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From: stt@houdini.camb.inmet.com (Tucker Taft)
Subject: Re: Ada to C/C++ translator needed
Date: 1996/10/15
Message-ID: <DzAwG0.HJJ.0.-s@inmet.camb.inmet.com>#1/1
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references: <53fnp2$8bs$1@goanna.cs.rmit.edu.au>
organization: Intermetrics, Inc.
newsgroups: comp.lang.ada
Date: 1996-10-15T00:00:00+00:00
List-Id: <comp.lang.ada>


Richard A. O'Keefe (ok@goanna.cs.rmit.edu.au) wrote:
: ...
: Since this is comp.lang.ada, perhaps someone could explain clearly
: whether an Ada compiler is allowed to perform this kind of optimisation
: given

: 	type Vector is array (Positive range <>) of Float;

: 	procedure Vec_Add(A: out Vector; B, C: in Vector) is
: 	begin
: 	    for I in A'Range loop A(I) := B(I) + C(I); end loop;
: 	end Vec_Add;

: I used to think that calls like

: 	declare
: 	    X, Y: Vector(3) := (1..3 => 1.0);
: 	begin
: 	    Vec_Add(X(2..3), X(1..2), Y(1..2));
: 	end;

: were forbidden, because the result depended on whether 'in Vector' was
: passed by copy or reference, but I probably got that wrong, and I am
: even less sure of exactly what the rule is in Ada 95.  A factor of 3
: speedup is worth having...

The rules regarding aliasing between parameters are given in RM95-6.2(12).
The generated code is allowed to result in the reading of an "old" value 
if there is aliasing between the parameters, and the code as written
would imply a write of an object by one access path followed by a read of 
the object by some other access path.  This implies that a loop
may be unrolled, and then different iterations interleaved by moving
certain of the reads earlier.

: Richard A. O'Keefe; http://www.cs.rmit.edu.au/%7Eok; RMIT Comp.Sci.

-Tucker Taft   stt@inmet.com   http://www.inmet.com/~stt/
Intermetrics, Inc.  Cambridge, MA  USA