Re: powerpc & unaligned block moves with fp registers

[David Edelsohn <dje at watson dot ibm dot com>]

>>>>> Richard Kenner writes:

Richard> No.  DImode seems the right mode to me.

Richard> I have not been following this discussion enough to understand what the
Richard> problem is so I can't suggest a solution.

	GCC uses DImode for a structure such as

struct x { char c[8]; };

which means that any loads and stores use DImode.  On some architectures,
unaligned 64-bit loads and stores can cause alignment exceptions trapping
to the OS to perform the load in pieces.

	The problem is that GCC uses DImode even for memory-to-memory
moves, such as

int quux (struct x* a, struct x* b) {
   *a = *b;
}

which is very inefficient.

	I believe that GCC should use a block move for this type of
unaligned copy.  IBM's compiler uses a block move.  The question is where
to hook this in.

	A hook recently was added to the PowerPC MD to notice DImode
memory-to-memory moves and split it into preferred moves.  I think that
GCC should use the SLOW_UNALIGNED_ACCESS macro to make this choice in
common code early in the decision process of how to perform the move
instead of the architecture-dependent code cracking the DImode move into
pieces late in the process. 

	I was tracing back the source of the DImode label on the
structure.  The DImode comes from the record layout.  If the record
remains DImode so that it is loaded into a register, then we need to
intercept the assignment sometime before emit_move_insn() is called and
choose the mode used for the move.

David