Re: gen_lowpart called where 'truncate' needed?

[Adam Nemet <adambnemet at gmail dot com>]

Ian Lance Taylor <iant@google.com> writes:
> Mat Hostetter <mhostetter@tilera.com> writes:
>
>> Since the high bits are already zero, that would be less efficient on
>> most platforms, so guarding it with something like this would probably
>> be smarter:
>>
>>   if (targetm.mode_rep_extended (mode, GET_MODE(x)) == SIGN_EXTEND)
>>     return simplify_gen_unary (TRUNCATE, mode, x, GET_MODE (x));
>>
>> I'm happy to believe I'm doing something wrong in my back end, but I'm
>> not sure what that would be.  I could also believe these are obscure
>> edge cases no one cared about before.  Any tips would be appreciated.
>
> Interesting.  I think you are in obscure edge case territory.  Your
> suggestion makes sense to me, and in fact it should probably be put
> into gen_lowpart_common.

FWIW, I disagree.  Firstly, mode_rep_extended is a special case of
!TRULY_NOOP_TRUNCATION so the above check should use that.  Secondly, in
MIPS we call gen_lowpart to convert DI to SI when we know it's safe.  In
this case we always want a subreg not a truncate for better code.  So I
don't think gen_lowpart_common is the right place to fix this.

I think the right fix is to call convert_to_mode or convert_move in the
expansion code which ensure the proper truncation.

Adam