[RFC, ivopts] fix bugs in ivopts address cost computation

[Sandra Loosemore]

My colleagues and I have been working on the GCC port for the Qualcomm
Hexagon.  Along the way I noticed that we were getting poor results
from the ivopts pass no matter how we adjusted the target-specific RTX
costs.  In many cases ivopts was coming up with candidate use costs
that seemed completely inconsistent with the target cost model.  On
further inspection, I found what appears to be a whole bunch of bugs
in the way ivopts is computing address costs:

(1) While the address cost computation is assuming in some situations
that pre/post increment/decrement addressing will be used if
supported by the target, it isn't actually using the target's address
cost for such forms -- instead, just the cost of the form that would
be used if autoinc weren't available/applicable.

(2) The computation to determine which multiplier values are supported
by target addressing modes is constructing an address rtx of the form
(reg * ratio) to do the tests.  This isn't a valid address RTX on
Hexagon, although both (reg + reg * ratio) and (sym + reg * ratio)
are.  Because it's choosing the wrong address form to probe with, it
thinks that the target doesn't support multipliers at all and is
incorrectly tacking on an extra cost for them.  I also note that it's
assuming that the same set of ratios are supported by all three
address forms that can potentially include them, and that all valid
ratios have the same cost.

(3) The computation to determine the range of valid constant offsets
for address forms that can include them is probing the upper end of
the range using constants of the form ((1<<n) - 1).  On Hexagon, the
offsets have to be aligned appropriately for the mode, so it's
incorrectly rejecting all positive offsets for non-char modes.  And
again, it's assuming that the same range of offsets are supported by
all address forms that can legitimately include them, and that all
valid offsets have the same cost.  The latter isn't true on Hexagon.

(4) The cost adjustment for converting the symbol_present address to a
var_present address seems overly optimistic in assuming that the
symbol load will be hoisted outside the loop.  I looked at a lot of
code where this was not happening no matter how expensive I made the
absolute addressing forms in the target-specific costs.  Also, if
subsequent passes actually do hoist the symbol load, this requires an
additional register to be available over the entire loop, which ivopts
isn't accounting for in any way.  It seems to me that this adjustment
shouldn't be attempted when the symbol_present form is a legitimate
address (because subsequent passes are not doing the anticipated
optimization in that case).  There's also a bug present in the cost
accounting: it's adding the full cost of the symbol + var addition,
whereas it should be pro-rated across iterations (see the way this is
handled for the non-address cases in get_computation_cost_at).

(5) The documentation of TARGET_ADDRESS_COST says that when two
(legitimate) address expressions have the same lowest cost, the one
with the higher complexity is used.  But, the ivopts code is doing the
opposite of this and choosing the lower complexity as the tie-breaker.
(Actually, it's also computing the complexity without regard to
whether the address rtx is even legitimate on the target.)

(6) The way get_address_costs is precomputing and caching a complete
set of cost data for each addressing mode seems incorrect to me, in
the general case.  For example, consider MIPS where MIPS16 and MIPS32
functions can be freely intermixed in the same compilation unit.  On
Hexagon, as I've noted, the assumption that all valid multiplier and
offset values have the same cost is also invalid.  The current code is
already precomputing 16 sets of address costs for each mode, plus
probing 128 addresses with multipliers for validity, and similarly
probing up to 32 or so constant offsets for validity, so I'm pretty
skeptical that precomputing and caching even more permutations would
be worthwhile.

(7) If the computed address cost turns out to be 0, the current code
(for some unknown reason) is turning that into 1, which can screw up
the relative costs of address computations vs other operations like
addition.  

I've come up with the attached patch to try to fix these things.  The
biggest change is that I have discarded the code for precomputing and
caching costs and instead go straight to querying the target back end
for the cost for the specific address computation we're handed; this
makes the code a lot simpler.  I would kind of like to get rid of
multiplier_allowed_in_address_p too, but it's being used in a couple
places other than the address computation and it seemed better not to
mess with that for now.  The other fixes are pretty straightforward.

I bootstrapped and regression-tested the patch on x86_64.  I haven't
tried to benchmark the performance effect of the patch on anything
other than Hexagon; there I found that, once ivopts actually started
paying attention to the target address costs function, it needed to be
re-tuned.  So, it wouldn't surprise me if other back ends could
benefit from some tweaking as well, depending on the extent to which
they're affected by the bugs I listed above.

Comments, complaints, proposals for alternate fixes, etc?  Or OK to
commit?

-Sandra


2012-06-05  Sandra Loosemore <sandra@codesourcery.com>

	gcc/
	* tree-ssa-loop-ivopts.c (comp_cost): Make complexity field signed.
	Update comments to indicate this is for addressing mode complexity.
	(new_cost): Make signedness of parameters match comp_cost fields.
	(compare_costs): Prefer higher complexity, not lower, per documentation
	of TARGET_ADDRESS_COST.
	(multiplier_allowed_in_address_p): Use (+ (* reg1 ratio) reg2) to
	probe for valid ratios, rather than just (* reg1 ratio).
	(get_address_cost): Rewrite to eliminate precomputation and caching.
	Use target's address cost for autoinc forms if possible.  Only attempt
	sym_present -> var_present cost conversion if the sym_present form
	is not legitimate; amortize setup cost over loop iterations.
	Adjust complexity computation.
	(get_computation_cost_at): Adjust call to get_address_cost.  Do not
	mess with complexity for non-address expressions.
	(determine_use_iv_cost_address): Initialize can_autoinc.
	(autoinc_possible_for_pair): Likewise.

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