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From: "wiljan" <W.Derks@nl.cis.philips.com>
Subject: Re: Byte/Bit Twiddling in Ada
Date: 1997/02/15
Message-ID: <01bc1b6e$f5d65c30$3e208b82@wd>#1/1
X-Deja-AN: 219015090
References: <5dvfnn$olj@neocad.xilinx.com> <33048844.7AEB@elca-matrix.ch>
 <dewar.856015429@merv>
Organization: Philips Electronics N.V.
Newsgroups: comp.lang.ada
Date: 1997-02-15T00:00:00+00:00
List-Id: <comp.lang.ada>


Robert Dewar <dewar@merv.cs.nyu.edu> wrote in article
<dewar.856015429@merv>...
> GNAT certainly is in the business of recognizing the CPU instruction set!
> That's what the code generation of gcc is all about.
> 
[snip]
> 
> GNAT *does* generate good code for messing with individual bits of a bit
> packed array, which seems the most common usage.
> 
I do not agree on that Robert, Plese check out the following program:

with Ada.Text_Io; use Ada.Text_Io;
procedure testsetcomp is

   type set is array(0..31) of boolean;
   pragma pack (set);

   A, B : Set;
begin
   A := (5..10=>True, others=>False);
   B := (others=>False);
   B (5..10) := (others=>True);
   if A <= B then
      Put_Line ("A<=B");
   else
      Put_Line ("not A<=B");
   end if;
end testsetcomp;

When I check the code generated using gnat -S -O2 -gnatp on above source
I find the compiler generating loops for all kind of things.
It generates loops for the variable initialisation.
It also generates loops to perform the A<=B operation.
I think this is normal usage of packed arrays. I actually use them a lot as
if
they where sets.
Anyway GNAT generates very bad code for this. Somehow it is not a lot
of knowledge about the packed arrays. I whould expect GNAT to
generate only code for the call to:
      Put_Line ("A<=B");
That all which is actually needed in above code.

Wiljan