[RTL, i386] Use subreg instead of UNSPEC_CAST

[Marc Glisse]

On Tue, 19 Mar 2013, Richard Henderson wrote:

> On 03/19/2013 08:47 AM, Marc Glisse wrote:
>>  (define_insn_and_split "avx_<castmode><avxsizesuffix>_<castmode>"
>>    [(set (match_operand:AVX256MODE2P 0 "nonimmediate_operand" "=x,m")
>> -	(unspec:AVX256MODE2P
>> -	  [(match_operand:<ssehalfvecmode> 1 "nonimmediate_operand" "xm,x")]
>> -	  UNSPEC_CAST))]
>> +	(subreg:AVX256MODE2P
>> +	  (match_operand:<ssehalfvecmode> 1 "nonimmediate_operand" "xm,x") 0))]
>>    "TARGET_AVX"
>>    "#"
>>    "&& reload_completed"
>>    [(const_int 0)]
>
> I'm not fond of this, primarily because I believe the pattern should
> not exist at all.

Sure, removing it would be even better.

> One of the following is true:
>
>  (1) reload needs working around (thus all the reload_completed nonsense)
> or
>  (2) the entire pattern is useless and would be subsumed by mov<mode>
> or
>  (3) the entire pattern is useless and is *already* subsumed by
>      mov<mode>, since mov is earlier in the md file, making this
>      pattern dead code.

We need something to expand _mm256_castpd128_pd256 to. I tried making it a 
define_expand (with the subreg pattern, and keeping the {} part intact), 
but that gives check_rtl errors in lra. I then tried to remove the REG_P 
condition and use simplify_gen_subreg or gen_lowpart, but the first one 
gives unrecognizable insn at -O0 (same as removing the {} part completely) 
(it seems happier at -O1), while the second ICEs (gen_lowpart_common 
returns 0) for any -Ox except -O0. As must be obvious from this paragraph, 
I just tried a few random bad ideas... and when none worked I posted the 
minimal patch that worked.

Do you at least agree that vector-vector subregs make sense, or is that 
part wrong as well?

-- 
Marc Glisse