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

[Konstantin Vladimirov]

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?

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With best regards, Konstantin