The situation comes up where no or a partial overlap of registers
permits optimal code - since this can avoid using scratch register
Thus no overlap OR partial overlap is preferred (or required)
Using nothing leaves overlap without preference - full, partial,none
Using = gives the least preffered case - full
Using & gives only the no-overlapping case - none
Ideally "NOT"= is required - which I would hope the register allocator
would quite like too.
Dave Korn wrote:
Ian Lance Taylor wrote:
Andrew Hutchinson <andrewhutchinson@> writes:
I can use "=" modifier to make operands use same register and early
clobber "&" to avoid overlaps.
Is it possible to have or construct a contraint that permits partial
overlap operands. (which neither = or & would allow)
The case would be wide types taking multiple hard registers.
eg Input r20..23 Output r22..25
There is no such constraint today. I suppose it would be possible to
define such a constraint if it seemed useful.
Maybe I'm misunderstanding, but I thought that was already the default if
you use neither "=" to specify full overlap nor "&" for no overlap?
Frex, a lot of ABIs specify that DImode values stored in pairs of SImode
registers must always use an odd-even register pair (using a test in
HARD_REGNO_MODE_OK), but when I was working on a custom port that allowed them
in any register pair, GCC would happily generate partially overlapping movdi
instructions such as (set:DI (reg:DI 5) (reg:DI 6)) (i.e., move r6/7 -> r5/6).
This hasn't changed since 3.3, has it?
cheers,
DaveK
