I don't understand the use of 'match_dup'

[ZengNan]

I don't understand the use of match_dup, even if I've read GCC Internals. For example, in mips.md,
 
(define_expand "seq"
[(set (match_operand:SI 0 "register_operand" "=d")
 (eq:SI (match_dup 1) (match_dup 2)))]
....
 
and in i386.md,
 
(define_expend "seq"
[(match_dup 1)
 (set (match_operand:QI 0 "register_operand" "")
        (eq:QI (cc0) (const_int 0)))]
......
 
What does "match_dup" means in the two patterns?


	

	
		
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