r206324 - [Allocator] Make the ContentCache object actually carry the 8-byte

[Chandler Carruth]

On Wed, Apr 16, 2014 at 2:31 AM, Evgeniy Stepanov <eugeni.stepanov at gmail.com
> wrote:

> I see a flood of warnings in GCC 4.8 build about unused attribute
> being unused. Looks like GCC does not need or understand this
> attribute on member declarations. Do we need a new macro like
> LLVM_ATTRIBUTE_UNUSED_MEMBER ?
>

Someone else mentioned this, but I can't reproduce it. I suspect it just
needs the attribute in a different place, but I don't know how to debug it
because I don't see it... Let me try more extreme measures.


>
> [44/588] Building CXX object
> tools/clang/lib/Lex/CMakeFiles/clangLex.dir/PreprocessorLexer.cpp.o
> In file included from ../tools/clang/lib/Lex/PreprocessorLexer.cpp:15:0:
> ../tools/clang/include/clang/Basic/SourceManager.h:100:56: warning:
> ‘__unused__’ attribute ignored [-Wattributes]
>      LLVM_ATTRIBUTE_UNUSED llvm::AlignedCharArray<8, 1> NonceAligner;
>                                                         ^
>
> On Wed, Apr 16, 2014 at 1:34 AM, Chandler Carruth <chandlerc at gmail.com>
> wrote:
> > Author: chandlerc
> > Date: Tue Apr 15 16:34:12 2014
> > New Revision: 206324
> >
> > URL: http://llvm.org/viewvc/llvm-project?rev=206324&view=rev
> > Log:
> > [Allocator] Make the ContentCache object actually carry the 8-byte
> > alignment constraint rather than using the allocator function's over
> > alignment "feature". This was the only use of the "feature" which I plan
> > to remove next. =] Attaching the alignment to the type seems cleaner and
> > more principled anyways.
> >
> > Modified:
> >     cfe/trunk/include/clang/Basic/SourceManager.h
> >     cfe/trunk/lib/Basic/SourceManager.cpp
> >
> > Modified: cfe/trunk/include/clang/Basic/SourceManager.h
> > URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/SourceManager.h?rev=206324&r1=206323&r2=206324&view=diff
> >
> ==============================================================================
> > --- cfe/trunk/include/clang/Basic/SourceManager.h (original)
> > +++ cfe/trunk/include/clang/Basic/SourceManager.h Tue Apr 15 16:34:12
> 2014
> > @@ -44,6 +44,7 @@
> >  #include "llvm/ADT/IntrusiveRefCntPtr.h"
> >  #include "llvm/ADT/PointerIntPair.h"
> >  #include "llvm/ADT/PointerUnion.h"
> > +#include "llvm/Support/AlignOf.h"
> >  #include "llvm/Support/Allocator.h"
> >  #include "llvm/Support/DataTypes.h"
> >  #include "llvm/Support/MemoryBuffer.h"
> > @@ -89,6 +90,15 @@ namespace SrcMgr {
> >        DoNotFreeFlag = 0x02
> >      };
> >
> > +    // Note that the first member of this class is an aligned character
> buffer
> > +    // to ensure that this class has an alignment of 8 bytes. This
> wastes
> > +    // 8 bytes for every ContentCache object, but each of these
> corresponds to
> > +    // a file loaded into memory, so the 8 bytes doesn't seem terribly
> > +    // important. It is quite awkward to fit this aligner into any
> other part
> > +    // of the class due to the lack of portable ways to combine it with
> other
> > +    // members.
> > +    llvm::AlignedCharArray<8, 1> NonceAligner LLVM_ATTRIBUTE_UNUSED;
> > +
> >      /// \brief The actual buffer containing the characters from the
> input
> >      /// file.
> >      ///
> > @@ -224,6 +234,11 @@ namespace SrcMgr {
> >      ContentCache &operator=(const ContentCache& RHS)
> LLVM_DELETED_FUNCTION;
> >    };
> >
> > +  // Assert that the \c ContentCache objects will always be 8-byte
> aligned so
> > +  // that we can pack 3 bits of integer into pointers to such objects.
> > +  static_assert(llvm::AlignOf<ContentCache>::Alignment >= 8,
> > +                "ContentCache must be 8-byte aligned.");
> > +
> >    /// \brief Information about a FileID, basically just the logical file
> >    /// that it represents and include stack information.
> >    ///
> >
> > Modified: cfe/trunk/lib/Basic/SourceManager.cpp
> > URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Basic/SourceManager.cpp?rev=206324&r1=206323&r2=206324&view=diff
> >
> ==============================================================================
> > --- cfe/trunk/lib/Basic/SourceManager.cpp (original)
> > +++ cfe/trunk/lib/Basic/SourceManager.cpp Tue Apr 15 16:34:12 2014
> > @@ -436,12 +436,8 @@ SourceManager::getOrCreateContentCache(c
> >    ContentCache *&Entry = FileInfos[FileEnt];
> >    if (Entry) return Entry;
> >
> > -  // Nope, create a new Cache entry.  Make sure it is at least 8-byte
> aligned
> > -  // so that FileInfo can use the low 3 bits of the pointer for its own
> > -  // nefarious purposes.
> > -  unsigned EntryAlign = llvm::AlignOf<ContentCache>::Alignment;
> > -  EntryAlign = std::max(8U, EntryAlign);
> > -  Entry = ContentCacheAlloc.Allocate<ContentCache>(1, EntryAlign);
> > +  // Nope, create a new Cache entry.
> > +  Entry = ContentCacheAlloc.Allocate<ContentCache>();
> >
> >    if (OverriddenFilesInfo) {
> >      // If the file contents are overridden with contents from another
> file,
> > @@ -468,12 +464,8 @@ SourceManager::getOrCreateContentCache(c
> >  ///  memory buffer.  This does no caching.
> >  const ContentCache*
> >  SourceManager::createMemBufferContentCache(const MemoryBuffer *Buffer) {
> > -  // Add a new ContentCache to the MemBufferInfos list and return it.
>  Make sure
> > -  // it is at least 8-byte aligned so that FileInfo can use the low 3
> bits of
> > -  // the pointer for its own nefarious purposes.
> > -  unsigned EntryAlign = llvm::AlignOf<ContentCache>::Alignment;
> > -  EntryAlign = std::max(8U, EntryAlign);
> > -  ContentCache *Entry = ContentCacheAlloc.Allocate<ContentCache>(1,
> EntryAlign);
> > +  // Add a new ContentCache to the MemBufferInfos list and return it.
> > +  ContentCache *Entry = ContentCacheAlloc.Allocate<ContentCache>();
> >    new (Entry) ContentCache();
> >    MemBufferInfos.push_back(Entry);
> >    Entry->setBuffer(Buffer);
> >
> >
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