutputSection>> OutputSections;
>>>> >> > + llvm::SpecificBumpPtrAllocator<OutputSection> CAlloc;
>>>> >> > + std::vector<OutputSection *> OutputSections;
>>>> >> > Chunk *ImportAddressTable = nullptr;
>>>> >> > uint32_t ImportDirectoryTableSize = 0;
>>>> >> > uint32_t ImportAddressTableSize = 0;
>>>> >> >
>>>> >> >
>>>> >> > _______________________________________________
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>>>> >
>>>> >
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