Re: [Patch,AVR]: Fix PR36467, PR49687 (better widening mul)

[Richard Henderson <rth at redhat dot com>]

On 07/19/2011 03:21 AM, Georg-Johann Lay wrote:
>> Yes, it is sad that the backends have to work around the fact
>> that sign/zero_extension of constants is invalid rtl.
> 
> Why is that invalid?
> 
> (set (reg:HI A)
>      (const_int 1000))
> 
> (set (reg:SI B)
>      (mult:SI (zero_extend:SI (reg:HI C))
>               (zero_extend:SI (reg:HI A))))
> 
> 
> If a target hat native support for
> 
> (set (reg:SI B)
>      (mult:SI (zero_extend:SI (reg:HI C))
>               (const_int 1000)))
> 
> then a combine pattern is straight forward.

Yes.  And the result you show is quite valid and correct.

What isn't correct is

	(zero_extend:HI (const_int 100000))

because const_int does not have a mode, and we no longer
know what the source mode is for the extension.


r~