Re: SUBREGs and modes

[Geoff Keating <geoffk at cygnus dot com>]

Jan Hubicka <jh@suse.cz> writes:

> Don't know - perhaps it would be better to wait for the SUBREG_BYTE
> patches.  The prbolem I am having is that we prohibit SUBREGs of
> const_int s lurking around the RTL stream and currently gcse is
> creating them, since it fails to simplify the SUBREGs of constants
> substitued to registers with NON-SImode and WORDNUM nonzero. This
> also disables any const propagation on these beast too - in the CSE,
> gcse and combine :(

Actually, we don't.  It used to be that when cross-compiling you could
get constructs like:

(subreg:SF (const_int 0x40800000) 0)

but a patch I committed about two weeks ago causes this case to only
happen when the FP emulator is not in use, which at present only
happens on VMS.

Now, I wouldn't _recommend_ such things.  As you've noticed, it hurts
optimisation.  gen_lowpart_common is the routine that knows how to fix
them.

Also, the reason for the patch is that PREFERRED_RELOAD_CLASS doesn't
work properly in the presence of such subregs, and it was easier to
ban them than to fix it.
-- 
- Geoffrey Keating <geoffk@cygnus.com>