[llvm-commits] [llvm-gcc-4.2] r51866 - /llvm-gcc-4.2/trunk/gcc/llvm-types.cpp

[Duncan Sands]

Hi Dale,

> This patch breaks FrontendC/2008-01-28-UnionSize.c and FrontendC/ 
> 2007-09-28-PackedUnionMember.c on Darwin x86-32.

sorry about that, I don't understand why I didn't see it when testing
because I sure see it now (x86-64)!  The problem is that gcc says that
this type

  union {
    struct opaque1 *f3;
    struct opaque2 *f4;
    struct {
      struct opaque3 *f5;
      unsigned short  f6;
    } f7;
  };

is 16 byte aligned but has a size of 10 bytes.  I thought gcc sizes
were always a multiple of the alignment?  Anyway, ConvertUNION chooses
to represent this using "struct opaque1 *" (the reason being that f7
is packed, dunno why, so has alignment 1, which is less than the alignment
of the pointers).  It then observes that the size (8 bytes) is less than
the gcc size (10 bytes) so tries to pad by adding 2 bytes, resulting in
  { opaque *, [2 x i8] }
This actually has an ABI size of 16 bytes - it is 10 bytes wide but is
8 bytes aligned -> 16 bytes.  This is not the same as the gcc type size.

I'm rather taken aback that gcc is producing type sizes that are not a
multiple of the alignment.  I also don't understand why the alignment
is 16.

I guess most of these oddities are related to the "#pragma pack(push, 2)"
on the first line of 2007-09-28-PackedUnionMember.c.  What does that mean?
And if things are packed, shouldn't the union be converted to a packed type
or something?

Ciao,

Duncan.