Re: Destructive comparison

[Michael Hope <michaelh at juju dot net dot nz>]

Thanks, that worked.  I ended up using:

(define_insn "cmpcc_xor"
  [(set (match_operand:CC 0 "register_operand" "=C")
	(compare:CC
	 (not:SI (xor:SI (match_operand:SI 1 "register_operand" "%r")
			 (match_operand:SI 2 "register_operand" "b")))
	 (const_int 0)))
  (set (match_operand:SI 3 "register_operand" "=1")
	(not:SI (xor:SI (match_dup 1) (match_dup 2))))]
  ""
  "XOR, %1"
)

The important thing was in the generation.  The XOR is two operand but
I needed to supply a third pretend operand using:

      emit_insn (gen_cmpcc_(cc_reg, x, y, gen_reg_rtx(SImode)));

Using a match_dup instead of operand 3 above, or supplying 'x' twice,
lead to the compiler not noticing the change.

-- Michael

2009/5/18 Jim Wilson <wilson@codesourcery.com>:
> Michael Hope wrote:
>>
>> ?* Using a define_insn to mark it as both a destructive xor and
>> compare in parallel, such as:
>
> When a compare is in a parallel, the compare must be the first operation.
> ?You have it second. ?This kind of pattern should work. ?You can find many
> examples of it in the sparc.md file for instance. ?Of course, in this case,
> they aren't generated at RTL generation time. They are generated at combine
> time. ?Still, I'd expect this to work, though there might be some early RTL
> optimization passes that are not prepared to handle it.
>
> See for instance the cmp_cc_xor_not_set pattern in the sparc.md file, which
> is similar to what you want.
>
> Jim
>