How to explicitly state in md pattern that I expect bitwise zero target?

[Andrew Pinski]

On Fri, Oct 26, 2012 at 1:49 AM, Konstantin Vladimirov
<konstantin.vladimirov@gmail.com> wrote:
> Hi,
>
> Consider I have pattern like:
>
> (define_insn "big_combined_one"
>   [(set (match_operand:SI 0 ...)
>         (ior:SI (and:SI (lshiftrt:SI (match_operand:SI 1 ...)
>                                      (match_operand:SI 2 ...))
>                         (match_operand:SI 3 ...))
>                 (and:SI (lshiftrt:SI (match_dup 1)
>                                      (match_operand:SI 4 ...))
>                         (match_operand:SI 5 ...))))]
>
> I can match it as one instruction, only if target operand is proven to
> be bitwise zero, that is very often case for this pattern.
>
> But how can I decide if it is so?

What do you mean the target operand is proven to be bitwise zero?
This pattern sets all of operand 0 and does touch use it.  I think you
want to have an explicit use of the operand in the pattern.

Thanks,
Andrew Pinski