On Wed, May 06, 2015 at 04:03:00PM +0100, Richard Sandiford wrote: > Jeff Law <l...@redhat.com> writes: > > So my worry here is that folks writing these loops to iterate over modes > > are going to easily miss the != VOIDmode terminator, or not know when to > > use GET_MODE_WIDER_SPECIAL. > > > > We can certainly go with the patch as-is since you've done the work to > > sort out which GET_MODE_WIDER to use and added the appropriate > > termination checks. But do we want to try to future proof this a > > little and define two iterators for folks to use rather than write the > > loops by hand every time and probably getting it wrong -- and wrong in > > such a way that it only breaks on PPC, forcing someone to regularly be > > fixing this stuff
If they miss the != VOIDmode, the program will hang since it will never exit the loop (VOIDmode is the wider type for VOIDmode). > Yeah. I might be wrong, but I don't think the emit-rtl.c code > that uses GET_MODE_WIDER_MODE_SPECIAL actually cares about one thing > being wider than another. It just wants to iterate over _all_ modes > of a particular class. So maybe we could just convert them to iterate > over everything between MIN_MODE_foo and MAX_MODE_foo (already defined > in insn-modes.h). Agree proper iterators would be even nicer though :-) The code in emit-rtl.c that uses GET_MODE_WIDER_MODE_SPECIAL just is initializing all of the types. There are 2 functions: init_derived_machine_modes -- this really doesn't need special, since it is just looking at the INT types, looking to define byte_mode, word_mode, and ptr_mode to appropriate integer types. init_emit_once -- this is initializing all of the special constants (-1.0, 0.0, 1.0, etc.) for each of the modes. The only one that really needs to use _SPECIAL is the floating point loop. When I was making the change months ago, I just used _SPECIAL in all of the places, just in case we ever needed to have an INT special type, or decimal float, etc. In addition in expr.c the function init_expr_target uses it to set up the conversion tables. Perhaps it is simpler just to have a target hook that says whether a given conversion can happen automatically (as opposed to explicitly if asked for by the user). -- Michael Meissner, IBM IBM, M/S 2506R, 550 King Street, Littleton, MA 01460-6245, USA email: meiss...@linux.vnet.ibm.com, phone: +1 (978) 899-4797
