Re: Fixing inconsistent uses of address costs

[Sandra Loosemore <sandra at codesourcery dot com>]

On 03/27/2015 03:43 AM, Kyrill Tkachov wrote:
> On 27/03/15 03:29, Bin.Cheng wrote:
> > [much snippage]
> >
> > As for tree ivopts, address cost is used in both ways.  For any
> > address computation that's invalid, it tries to legitimize it into two
> > parts, the first part results in alu instructions, the second part
> > results in address expression of different addressing modes.  Right
> > now the rtx cost (for the first part) and the address cost (for the
> > second part) are accumulated and compared altogether.  I do like the
> > idea split costs into different types because all elements are
> > entangled in single cost model, and it's hard to tune for specific
> > case.
>
> Thanks for explaining.
> I think it would be possible to make the comparisons there a bit more sane.
> If an address computation is split into alu instructions and a
> legitimate address
> then carry around the rtx cost of the alu part and the address. When the
> time
> comes to compare two computations, we create a more involved way of
> comparing.
> For example (would need benchmarking, of course):
> * Compare the rtx costs of the alu components and check the address
> preference for
> the legitimate address components using the new hook.
> * If the alu part costs are of equal rtx cost, pick the one which has
> the preferable legitimate address.
> * If expression 'a' has a more expensive alu component than 'b' but a more
> preferable address component, then use some tie breaker. Perhaps apply
> rtx costs
> on the address expression and compare those...

Just as an aside here, tree-ssa-loop-ivopts.c has a lot of other 
problems with how it's computing address costs, or it least it did when 
I last looked at it a few years ago:

https://gcc.gnu.org/ml/gcc-patches/2012-06/msg00319.html

Shortly after I posted that, Qualcomm lost interest in pushing the 
Hexagon port upstream or improving performance, so I lost interest in 
pursuing that patch when it was evident that it was going to take a lot 
of work to resolve the objections.  I think fixes for problems (2) and 
(3) there have since been pushed by other people, but I'm not sure about 
the others.

FWIW, I think a big part of the problem here is that the GCC internals 
documentation isn't very clear on where the cost of legitimizing an 
address (the "alu components" above) should be computed.  IIRC when I 
was last looking at current practice, most targets' implementation of 
TARGET_RTX_COSTS didn't make any attempt to account for the address cost 
in a MEM -- either adding the cost of legitimizing the address or the 
cost of the addressing mode itself (TARGET_ADDRESS_COST).  If 
TARGET_RTX_COSTS is supposed to do that, its documentation should say 
so.  Or maybe we need a separate hook like 
TARGET_LEGITIMIZE_ADDRESS_COST to capture the "alu components" cost.

-Sandra