From: Gary Benson <gbenson@redhat.com>
To: archer@sourceware.org
Cc: Yao Qi <qiyaoltc@gmail.com>
Subject: Re: C++ draft
Date: Tue, 28 Jun 2011 12:21:00 -0000 [thread overview]
Message-ID: <20110628122135.GB3005@redhat.com> (raw)
In-Reply-To: <BANLkTimpvfyY4hWO_WGxtpXqf8sLiQX6YwfX4fhyJ5Lz8nb3Qw@mail.gmail.com>
Yao Qi wrote:
> Do we plan to move gdbserver to C++? I think no, because some
> baremental boards have too few memory to hold a C++ application.
> So we are in a state that both C and C++ co-exist in GDB for
> some time. I don't think C and C++ co-existance is a problem,
> or, your plan is> about "make good use of C++ to replace some
> bad and error-prone stuffs in GDB, and keep the rest of GDB as
> it is". Is it right?
As I understand it, I don't know that there's any reason C++ has to
use more memory than C. Granted there are things like the STL that
generate vastly more code under the hood than you might expect, but
I don't think anybody is talking about using STL here.
I've spent the past few years working on HotSpot, which is written
using a similar subsection of C++ to what Tom is proposing (with the
exception that HotSpot does not use C++ exceptions). My experience
has been that the overheads of C++ are as minimal as the can be. A
data structure in a virtualized class, for example, is larger than
that same data structure in a struct by only a single pointer. In
cases where GDB is doing what C++ would anyway--for example, replacing
pointers to functions with C++ virtual functions--then we're doing
essentially the same thing, only with better readability and error-
detection.
A concrete example, hello world:
#include <stdio.h>
int
main (int argc, char *argv[])
{
puts ("Hello world");
return 0;
}
Save it as hello.c and compile with gcc, then save it as hello.cc
and compile with g++. The code for the main function is _exactly_
the same:
0000000000400514 <main>:
400514: 55 push %rbp
400515: 48 89 e5 mov %rsp,%rbp
400518: 48 83 ec 10 sub $0x10,%rsp
40051c: 89 7d fc mov %edi,-0x4(%rbp)
40051f: 48 89 75 f0 mov %rsi,-0x10(%rbp)
400523: bf 38 06 40 00 mov $0x400638,%edi
400528: e8 e3 fe ff ff callq 400410 <puts@plt>
40052d: b8 00 00 00 00 mov $0x0,%eax
400532: c9 leaveq
400533: c3 retq
400534: 90 nop
400535: 90 nop
400536: 90 nop
400537: 90 nop
400538: 90 nop
400539: 90 nop
40053a: 90 nop
40053b: 90 nop
40053c: 90 nop
40053d: 90 nop
40053e: 90 nop
40053f: 90 nop
The resulting executable is slightly larger (6562 bytes from 6433).
I'm not sure where this comes from or how it would scale from this
trivial example to a project the size of GDB, but the beauty of
Tom's plan is that the first step is to get the basic C code to
compile with a C++ compiler. Once that's done we can build the
same codebase with the different compilers and see where we're at.
Cheers,
Gary
--
http://gbenson.net/
next prev parent reply other threads:[~2011-06-28 12:21 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-06-27 21:08 Tom Tromey
2011-06-27 21:23 ` [Archer] " Joel Brobecker
2011-06-29 19:55 ` Tom Tromey
2011-06-29 21:47 ` [Archer] " Joel Brobecker
2011-06-28 4:33 ` Matt Rice
2011-06-28 8:21 ` Yao Qi
2011-06-28 12:21 ` Gary Benson [this message]
2011-06-28 17:08 ` Matt Rice
2011-06-28 17:36 ` Jan Kratochvil
2011-06-29 20:08 ` Tom Tromey
2011-06-29 20:06 ` Tom Tromey
2011-06-30 6:06 ` Yao Qi
2011-09-26 21:01 ` Jim Blandy
2011-06-29 18:52 ` Stan Shebs
2011-06-29 19:03 ` [Archer] " Joel Brobecker
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