Re: [Qemu-devel] [PATCH 08/12] tcg-s390: Remove constraint letters for and

[Richard Henderson]

On 03/28/2013 08:03 AM, Aurelien Jarno wrote:
>> Since we have a free temporary and can always just load the constant, we
>> > ought to do so, rather than spending the same effort constraining the 
>> > const.
> Is it really a good idea doing so? If a constraint can't be satisfied
> the TCG code will also load the constant in a register, with the
> difference that the register is not trashed and might be reused later 
> instead of reloading the constant again. Of course it means one more
> register available, but the S390 target doesn't really have issues
> with the number of available registers.
> 

My main thinking is along the lines you yourself pointed out when the code
was first written -- it's really quite hard to figure out what constants are
implementable for AND.

It gets even worse with a patch further in the series that uses ROTATE AND
INSERT SELECTED BITS.

It's complicated enough that it *seems* better to just go ahead and accept
all constants.  Even from a maintainence point of view -- we no longer have
to have two big functions match up.


r~