[Freeciv-Dev] Re: [PATCH] is_normal_tile function

[Raimar Falke <hawk@xxxxxxxxxxxxxxxxxxxxxxx>]

On Fri, Aug 17, 2001 at 09:40:28AM -0400, Ross W. Wetmore wrote:
> At 11:41 AM 01/08/17 +0200, Raimar Falke wrote:
> >On Thu, Aug 16, 2001 at 08:15:04PM -0400, Ross W. Wetmore wrote:
> >> At 09:32 AM 01/08/16 +0200, Raimar Falke wrote:
> [...]
> >> I know you hate this, but if you look at most machine instruction sets
> >> they have indexing operations that are perverted into 3-way adds by gcc
> >> and other compilers. So, ...
> [...]
> >> This also means that when people cleanup map.xsize-1 they are usually 
> >> wasting their time :-).
> >
> >I don't understand this one.
> 
> In 386 assembler, the instruction is LEA disp(index,base). But the
> equivalent exists on most other platforms because hardware indexed array
> operations are a big payoff.
> 
> Compilers fold constants into the disp field, and can add an index to a
> base as well. This means  x - (map.xsize-1) is LEA 1(-map.xsize,x) with the
> condition set for the branch. Or something like this ...
> 
> Constant additions or subtractions from a base value as are typical in most
> array index computations are very fast single instructions. So you
> shouldn't worry about a few being used in any computation as the compiler
> will find them.

I know this (ab)use of lea from the m68k. I didn't and still don't
understand what you mean with "when people cleanup map.xsize-1".

        Raimar

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