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Avoiding subregs of PSImode when word_mode is HImode
- From: Peter Bigot <bigotp at acm dot org>
- To: gcc at gcc dot gnu dot org
- Date: Wed, 20 Jun 2012 07:25:45 -0500
- Subject: Avoiding subregs of PSImode when word_mode is HImode
I'm running into issues with the MSP430 and use of PSImode in packed
structures. Here the native word size is HImode, but all general purpose
registers are capable of holding and operating on PSImode (=20 bit) values.
Due to instruction set limitations, on this architecture:
(subreg:HI (reg:PSI 0))
is fine, but
(subreg:HI (reg:PSI 2))
is inexpressible. The upper 4 bits can only be accessed with multiple shift
instructions.
CANNOT_CHANGE_MODE_CLASS doesn't provide the control to express this
restriction on subregs. The only way I've found to tell gcc this is to hack
validate_subreg to rule out non-zero offsets on PSImode.
Is there a non-hack way to tell GCC about this limitation?
More generally, is there a way to tell GCC not to try to decompose SImode
operations into subregs even though word_mode is HImode? I've got
HARD_REGNO_NREGS, HARD_REGNO_NREGS_HAS_PADDING, and REGMODE_NATURAL_SIZE set
up as accurately as I can:
#define MSP430_MODE_NREGS(MODE) \
(PSImode == (MODE) \
? 1 \
: ((GET_MODE_SIZE (MODE) + UNITS_PER_WORD - 1) / UNITS_PER_WORD))
#define HARD_REGNO_NREGS(REGNO, MODE) MSP430_MODE_NREGS(MODE)
#define HARD_REGNO_MODE_OK(REGNO, MODE) 1
#define MODES_TIEABLE_P(MODE1, MODE2) \
((PSImode != (MODE1) || PSImode == (MODE2)) \
&& (PSImode != (MODE2) || 1 == MSP430_MODE_NREGS (MODE1)))
#define HARD_REGNO_NREGS_HAS_PADDING(REGNO, MODE) (PSImode == (MODE))
#define HARD_REGNO_NREGS_WITH_PADDING(REGNO, MODE) (PSImode == (MODE)
? 2 : MSP430_MODE_NREGS(MODE))
#define REGMODE_NATURAL_SIZE(MODE) (PSImode == (MODE) ?
2*UNITS_PER_WORD : UNITS_PER_WORD)
Thanks.
Peter