Proposal: adding two zeros to the integer cost to calibrate better.

[Rask Ingemann Lambertsen]

On Wed, Jun 27, 2007 at 09:41:08PM +0200, J.C. Pizarro wrote:

> I recommend to add 2 zeros to the integer costs as if those are 2 decimal 
> zeros,
> for example,
> 
> insn_cost 5: 1200     // it's 12.00
> insn_cost 6: 800      // it's 8.00
> insn_cost 7: 400      // it's 4.00
> insn_cost 8: 433      // it's 4.33   little costly than 7th, +x.xx%
> better calibrating.
> insn_cost 9: 466      // it's 4.66   little costly than 8th, +x.xx%
> better calibrating.
> insn_cost 10: 500      // it's 5.00

   We already multiply the costs by four (COSTS_N_INSNS()), so you can
already add or subtract one to fine tune costs.

   A much worse problem is that e.g. combine only passes small fragments of
an insn to rtx_cost(), which means that the back end had to guess what the
insn looked like to begin with, rather than simply looking at the rtx passed
to it. It is a garbage in, garbage out system.

-- 
Rask Ingemann Lambertsen