Constraint modifier for partially overlaping operands

[Andrew Hutchinson]

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
>
>