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Re: [RFC: Patch 1/6 v2] New target hook: max_noce_ifcvt_seq_cost

On 06/21/2016 09:50 AM, James Greenhalgh wrote:

On Fri, Jun 03, 2016 at 12:39:42PM +0200, Richard Biener wrote:
On Thu, Jun 2, 2016 at 6:53 PM, James Greenhalgh
<> wrote:


This patch introduces a new target hook, to be used like BRANCH_COST but
with a guaranteed unit of measurement. We want this to break away from
the current ambiguous uses of BRANCH_COST.

BRANCH_COST is used in ifcvt.c in two types of comparisons. One against
instruction counts - where it is used as the limit on the number of new
instructions we are permitted to generate. The other (after multiplying
by COSTS_N_INSNS (1)) directly against RTX costs.

Of these, a comparison against RTX costs is the more easily understood
metric across the compiler, and the one I've pulled out to the new hook.
To keep things consistent for targets which don't migrate, this new hook
has a default value of BRANCH_COST * COSTS_N_INSNS (1).


How does the caller compute "predictable"?  There are some archs where
an information on whether this is a forward or backward jump is more
useful I guess.  Also at least for !speed_p the distance of the branch is
important given not all targets support arbitrary branch offsets.

Just through a call to predictable_edge_p. It isn't perfect. My worry
with adding more details of the branch is that you end up with a nonsense
target implementation that tries way too hard to be clever. But, I don't
mind passing the edge through to the target hook, that way a target has
it if they want it. In this patch revision, I pass the edge through.
There are so many things that can factor into this decision. But I suspect we get most of the benefit from a small amount of work (ie, using the prediction information we've already generated). If the target wants to override based on other factors, there's a mechanism for that, but I don't think it's likely to be heavily, if at all, used.

In this patch revision, I started by removing the idea that this costs
a branch at all. It doesn't, the use of this hook is really a target
trying to limit if-convert to not end up pulling too much on to the
unconditional path. It seems better to expose that limit directly by
explicitly asking for the maximum cost of an unconditional sequence we
would create, and comparing against seq_cost of the new RTL. This saves
a target trying to figure out what is meant by a cost of a branch.
Seems sensible. Essentially you're just asking a different but related (and more direct) question that side-steps the need to be able to compare branch cost with costs of other insns.

A target maintainer may have a sense of how branch cost relates to normal insns on their target and they may choose to define the hooks in those terms (though I would discourage that as it re-introduces the precise problem we're trying to get around).

Having done that, I think I can see a clearer path to getting the
default hook implementation in shape. I've introduced two new params,
which give maximum costs for the generated sequence (one for a "predictable"
branch, one for "unpredictable") in the speed_p cases. I'm not expecting it
to be useful to give the user control in the case we are compiling for
size - whether this is a size win or not is independent of whether the
branch is predictable.
Maybe not for MIPS :-)  But I think that should be tabled.

For the default implementation, if the parameters are not set, I just
multiply BRANCH_COST through by COSTS_N_INSNS (1) for size and
COSTS_N_INSNS (3) for speed. I know this is not ideal, but I'm still short
of ideas on how best to form the default implementation. This means we're
still potentially going to introduce performance regressions for targets
that don't provide an implementation of the new hook, or a default value
for the new parameters. It does mean we can keep the testsuite clean by
setting parameter values suitably high for all targets that have
conditional move instructions.
But I think that's an OK place to be -- I don't think it's sensible for you to have to figure that out for every target. It's something the target maintainers ought to be able to guesstimate more accurately. My only objection is conceptual based on mixing BRANCH_COST & COSTS_N_INSNS. But there may simply not be another way to set the default.

The new default causes some changes in generated conditional move sequences
for x86_64. Whether these changes are for the better or not I can't say.
You might consider passing those along to Uros to get his opinion.

This first patch introduces the two new parameters, and uses them in the
default implementation of the target hook.

Bootstrapped on x86_64 and aarch64 with no issues.



2016-06-21  James Greenhalgh  <>

	* target.def (max_noce_ifcvt_seq_cost): New.
	* doc/ (TARGET_MAX_NOCE_IFCVT_SEQ_COST): Document it.
	* doc/tm.texi: Regenerate.
	* targhooks.h (default_max_noce_ifcvt_seq_cost): New.
	* targhooks.c (default_max_noce_ifcvt_seq_cost): New.
	* doc/invoke.texi: Document new params.
This is fine IMHO.


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