[llvm-commits] [126245] Fix http://lists.cs.uiuc.edu/pipermail/llvm-commits/ Week-of-Mon-20070416/047943.html

[Duncan Sands]

Hi Devang, I just posted a fix which fixes some other issues too.
A few lines earlier:

    if (StartOffsetFromByteBoundry != 0) {
      // New field does not start at byte boundry.
      PadBits = StartOffsetInBits - (FirstUnallocatedByte*8);
      PadBytes = PadBits/8 + 1;
    }

    PadBytes += StartOffsetInBits/8-FirstUnallocatedByte;

If StartOffsetFromByteBoundry != 0, then PadBytes gets added to twice,
setting it to 2*PadBits/8 + 1.

>        Pad = ArrayType::get(Pad, PadBytes);
>      Info.addElement(Pad, FirstUnallocatedByte, PadBytes);
>      FirstUnallocatedByte = StartOffsetInBits/8;
> -    // This field will use some of the bits from this PadBytes.
> -    FieldSizeInBits = FieldSizeInBits - (PadBytes*8 - PadBits);
> +    // This field will use some of the bits from this PadBytes, if
> +    // starting offset is not at byte boundry.
> +    if (StartOffsetFromByteBoundry != 0)
> +      FieldSizeInBits = FieldSizeInBits - (8 - PadBits);

Here PadBits can be bigger than 8, which will lead to a bad result :)
Even if it is less than 8 this can create a negative FieldSizeInBits.
For example, if FieldSizeInBits=1, StartOffsetFromByteBoundry=1, then
typically PadBits=1, meaning you are setting FieldSizeInBits to -6 here!

>    }
>  
>    // Now, Field starts at FirstUnallocatedByte and everything is aligned.

Best wishes,

Duncan.