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Re: [RFC] PR tree-optimization/67628: Make tree ifcombine more symmetric and interactions with dom

Hi all,

On 23/09/15 11:10, Richard Biener wrote:

On Wed, 23 Sep 2015, Kyrill Tkachov wrote:


On 23/09/15 10:09, Pinski, Andrew wrote:

On Sep 23, 2015, at 1:59 AM, Kyrill Tkachov <kyrylo.tkachov@arm.com>
wrote:



On 22/09/15 20:31, Jeff Law wrote:

On 09/22/2015 07:36 AM, Kyrill Tkachov wrote:
Hi all,
Unfortunately, I see a testsuite regression with this patch:
FAIL: gcc.dg/pr66299-2.c scan-tree-dump-not optimized "<<"

The reduced part of that test is:
void
test1 (int x, unsigned u)
{
     if ((1U << x) != 64
         || (2 << x) != u
         || (x << x) != 384
         || (3 << x) == 9
         || (x << 14) != 98304U
         || (1 << x) == 14
         || (3 << 2) != 12)
       __builtin_abort ();
}

The patched ifcombine pass works more or less as expected and produces
fewer basic blocks.
Before this patch a relevant part of the ifcombine dump for test1 is:
;;   basic block 2, loop depth 0, count 0, freq 10000, maybe hot
     if (x_1(D) != 6)
       goto <bb 6>;
     else
       goto <bb 3>;

;;   basic block 3, loop depth 0, count 0, freq 9996, maybe hot
     _2 = 2 << x_1(D);
     _3 = (unsigned intD.10) _2;
     if (_3 != u_4(D))
       goto <bb 6>;
     else
       goto <bb 4>;


After this patch it is:
;;   basic block 2, loop depth 0, count 0, freq 10000, maybe hot
     _2 = 2 << x_1(D);
     _3 = (unsigned intD.10) _2;
     _9 = _3 != u_4(D);
     _10 = x_1(D) != 6;
     _11 = _9 | _10;
     if (_11 != 0)
       goto <bb 5>;
     else
       goto <bb 3>;

The second form ends up generating worse codegen however, and the
badness starts with the dom1 pass.
In the unpatched case it manages to deduce that x must be 6 by the
time
it reaches basic block 3 and
uses that information to eliminate the shift in "_2 = 2 << x_1(D)"
from
basic block 3
In the patched case it is unable to make that call, I think because
the
x != 6 condition is IORed
with another test.

I'm not familiar with the internals of the dom pass, so I'm not sure
where to go looking for a fix for this.
Is the ifcombine change a step in the right direction? If so, what
would
need to be done to fix the issue with
the dom pass?

I don't see how you can reasonably fix this in DOM.  if _9 or _10 is
true, then _11 is true.  But we can't reasonably record any kind of
equivalence for _9 or _10 individually.

If the statement
_11 = _9 | _10;

Were changed to

_11 = _9 & _10;

Then we could record something useful about _9 and _10.



I suppose what we want is to not combine basic blocks if the sequence
and conditions of the basic blocks are
such that dom can potentially exploit them, but how do we express
that?

I don't think there's going to be a way to directly express that.  You
could essentially claim that TRUTH_OR is more expensive than TRUTH_AND
because of the impact on DOM, but that in and of itself may not resolve
the situation either.

I think the question we need to answer is whether or not your changes
are generally better, even if there's specific instances where they make
things worse.  If the benefits outweigh the negatives then we can xfail
that test.

Ok, I'll investigate and benchmark some more.
Andrew, this transformation to ifcombine (together with the restriction
that the inner condition block
has to be a single comparison) was added by you with r204194.
Is there a particular reason for that restriction and why it is applied to
the inner block and not either?

My reasoning at the time was there might be an "expensive" instruction or
one that might trap (I did not check to see if the other part of the code
was detecting that).
The outer block did not need any checks as we have something like
...
If (a)
    If (b)

Or
....
If (a)
    Goto f
else if (b)
   ....
Else
{
F:
....
}

And there was no need to check what was before the if (a) part just what is
in between the two ifs.

Ah, because the code in outer_cond_bb would have to be executed anyway whether
we perform the conversion or not, right?

All ifcombine transforms make the outer condition unconditionally
true/false thus the check should have been on whether the outer
cond BB is "empty".  Which would solve your problem, right?

Note that other transforms (bit test recognition) don't care (sth
we might want to fix?).

In general this needs a better cost function, maybe simply use
estimate_num_insns with speed estimates and compare against a
new --param.


So I've played around with that code and I think I'd like to make it a bit
more intricate. Just comparing against estimate_num_insns is too coarse-grained and
is just a comparison by a magic number number and I've been struggling to make this
aggressive enough without pulling in too much code into the unconditional path.

As far as aarch64 is concerned I want to make this transformation more aggressive when
it can facilitate conditional comparison generation during expand. This means that I want
to allow the cases where the inner block contains comparisons, combined using logical operations
like TRUTH_AND_EXPR, TRUTH_IOR_EXPR, TRUTH_NOT_EXPR, their bitwise variants etc.
The expand code in ccmp.c knows how to handle these chains of comparisons and emit the appropriate
conditional compare patterns. If, however, the inner block contains other types of operations like
arithmetic ops, pointer dereferencing etc I'd want to be conservative to avoid pulling in operations
that don't facilitate the ccmp.c expansions.

So what I'm proposing is:
- If a target supports conditional comparisons through TARGET_GEN_CCMP_FIRST
(currently only aarch64) then we allow the aforementioned comparisons+logical-combinations blocks.
If the block also contains other types of operations we apply the estimate_num_insns cost comparison
with the default value for the comparison picked to be such so that it changes codegen the least from
the current situation i.e. one instruction. This value will be a new param that targets can increase
if they want to.

- If the target does not support conditional comparisons we follow only the second scheme (the conservative
estimate_num_insns comparison). This should cause the minimal codegen difference for the targets that
don't support conditional compares (which is all targets but aarch64) while allowing them to scale
the aggressiveness of this transformation if their benchmarking shows it to be beneficial.


 I believe such a scheme would avoid pulling in too much code that doesn't
facilitate conditional compares generation into the unconditional path and would minimise
impact on existing targets that don't do conditional compares.

Would that be an acceptable plan for the ifcombine_ifandif transformation in
tree-ssa-ifcombine.c (pending benchmarking, of course)?

Thanks,
Kyrill



Thanks,
Richard.


Thanks,
Kyrill


What I mean by expensive for an example is division or some function call.

Thanks,
Andrew



Thanks,
Kyrill




jeff
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