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Re: Conditional negation elimination in tree-ssa-phiopt.c



On 12/08/14 15:22, Jeff Law wrote:
On 08/12/14 04:31, Kyrill Tkachov wrote:
On 12/08/14 10:39, Richard Biener wrote:
On Mon, Aug 11, 2014 at 9:56 PM, Jeff Law <law@redhat.com> wrote:
On 08/11/14 07:41, Kyrill Tkachov wrote:
I haven't been able to get combine to match the comparison+xor+neg+plus
RTL and it seems like it would be just a workaround to undo the
tree-level transformation.
Yea, it'd just be a workaround, but it's probably the easiest way to
deal
with this problem.  Can you describe in further detail why you
weren't able
to get this to work?
Too many instructions to combine I guess.  You might want to add
intermediate "combine" insn-and-splits.  If that's still a no-go then
read on.
My guess was too many insns as well..  But that's often solvable.

  From the combine dump I can see that it tried to combine up to:
(set (reg/i:SI 0 x0)
      (plus:SI (xor:SI (neg:SI (reg:SI 84 [ D.2565 ]))
              (reg:SI 73 [ D.2564 ]))
          (reg:SI 84 [ D.2565 ])))
And did it find a match for this?   What happens if (just for testing
purposes), you create a pattern for this?  Does combine then try
something even more complex, possibly getting your conditional negation?

I managed to get combine to recognise this pattern:
(set (match_operand:GPI 0 "register_operand" "=r")
(plus:GPI (xor:GPI (neg:GPI (match_operand:GPI 1 "register_operand" "r"))
                           (match_operand:GPI 2 "register_operand" "r"))
                  (match_dup 1)))

Now what I need is for operand 1 to instead be a cc_reg comparison, but when I change operand 1 to this pattern:

(set (match_operand:GPI 0 "register_operand" "=r")
(plus:GPI (xor:GPI (neg:GPI (match_operator:GPI 1 "aarch64_comparison_operator" [(match_operand:CC 2 "cc_register" "") (const_int 0)]))
                           (match_operand:GPI 3 "register_operand" "r"))
                  (match_dup 1)))

This doesn't match. Is there any way to express this in a combineable pattern?

Thanks,
Kyrill




On the other hand, I did manage to write a peephole2 that detected the
sequence of compare+neg+xor+plus and transformed it into the
if_then_else form that our current conditional negation pattern has,
although I'm not sure how flexible this is.
Probably not very.  We really should be looking at combine.  In fact, I
would argue that we should be looking at combine regardless of whether
or not we twiddle expansion as humans or machine generated code could
look like this...

jeff





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