[lld] r238936 - COFF: Change OutputSections' type from vector<unique_ptr<T>> to vector<T*>.
Rui Ueyama
ruiu at google.com
Wed Jun 3 10:39:47 PDT 2015
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.
Reading only this patch could be a bit confusing, but this pattern appears
many times in other places, and doing this makes things more consistent.
File objects owns a lot of chunks and symbols (that are allocated using
BumpPtrAllocator or placement new for performance reason), and we have
naked pointers to them.
> 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.
That is true, but this small code doesn't have to be bullet-proof.
Consistency and clarity are more important in this case.
> ~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
> >> > 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
> >> >
> >> >
> ==============================================================================
> >> > --- 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
> >> >
> >> >
> ==============================================================================
> >> > --- 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
> >
> >
>
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