On 25/09/15 10:49, Richard Biener wrote:
On Fri, 25 Sep 2015, Kyrill Tkachov wrote:
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.tkac...@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 the inner block only contains stmts feeding a condition? As we're
combining that condition with the outer one and the result simplified(?)
it makes sense to allow that as "empty" block generally.
My concern is that those stmts feeding the condition will
now be executed unconditionally (rather than only after jumping
to the inner_cond_bb) so we might end up doing redundant work
if the the outer condition would actually have jumped to the else block
rather than to the inner one.
Kyrill
No?
Thanks,
Richard.
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
