A concern about mixing C and C++

[ziman137]

Hello all,

I have a question and am seeking for some advice.

I am currently working to implement an algorithmic library. Because
the performance is the most important factor in later applications, I
decide to write it in C instead of C++. However, I thought it might be
convenient to use some C++ code at some misc places. I'm aware that, I
could always use the C++ compiler to get it work.

My concerns are:

1) Would the way I mix C and C++ code have any potential drawbacks to
the performance?
2) Would the way I mix C and C++ code have any potential drawbacks for
the future users to use the library?

My intention for choosing C interface instead of C++ OOD is to gain the
maximum performance as possible, yet I still like to use some C++
coding features (e.g., "const", reference instead of pointers, ...).

Thanks,

Gary

[jacob navia]

Keith Thompson a écrit :

Richard Heathfield <in*****@invali d.invalidwrites :

>>jacob navia said:

>>>There may be situations
where C++ templates outperform assembly language,

Jacob's talking through his hat again. His claim is easily disproved. All we
have to do is tell the compiler to generate assembly language from the C++
template code. We now have assembly language that performs exactly as well
as the C++ template code. And, given an experienced assembly language
programmer (to match the experienced C++ programmer who produces such
astoundingl y quick code), we can almost certainly find an optimisation,
however trivial, that will make the assembly language version at least a
little faster than the C++ template version from which it was generated.

On the other hand, for any language at a higher level than assembler,
the compiler may find opportunities for optimizations that someone
creating hand-written assembler wouldn't find, or would choose not to
use.

For example, if I'm writing low-level code that I expect to be
maintained, I'm not likely to perform a pervasive optimization that
works only if an array size is a power of 2.

A skilled assembly language programmer is likely to be much smarter
than an optimizing compiler, but an optimizer has a different set of
tradeoffs to consider. A programmer will perform optimizations over
time as he's working on a piece of code; an optimizing compiler will
redo all its work from scratch every time it's invoked.

Yes, you can use an optimizing compiler as a way to generate assembly
language -- but that argument doesn't apply to hand-written assembly
language.

The things I had in mind is that templates expose to the
compiler details that allows more optimizations than what
an assembly language programmer would do. Specifically it would
be highly surprising that he/she writes the same optimized version
for a qsort search for each possible type of argument that the
function could receive. Most likely it will design a general
qsort interface (like C offers) that will be highly optimized
but never as optimized as a specially tailored qsort for each
input type.

jacob

[Ark]

Ian Collins wrote:

Ark wrote:

>Ian Collins wrote:

>>Ark wrote:

Ian Collins wrote:

C++ gives the developer a choice between the C way of doing something
and the C++ way.
>
Ehmmm... please help me here: I am not a C++ guy.
AFAIK, C++ does not allow all the C way (example: tentative
declarations ).
Then, if I write C-style in C++, at the very minimum the C++ compiler
doesn't know e.g. about any exceptions that may be thrown in a function
I call and which is in a different translation unit. So it just has to
bring in the heavy machinery in just in case. Am I missing something?

If my understanding of tentative declarations is correct, they are
synonymous with forward declarations in C++ due to the differing use of
'struct' in a declaration, that is:

struct X;

forward declares the type X.

The compiler doesn't have to bother with unexpected exceptions when
compiling idiomatic C code, any unhandled exception can be caught by
whatever calls main().

As a trivial example, on my platform the following generated essentially
the same code when compiled as C or C++:

extern int fn(void);
extern void fn1(int);

typedef struct { int n; } X;

int main(void)
{
X x;

x.n = fn();

fn1( x.n );
}

1. Tentative declarations of variables are (in C) indistinguishab le from
definitions until the end of the translation unit; they AFAIK expressly
prohibited in C++. E.g., a (const) circular data structure like
typedef struct X {int x, const struct X *next} X;

typedef struct X {int x; const struct X *next;} X;

Is valid C but not C++.

typedef struct X {int x; const X *next;} X;

Is valid C++ but not C.

typedef struct X_t {int x; const struct X_t *next;} X;

Is valid C++ and C.

>const X x;
const X y;
const X z = {5, &x};
const X y = {6, &z};
const X x = {7, &y};
is a valid C but not C++.

Correct , C++ prohibits uninitialised consts. Easily fixed for both thus:

extern const X x;
extern const X y;
const X z = {5, &x};
const X y = {6, &z};
const X x = {7, &y};

>2. Interesting example; just curious what the differences are and
whether it matters that your function is called main, not, say, foo.

The C++ compiler pushed one extra register on the stack. The function
name does not make any difference.

1. It is not an easy fix if I want x, y, z to also be static. (Actually,
regardless of whether they are const).
2. It's interesting to understand /why/ C++ uses more stack. Is it, by
any chance, for passing (e.g. passing through) a pointer to an exception
object? In any event, those things get compounded... definitely a
consideration for resource-constrained environments...
- Ark

[Malcolm]

"jacob navia" <ja***@jacob.re mcomp.frwrote in message
news:44******** *************@n ews.orange.fr.. .

Keith Thompson a écrit :

>Richard Heathfield <in*****@invali d.invalidwrites :

>>>jacob navia said:
There may be situations
where C++ templates outperform assembly language,

Jacob's talking through his hat again. His claim is easily disproved. All
we have to do is tell the compiler to generate assembly language from the
C++ template code. We now have assembly language that performs exactly as
well as the C++ template code. And, given an experienced assembly
language programmer (to match the experienced C++ programmer who produces
such astoundingly quick code), we can almost certainly find an
optimisation , however trivial, that will make the assembly language
version at least a little faster than the C++ template version from which
it was generated.

On the other hand, for any language at a higher level than assembler,
the compiler may find opportunities for optimizations that someone
creating hand-written assembler wouldn't find, or would choose not to
use.

For example, if I'm writing low-level code that I expect to be
maintained, I'm not likely to perform a pervasive optimization that
works only if an array size is a power of 2.

A skilled assembly language programmer is likely to be much smarter
than an optimizing compiler, but an optimizer has a different set of
tradeoffs to consider. A programmer will perform optimizations over
time as he's working on a piece of code; an optimizing compiler will
redo all its work from scratch every time it's invoked.

Yes, you can use an optimizing compiler as a way to generate assembly
language -- but that argument doesn't apply to hand-written assembly
language.

The things I had in mind is that templates expose to the
compiler details that allows more optimizations than what
an assembly language programmer would do. Specifically it would
be highly surprising that he/she writes the same optimized version
for a qsort search for each possible type of argument that the
function could receive. Most likely it will design a general
qsort interface (like C offers) that will be highly optimized
but never as optimized as a specially tailored qsort for each
input type.

I did see some benchmarks for the standard template library, and they were
better than C equivalents. I thought that maybe the time had come to go to
C++.
What I forgot was that the syntax was difficult, and in fact a step too far,
with an already difficult and overloaded language. No one could use the STL
constructs effectively. So the experiment was short-lived, and now I have
virtually given up on C++ altogether.
--
www.personal.leeds.ac.uk/~bgy1mm
freeware games to download.

[Stephen Sprunk]

"ziman137" <ga********@yah oo.comwrote in message
news:11******** *************@m 73g2000cwd.goog legroups.com...

I am currently working to implement an algorithmic library. Because
the performance is the most important factor in later applications,
I
decide to write it in C instead of C++. However, I thought it might
be
convenient to use some C++ code at some misc places. I'm aware
that, I could always use the C++ compiler to get it work.

There's two basic approaches to mixing C and C++:

1. Write the code in C and provide (optional) C++ wrappers.
2. Write the code in C++ and provide (optional) C wrappers.

Once you use _any_ C++isms in your library, you're forever chained to
the portability problems that C++ still encounters, limit yourself to
systems with C++ available, etc. Once you've paid that price, you
might as well write the whole library in C++ and then it's off-topic
for clc (though clc++ will probably be happy to help with the C
wrappers).

My intention for choosing C interface instead of C++ OOD is to gain
the
maximum performance as possible, yet I still like to use some C++
coding features (e.g., "const", reference instead of pointers, ...).

If you're going to write the library in the subset of C++ that looks
like C, you might as well write it in C and forgo the syntactic sugar
that C++ provides (except, perhaps, in some C++ wrappers). If you
can't give up C++'s syntactic sugar, then write the library in _real_
C++ (not some ugly common subset of the two) and provide C wrappers.

Mixing languages in the same project is considered bad form unless you
have a really strong reason, and "const" or "references " are IMHO not
a reason worth dealing with all the headaches of that "solution".

S

--
Stephen Sprunk "God does not play dice." --Albert Einstein
CCIE #3723 "God is an inveterate gambler, and He throws the
K5SSS dice at every possible opportunity." --Stephen Hawking

--
Posted via a free Usenet account from http://www.teranews.com

[Al Balmer]

On Sun, 30 Jul 2006 10:42:21 +1200, Ian Collins <ia******@hotma il.com>
wrote:

>Richard Heathfield wrote:

>Keith Thompson said:

<snip>

>>>On the other hand, for any language at a higher level than assembler,
the compiler may find opportunities for optimizations that someone
creating hand-written assembler wouldn't find, or would choose not to
use.

Sure, but perhaps you missed my point, which is that the /starting point/
for the assembly language programmer would be the super-efficient C++
template code, expressed in assembly language form. Given that starting
base, (a) it's bound to be as fast as the C++ template code, because it's
the same code that the C++ template code is compiled to - which is in
itself sufficient to destroy Mr Navia's point, and (b) it would be quite
surprising if the skilled assembly language programmer couldn't find /one/
hand-optimisation to make the asm form just a tiny smidgenette faster.

I don't know if you do this, but I often find I can obtain more
productive optimisations by using the generated assembly language to
tune the high level language code rather than hand editing the assembly.

When you do that, are you not tuning your code to a particular
compiler? No worse than tuning the assembler code, of course, and less
detrimental to portability. For the ultimate in (non-portable)
efficiency, you might do another iteration of the process.

>
One area optimising compilers have an edge and will continue to improve
is global optimisations, there is only so much code a human can retain
in working memory. We can often do better with micro-optimisations, but
the compiler is better equipped to see the big picture.

Even if the programmer can see the big picture, the compiler can do it
systematically and much more quickly.

--
Al Balmer
Sun City, AZ

[Ian Collins]

Al Balmer wrote:

On Sun, 30 Jul 2006 10:42:21 +1200, Ian Collins <ia******@hotma il.com>
wrote:

>>Richard Heathfield wrote:

>>>Keith Thompson said:

<snip>

On the other hand, for any language at a higher level than assembler,
the compiler may find opportunities for optimizations that someone
creating hand-written assembler wouldn't find, or would choose not to
use.
Sure, but perhaps you missed my point, which is that the /starting point/
for the assembly language programmer would be the super-efficient C++
template code, expressed in assembly language form. Given that starting
base, (a) it's bound to be as fast as the C++ template code, because it's
the same code that the C++ template code is compiled to - which is in
itself sufficient to destroy Mr Navia's point, and (b) it would be quite
surprising if the skilled assembly language programmer couldn't find /one/
hand-optimisation to make the asm form just a tiny smidgenette faster.

I don't know if you do this, but I often find I can obtain more
productive optimisations by using the generated assembly language to
tune the high level language code rather than hand editing the assembly.

When you do that, are you not tuning your code to a particular
compiler? No worse than tuning the assembler code, of course, and less
detrimental to portability. For the ultimate in (non-portable)
efficiency, you might do another iteration of the process.

Like most low level optimisations, I tend to do this on embedded
systems, where portability isn't too much of an issue (I've found it
more cost effective to throw faster hardware at desktop applications
rather than fiddle with low level optimisations). The results can be
dramatic, one compiler I used generated diabolical code with loops like

for( i = 0; i < size; ++i )
{
doStuffWith( array[i] );
}

But good tight code with

int* p = array;
int* end = array+size;

while( p != end )
{
doStuffWith( *p++ );
}

--
Ian Collins.

[Al Balmer]

On Mon, 31 Jul 2006 19:21:07 +1200, Ian Collins <ia******@hotma il.com>
wrote:

>Like most low level optimisations, I tend to do this on embedded
systems, where portability isn't too much of an issue (I've found it
more cost effective to throw faster hardware at desktop applications
rather than fiddle with low level optimisations). The results can be
dramatic, one compiler I used generated diabolical code with loops like

for( i = 0; i < size; ++i )
{
doStuffWith( array[i] );
}

But good tight code with

int* p = array;
int* end = array+size;

while( p != end )
{
doStuffWith( *p++ );
}

I can believe it - I've seen some strange things. Sometimes compilers
used for embedded devices seem to assume that the user will tweak the
assembler :-)

I don't do much embedded programming myself anymore, but the hardware
guy down the hall yells for help quite often, so I still see a lot of
it. We often sit in his office and massage C source until the
assembler looks the way he wants it.

--
Al Balmer
Sun City, AZ

[Frederick Gotham]

Ian Collins posted:

One compiler I used generated diabolical code with loops like

for( i = 0; i < size; ++i )
{
doStuffWith( array[i] );
}

But good tight code with

int* p = array;
int* end = array+size;

while( p != end )
{
doStuffWith( *p++ );
}

If I know the the loop must execute at least once, then I choose:

T *p = array;

T const *const p_over = array + len;

do Process(*p++);
while(p_over != p);

--

Frederick Gotham

[Dave Thompson]

On Sun, 30 Jul 2006 17:07:12 +1200, Ian Collins <ia******@hotma il.com>
wrote:

Ark wrote:

1. Tentative declarations of variables are (in C) indistinguishab le from
definitions until the end of the translation unit; they AFAIK expressly
prohibited in C++. E.g., a (const) circular data structure like
typedef struct X {int x, const struct X *next} X;

typedef struct X {int x; const struct X *next;} X;

Is valid C but not C++.

Wrong. In C++ it is a 'benign' redefinition of the typename, which is
allowed. For essentially this reason.

typedef struct X {int x; const X *next;} X;

Is valid C++ but not C.

typedef struct X_t {int x; const struct X_t *next;} X;

Is valid C++ and C.

Both right.

const X x;
const X y;
const X z = {5, &x};
const X y = {6, &z};
const X x = {7, &y};
is a valid C but not C++.

Correct , C++ prohibits uninitialised consts. Easily fixed for both thus:

Even for nonconst C++ prohibits the declare-now define-later approach
without (explicit) extern, which as you note fixes the problem.

extern const X x;
extern const X y;
const X z = {5, &x};
const X y = {6, &z};
const X x = {7, &y};

However, this does not work for static=internal-linkage variables.
C++ does not allow /*qual?*/ static X x;
then /*qual*/ /*static*/ X x = init;

Or, write a ctor or other factory routine for X and:
const X z = X /* or makeX */ (5, &x);
etc.

For this particular case in EITHER C or C++
I might also consider something like:
const /*struct*/ X array [3] =
{ { 5, &array[2] }, { 6, &array[1] }, { 7, &array[0] } };
#define x array[2] // only if x is a more unique name
or in C++ X & x (&array[2])
etc.

- David.Thompson1 at worldnet.att.ne t