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

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Proposal: adding two zeros to the integer cost ...
\ J.C. Pizarro (27 Jun 2007)
. \ Rask Ingemann Lambertsen (28 Jun 2007)

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Subject:	Re: Proposal: adding two zeros to the integer cost to calibrate better.
Group:	Gcc
From:	Rask Ingemann Lambertsen
Date:	28 Jun 2007

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