ANN: Paraffin 4.2, Parallelism Generics

[Brad Moore]

This is a minor update to the Paraffin release 4.1 that was released a
couple days ago.

This release (4.2) is geared towards getting the Ravenscar version of
the FFT (Fast Fourier Transform) paraffinalia app to work on Windows,
and other targets where synchronous barriers are implemented by
protected types.

To achieve this, a new package, called
Parallel.Simple_Synchronous_Barriers was introduced.

This package provides the services of a barrier when only two tasks
need to wait on the barrier, and is implemented using Ada's suspension
objects, rather than a protected type.

The issue is that on Windows, Ada.Synchronous_Barriers is implemented
by a protected object in GNAT. Ravenscar disallows protected objects
to be declared at a nested level, which is needed by the FFT app.
Since only two tasks ever need to wait on a barrier in the FFT
algorithm, a specialized implementation of a barrier can be constructed
using Ada's suspension objects.

This specialized barrier satisfies the Ravenscar restrictions, but also
is lighter weight, and was deemed worthwhile for use for the
non-ravenscar version of fft.

As a result of this change, the project file test_ravenscar_fft_linux
was renamed to test_ravenscar_fft, since it should now be portable to
all targets.

The latest stable release and older releases may be downloaded from;

https://sourceforge.net/projects/paraffin/files/

For those who want the current development versions of the source they
can download using git (http://git-scm.com/) by issuing the following
commands;

mkdir sandbox
cd sandbox
git clone git://git.code.sf.net/p/paraffin/code paraffin-code

[volker...@googlemail.com]

Dear Brad,

i tried to compile the Praffin 4.2 test-projects and got this compile error:

parallel.ads:50:06: must use private with clause for preelaborated unit "Multiprocessors"

Compiler: GNAT GPL 2013 / x86_64-darwin

Any hints?

BW,
Volkert

[Brad Moore]

Yes, I suspect you have just downloaded the new GNAT 2013, as I have.

I am seeing the same issue. The 2012 GNAT did not complain and compiled
the code, but the code actually should not have compiled. The 2013 GNAT
is detecting the problem.

The problem is because the package spec, parallel.ads has the
Remote_Types pragma, but it also with's the package
System.Multiprocessors, which is not a Remote_Types, Shared_Passive, or
Pure package. A Remote_Types package spec can only depend on one of the
above types of package specs.

To fix the problem, delete (or comment out) the Remote Types pragma in
parallel.ads.

Your code should then compile.

I was a bit mystified when I first saw the error message.

I then remembered an AI
http://www.ada-auth.org/cgi-bin/cvsweb.cgi/ai05s/ai05-0206-1.txt?rev=1.6

Which was one I was particularly involved with, where the rules were
relaxed so that a Remote_Types package spec could depend on a
preelaborated packages so long as the with clause that mentions the
preelaborated package is a private with clause.

The error message is suggesting that the with for System.Multiprocessors
should be changed to be a private with, but that
wouldn't work, because the public part of parallel.ads uses declarations
from System.Multiprocessors.

Hopefully Adacore will improve the error message for GNAT 2014, but it
is reporting an actual error in the code. It is not a compiler bug.
Ideally the error message would mention that the Remote_Types pragma is
triggering the error.

I also found another set of problems in the code. The 2013 compiler is
now checking that the aliased keyword must be present on parameters of
overriding subprograms if it was specified on a parameter of the
interface. It wasn't doing that in 2012.

I have checked in the changes onto sourceforge, which you can retrieve
using git, but I have not yet created a new bugfix release, which I will
do as soon as I get a chance.

Thanks for reporting the error.

Brad

[volker...@googlemail.com]

Brad, thank you for the explanation. It compiles fine now ...