This is the mail archive of the mailing list for the GCC project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: set_src_cost lying comment

> These are good examples of things the costing model 
> simply wasn't ever designed to consider  -- because they weren't 
> significant issues on the m68k, vax and other ports in the gcc-1 era.
> So I don't really know how to tell you to proceed -- I've considered the 
> costing models fundamentally flawed for many years, but haven't ever 
> tried to come up with something that works better.

I'm not sure I'd call it "fundamentally flawed" or that it wasn't
"designed to consider" these things.  I see the costing model as meant
as a *heuristic* for making choices, not as a precise metric for
anything.  Certainly, a lot of people worked on that model, including
both of us, during the gcc-2 RISC porting days.

I see the "flaw" as trying to use it for too much.  There's a limit to
the amount of decisions you can make with purely local data.  When
trying to decide whether a source should be a constant or a register,
you have to look at:

- the chances the reg-reg copy can be eliminated
- register pressure
- whether there's room to insn schedule around any conflicts

Unless you look at the whole picture, you're just guessing, which is, of
course, what a heuristic is all about!

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]