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Re: [PATCH, PR target/65103, 2/3] Propagate address constants into loops for i386


On 05/05/2015 05:05 AM, Ilya Enkovich wrote:
2015-04-21 8:52 GMT+03:00 Jeff Law <law@redhat.com>:
On 04/17/2015 02:34 AM, Ilya Enkovich wrote:

On 15 Apr 14:07, Ilya Enkovich wrote:

2015-04-14 8:22 GMT+03:00 Jeff Law <law@redhat.com>:

On 03/15/2015 02:30 PM, Richard Sandiford wrote:


Ilya Enkovich <enkovich.gnu@gmail.com> writes:


This patch allows propagation of loop invariants for i386 if
propagated
value is a constant to be used in address operand.  Bootstrapped and
tested on x86_64-unknown-linux-gnu.  OK for trunk or stage 1?



Is it necessary for this to be a target hook?  The concept doesn't seem
particularly target-specific.  We should only propagate into the
address
if the new cost is no greater than the old cost, but if the address
meets that condition and if propagating at this point in the pipeline
is
a win on x86, then wouldn't it be a win for other targets too?


I agree with Richard here.  I can't see a strong reason why this should
be a
target hook.

Perhaps part of the issue here is the address costing metrics may not
have
enough context to make good decisions.  In which case what context do
they
need?


At this point I don't insist on a target hook.  The main reasoning was
to not affect other targets. If we extend propagation for non constant
values different aspects may appear. E.g. possible register pressure
changes may significantly affect ia32. I just wanted to have an
instrument to play with a propagation on x86 not affecting other
targets. I don't have an opportunity to test possible performance
implications on non-x86 targets. Don't expect (significant)
regressions there but who knows...

I'll remove the hook from this patch. Will probably introduce it later
if some target specific cases are found.

Thanks,
Ilya


Jeff


Here is a version with no hook.  Bootstrapped and tested on
x86_64-unknown-linux-gnu.  Is it OK for trunk?

Thanks,
Ilya
--
gcc/

2015-04-17  Ilya Enkovich  <ilya.enkovich@intel.com>

         PR target/65103
         * fwprop.c (forward_propagate_into): Propagate loop
         invariants if a target says so.

gcc/testsuite/

2015-04-17  Ilya Enkovich  <ilya.enkovich@intel.com>

         PR target/65103
         * gcc.target/i386/pr65103-2.c: New.

It seems to me there's a key piece missing here -- metrics.

When is this profitable, when is it not profitable.   Just blindly undoing
LICM seems wrong here.

The first thought is to look at register pressure through the loop.  I
thought we had some infrastructure for this kind of query available. It'd
probably be wise to re-use it.  In fact, one might reasonably ask if LICM
should have hoisted the expression to start with.


I'd also think the cost of the constant may come into play here.  A really
cheap constant probably should not have been hoisted by LICM to start with
-- but the code may have been written in such a way that some low cost
constants are pulled out as loop invariants at the source level.  So this
isn't strictly an issue of un-doing bad LICM

So I think to go forward we need to be working on solving the "when is this
a profitable transformation to make".

This patch doesn't force propagation.  The patch just allows
propagation and regular fwprop cost estimation is used to compute if
this is profitable.  For i386 I don't see cases when we shouldn't
propagate. We remove instruction, reduce register pressure and having
constant in memory operand is free which is reflected in address_cost
hook.
Right, but you're blindly propagating. The right thing to do is look at some kind of metric to estimate when it's profitable to propagate the constant back in vs leave it hoisted out.

If you look at what Kugan is doing in cprop.c, that's exactly the approach he's taking -- looking at rtx costing to determine when to propagate the constant back into the loop. It could probably be made better with some knowledge of register pressure and looking at whether or not all uses (vs just some uses) of the constant will be propagated.

Jeff


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