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    • Duncan Sands's avatar
      Executive summary: getTypeSize -> getTypeStoreSize / getABITypeSize. · 44b8721d
      Duncan Sands authored
      The meaning of getTypeSize was not clear - clarifying it is important
      now that we have x86 long double and arbitrary precision integers.
      The issue with long double is that it requires 80 bits, and this is
      not a multiple of its alignment.  This gives a primitive type for
      which getTypeSize differed from getABITypeSize.  For arbitrary precision
      integers it is even worse: there is the minimum number of bits needed to
      hold the type (eg: 36 for an i36), the maximum number of bits that will
      be overwriten when storing the type (40 bits for i36) and the ABI size
      (i.e. the storage size rounded up to a multiple of the alignment; 64 bits
      for i36).
      
      This patch removes getTypeSize (not really - it is still there but
      deprecated to allow for a gradual transition).  Instead there is:
      
      (1) getTypeSizeInBits - a number of bits that suffices to hold all
      values of the type.  For a primitive type, this is the minimum number
      of bits.  For an i36 this is 36 bits.  For x86 long double it is 80.
      This corresponds to gcc's TYPE_PRECISION.
      
      (2) getTypeStoreSizeInBits - the maximum number of bits that is
      written when storing the type (or read when reading it).  For an
      i36 this is 40 bits, for an x86 long double it is 80 bits.  This
      is the size alias analysis is interested in (getTypeStoreSize
      returns the number of bytes).  There doesn't seem to be anything
      corresponding to this in gcc.
      
      (3) getABITypeSizeInBits - this is getTypeStoreSizeInBits rounded
      up to a multiple of the alignment.  For an i36 this is 64, for an
      x86 long double this is 96 or 128 depending on the OS.  This is the
      spacing between consecutive elements when you form an array out of
      this type (getABITypeSize returns the number of bytes).  This is
      TYPE_SIZE in gcc.
      
      Since successive elements in a SequentialType (arrays, pointers
      and vectors) need to be aligned, the spacing between them will be
      given by getABITypeSize.  This means that the size of an array
      is the length times the getABITypeSize.  It also means that GEP
      computations need to use getABITypeSize when computing offsets.
      Furthermore, if an alloca allocates several elements at once then
      these too need to be aligned, so the size of the alloca has to be
      the number of elements multiplied by getABITypeSize.  Logically
      speaking this doesn't have to be the case when allocating just
      one element, but it is simpler to also use getABITypeSize in this
      case.  So alloca's and mallocs should use getABITypeSize.  Finally,
      since gcc's only notion of size is that given by getABITypeSize, if
      you want to output assembler etc the same as gcc then getABITypeSize
      is the size you want.
      
      Since a store will overwrite no more than getTypeStoreSize bytes,
      and a read will read no more than that many bytes, this is the
      notion of size appropriate for alias analysis calculations.
      
      In this patch I have corrected all type size uses except some of
      those in ScalarReplAggregates, lib/Codegen, lib/Target (the hard
      cases).  I will get around to auditing these too at some point,
      but I could do with some help.
      
      Finally, I made one change which I think wise but others might
      consider pointless and suboptimal: in an unpacked struct the
      amount of space allocated for a field is now given by the ABI
      size rather than getTypeStoreSize.  I did this because every
      other place that reserves memory for a type (eg: alloca) now
      uses getABITypeSize, and I didn't want to make an exception
      for unpacked structs, i.e. I did it to make things more uniform.
      This only effects structs containing long doubles and arbitrary
      precision integers.  If someone wants to pack these types more
      tightly they can always use a packed struct.
      
      llvm-svn: 43620
      44b8721d
    • Bill Wendling's avatar
      Silence, accersed warning · b7cabbe2
      Bill Wendling authored
      llvm-svn: 43609
      b7cabbe2
  13. Oct 31, 2007
  14. Oct 30, 2007
  15. Oct 29, 2007
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