Re: [patch] Move lowering of switches to bit tests to GIMPLE

[Richard Henderson <rth at redhat dot com>]

On 07/21/2012 06:10 AM, Oleg Endo wrote:
> I think on SH the cost test in lshift_cheap_p with
> gen_rtx_ASHIFT (word_mode, const1_rtx, reg), speed_p);
> 
> will always 'fail', because of sh.c (shiftcosts):
>   /* There is no pattern for constant first operand.  */
>   if (CONST_INT_P (XEXP (x, 0)))
>     return MAX_COST;
> 
> On SH3 / SH4* / SH2A there is a dynamic shift that does "reg << reg" or
> "reg >> reg", which is not that expensive actually.  However, the
> constant "1" must be loaded into a register first.  I'm currently trying
> to brush up the shift code in SH a little bit and could add some things
> to handle the "const << reg" case.

Having to load the "1" into a register is something common to all targets.
I shouldn't think that SH should need to do anything special.  Indeed, that
conditional that you quote above looks very wrong.  I believe you should 
simply recurse (or let the rtx_cost driver recurse) to find the cost of
arguments to the shift.


r~