Re: Development process for i386 machine descriptions

[Uros Bizjak <ubizjak at gmail dot com>]

Hello!

> 1.) The processor_costs structure seems very limited, but seem very 
> easily to "fill in" but are these costs supposed to be best or worst 
> case? For instance, many instructions with different sized operands 
> vary in latency.

Instruction costs are further refined in config/i386.c, ix86_rtx_costs 
and the cost for various operand types is determined in several *_cost 
functions, scattered around i386.c file.

> 2.) I don't understand the meaning of the stringop_algs, scalar, 
> vector, and branching costs at the end of the processor_cost 
> structure. Could someone give me an accurate description?

stringop_algs is a structure that defines various algorithms for string 
processing functions (memcpy, memset, ...). This structure also defines 
size thresholds for various algorithms.

The costs at the end of a cost structure are used in autovectorization 
decisions, when -fvect-cost-model is in effect (please look at the ehd 
of i386.h where these values are used).

> 3.) The processor I am currently attempting to model is 
> single-issue/in-order with a simple pipeline. Stalls can occasionally 
> occur in the fetch/decode/translate, but the core is the latency of 
> instructions in the functional units in the execute stage. What 
> recommendations can anyone make to me for designing the DFA? Should it 
> just directly model the functional units latencies for certain insn types?
Hm, perhaps you should look into {athlon, geode, k6, pentium, ppro}.md 
files first. All these files define scheduling for various processors. 
I'm sure that quite some ideas can be harvested there.

Uros.