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Subject: Re: [Qemu-devel] [PATCH 4/5] tcg/i386: Use ANDN instruction
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To: Aurelien Jarno <aurelien@aurel32.net>
Cc: qemu-devel@nongnu.org

On 02/16/2014 12:12 PM, Aurelien Jarno wrote:
> On Fri, Jan 31, 2014 at 08:43:37AM -0600, Richard Henderson wrote:
>> > Note that the optimizer cannot simplify ANDC X,Y,C to AND X,Y,~C
>> > so we must handle constants in the implementation of andc.
> I do wonder if it actually won't be a better idea to add this
> simplification to the optimizer instead of adding it to the backend.
> 
> The best to do that would be to check with tcg_target_const_match to
> see if ANDC would accept such a constraint and to convert it to AND
> if not.
> 
> The same can probably be done for ORC.
> 

I suppose we could.  There are plenty of pairs for which this could apply, even
add/sub, though so far we've forced the backend to take care of that.

Perhaps if I have time I'll work something up for tcg_reg_alloc_op and see how
much savings we can achieve across the backends.  That'll tell us if it's worth
bothering with.


r~