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Re: [C Patch]: pr52543

On May 3, 2012, at 12:50 PM, Georg-Johann Lay wrote:
> It's hardly possible to write proper rtx_costs for SET:
> 1) What should be the cost of (const_int 1) if you don't see the
> machine mode? Is it QI, is it HI, is it SI or whatever?

You can choose to see the complete expression in its entirety and form the cost for it.

  (set (reg:DI 1) (const_int 1))

is a DI mode set of register 1, with the value 1.  This can have a different cost, from the perspective of the cost function from:

  (set (reg:SI 1 (const_int 1))

or even

  (set (reg:SI 1 (const_int 2))


  (set (reg:SI 2 (const_int 1))

though, little else in the compiler would help mitigate such differences.  Just return a non-zero value from the cost function.  If you return zero, you are saying that you don't care, or that the costs compose in a simplistic manner.  Try returning 1, and figuring out the total cost of the entire expression yourself, if the simplistic answer is wrong.

> 2) If the target will be a REG, what is the register class for the
> assignment?

Hard register has a class associated with it that can be found with REGNO_REG_CLASS (REGNO (x)).  If the register isn't a hard register, then, no class has been chosen for it yet, and the register allocator can choose any valid class it wants for the mode.  I suspect you're better off explaining the average cost (or maybe even the best case cost), when multiple choices exist.  In general, someone else should check later the true cost of the operation based upon the class and should be willing to influence the code generated (the class picked), so what you return for rtx_costs shouldn't matter too much.

> rtx_costs are called after reload, so it would be good to
> know. It would be good to know if it is a pseudo or hard reg.

HARD_REGISTER_NUM_P (REGNO (x)) will tell you if it is hard.  The inverse of this will tell you if is isn't hard, aka, a pseudo.

> And in many places the backend does not know where it is standing.
> Is it upon expanding? Prior or after combine? Or split1?

I think the idea is to give the cost of the rtl it asks for.

  (set (reg:SI 1) (const_int 0))

should have the same cost, before reload, after reload, during optimization, just before final, or at expand time.

> 3) Likewise, the costs of MEM are peeled of MEM and pass just
> the address without any information on the MEM like it's address
> space. Cost might highly depend on the address space involved.

Yes, that is why on my machine:

  (set (mem) (reg))

has one set of costs, and

  (set (reg) (mem))

has a completely different set of costs.  The address space, if it influences the cost, can be had with:


or a store, and:


for a load.

> The original PR is because split of mem:HI is fine -- if it reads
> from generic. And splitting mem:HI is complete bloat for other
> address spaces. Likewise for wider modes like PSI, SI, ...

The you will want to check the mode of the MEM, and address space involved.

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