[RFC] Try vector<bool> as a new representation for vector masks

Richard Henderson rth@redhat.com
Wed Sep 23 18:51:00 GMT 2015


On 09/23/2015 06:53 AM, Richard Biener wrote:
> I think independent improvements are
> 
>  1) remove (most) of the bool patterns from the vectorizer
> 
>  2) make VEC_COND_EXPR not have a GENERIC comparison embedded
> 
> (same for COND_EXPR?)

Careful.

The reason that COND_EXPR have embedded comparisons is to handle flags
registers.  You can't separate the setting of the flags from the using of the
flags on most targets, because there's only one flags register.

The same is true for VEC_COND_EXPR with respect to MIPS.  The base architecture
has 8 floating-point comparison result flags, and the vector compare
instructions are fixed to set fcc[0:width-1].  So again there's only one
possible output location for the result of the compare.

MIPS is going to present a problem if we attempt to generalize logical
combinations of these vector<bool>, since one has to use several instructions
(or one insn and pre-load constants into two registers) to get the fcc bits out
into a form we can manipulate.


r~



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