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



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.

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 ])))

What I need is for that reg 84 to be the result of the comparison, something like:
(ne (cc_reg) (const_int 0)) which I couldn't get combine to shove in there.


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.


OTOH a suitable place to "undo" would be smarter RTL expansion
that detects this pattern (and hope for TER to still apply - thus
no CSE opportunities going in your way).  For your testcase TER
allows

r_5 replace with --> r_5 = (int) _4;

_8 replace with --> _8 = _4 != 0;

_10 replace with --> _10 = -_9;

_11 replace with --> _11 = _10 ^ r_5;

_12 replace with --> _12 = _11 + _9;

which unfortunately already "breaks" because of the multi-use _9
and _4.

Now - the way out here is a GIMPLE pass right before expansion
that performs pattern detection and generates target specific code suitable
for optimal expand.  Fortunately we already have that with
pass_optimize_widening_mul (which does widen-mul and fma detection).

Maybe we can piggyback on this pass then? (probably renaming it to something more generic in the process...)


This is the place where you'd deal with such pattern, generating
a suitable compound operation (or two, dependent on expansion
and insn pattern details).  This of course usually requires new
tree codes or internal functions.  For more arcane stuff like
your conditional negate I'd prefer an internal function.

What is an internal function in this context? Is that like a target-specific builtin?


Of course you then have to add an optab or a target hook to
do custom internal function expansion.  I suppose for internal
functions a target hook would be nicer than a generic optab?

So something like TARGET_EXPAND_CONDITIONAL_NEGATION that gets called from the aforementioned GIMPLE pass when it finds a an appropriate sequence of compare+neg+xor+plus ?

Thanks for the pointers,
Kyrill


Thanks,
Richard.



What is the most acceptable way of disabling this transformation for a
target that has a conditional negation instruction?
In general, we don't want target dependencies in the gimple/ssa optimizers.

Jeff



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