[lld] r238936 - COFF: Change OutputSections' type from vector<unique_ptr<T>> to vector<T*>.
David Blaikie
dblaikie at gmail.com
Thu Jun 4 15:46:32 PDT 2015
On Thu, Jun 4, 2015 at 3:41 PM, Rui Ueyama <ruiu at google.com> wrote:
> Not sure if that's applicable here because this is a frequent pattern in
> this code base. If you read other files and then this file, this should not
> be strange at least.
>
I'd still be in favor of unque_ptr as the default unless there's extra
semantics/behavior that's desired of something else (basically:
performance). But it's not my codebase/project - so take that or leave it.
> As to the dtor, this bump pointer is SpecificBumpPtrAllocator, which I
> believe calls dtors.
>
I believe that only happens if you call destroyAll.
>
> On Thu, Jun 4, 2015 at 2:41 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>>
>>
>> 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;
>>>>>> >> >
>>>>>> >> >
>>>>>> >> > _______________________________________________
>>>>>> >> > llvm-commits mailing list
>>>>>> >> > llvm-commits at cs.uiuc.edu
>>>>>> >> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>>>>> >
>>>>>> >
>>>>>> _______________________________________________
>>>>>> llvm-commits mailing list
>>>>>> llvm-commits at cs.uiuc.edu
>>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> llvm-commits mailing list
>>>>> llvm-commits at cs.uiuc.edu
>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>>>>
>>>>>
>>>>
>>>
>>
>> _______________________________________________
>> llvm-commits mailing list
>> llvm-commits at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20150604/d0d3b259/attachment.html>
More information about the llvm-commits
mailing list