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problem with microsoft speed optimization

Hello.

I'm having a problem with the Visual Studio .net (2003) C++ speed
optimization, and hope someone can suggest a workaround.

My project includes many C++ files, most of which work fine with speed
optimization turned on. At least one does not however, though it does
work with size optimization turned on. I don't know specifically what
the optimizer is doing wrong, just that the output is incorrect. And I
know within about 10 lines where the problem is. I've tried
rearranging the offending lines (which are straight C code). I've
tried using 'volitle' data types in hopes of avoiding optimization.
I've looked for missing variable initializations , but everything looks
correct and straightforward .

One solution would be to have a precompiler directive which would
enable (or disable) the optimization only on specific files. Of course
I can manually turn the optimization on and off while recompiling
specific files, but I need something automated.

I can't post the code, but will try to follow up with a test case that
exhibits the same behavior.

Presumably the program would compile fine with g++, but unfortunately
that's not an option for this project.

Thanks very much.

Mark

Dec 29 '05 #1
21 2594

mjbackues at yahoo wrote:
Hello.

I'm having a problem with the Visual Studio .net (2003) C++ speed
optimization, and hope someone can suggest a workaround.

My project includes many C++ files, most of which work fine with speed
optimization turned on. At least one does not however, though it does
work with size optimization turned on. I don't know specifically what
the optimizer is doing wrong, just that the output is incorrect. And I
know within about 10 lines where the problem is. I've tried
rearranging the offending lines (which are straight C code). I've
tried using 'volitle' data types in hopes of avoiding optimization.
I've looked for missing variable initializations , but everything looks
correct and straightforward .

One solution would be to have a precompiler directive which would
enable (or disable) the optimization only on specific files. Of course
I can manually turn the optimization on and off while recompiling
specific files, but I need something automated.

I can't post the code, but will try to follow up with a test case that
exhibits the same behavior.

Presumably the program would compile fine with g++, but unfortunately
that's not an option for this project.


It sounds like you're asking about the particular behaviour of your
compiler's optimiser. If so, then you are asking the wrong group. There
is a VC++ .Net group suggested here.

http://www.parashift.com/c++-faq-lit...t.html#faq-5.9

Gavin Deane

Dec 29 '05 #2
OK, thanks.

Dec 29 '05 #3
mjbackues at yahoo wrote:
Hello.

I'm having a problem with the Visual Studio .net (2003) C++ speed
optimization, and hope someone can suggest a workaround.
This subject matter is off-topic in this group, which is solely
concerned with *standard* C++. See the FAQ for what is on-topic and for
some suggestions of better places to post:

http://www.parashift.com/c++-faq-lit...t.html#faq-5.9
My project includes many C++ files, most of which work fine with speed
optimization turned on. At least one does not however, though it does
work with size optimization turned on. I don't know specifically what
the optimizer is doing wrong, just that the output is incorrect. And I
know within about 10 lines where the problem is. I've tried
rearranging the offending lines (which are straight C code). I've
tried using 'volitle' data types in hopes of avoiding optimization.
I've looked for missing variable initializations , but everything looks
correct and straightforward .
People often blame the compiler for strange errors, but it usually
turns out to be user error, perhaps in an apparently unrelated part of
the program. Have you tried looking at the generated assembly code?
One solution would be to have a precompiler directive which would
enable (or disable) the optimization only on specific files. Of course
I can manually turn the optimization on and off while recompiling
specific files, but I need something automated.
Look up #pragma in your compiler documentation. Better yet, turn off
the optimizer altogether (except for inlining and such). See this
article, "Optimizati on: Your Worst Enemy":

http://www.codeproject.com/tips/optimizationenemy.asp

See also "Surviving the Release Version":

http://www.codeproject.com/debug/survivereleasever.asp
I can't post the code, but will try to follow up with a test case that
exhibits the same behavior.
You should have done this before posting. See the FAQ:

http://www.parashift.com/c++-faq-lit...t.html#faq-5.8

But, of course, if you had read the FAQ in advance, you would have
posted to a different newsgroup to begin with. ;-)
Presumably the program would compile fine with g++, but unfortunately
that's not an option for this project.


Such warantless speculation is entirely unhelpful. If possible, you
should try at least the problem spot in your code on that and any other
compiler you have at your disposal. You could use the same distilled
code that you'll be posting shortly, and that might generate some
helpful results.

Cheers! --M

Dec 29 '05 #4
Wow. Since you fancy yourself clever and obviously have time on your
hands....follow ing is a test program that works with gcc but not with
visual studio (either with or without optimization). The output using
'fprintf' is correct, whereas the output with 'fwrite' is not, and the
resulting file doesn't even have the right size. The 'fwrite' does
work however if the '3.14' in the preceeding calculation is replaced
with another pi approximation with more digits.

Useful responses from other people are also welcome.

#include <stdio.h>
#include <math.h>
#include <malloc.h>
#define PI 3.1415926535897 9

main(){
float* ph;
int i;
FILE *fid;

ph = (float*)malloc( sizeof(float)*1 28*1024*2);

for(i=0;i<128*1 024*2;i+=2){
ph[i]=(float)cos(3.1 4*(double)i*0.2 );
ph[i+1]=1.0;
}

fid=fopen("out1 ","w");
for(i=0;i<128*1 024*2;i++)
fprintf(fid,"%l f\n",ph[i]);
fclose(fid);

fid = fopen("out2","w ");
fwrite(ph,sizeo f(float),128*10 24*2,fid);
fclose(fid);

free(ph);
}

Dec 29 '05 #5
mjbackues at yahoo wrote:
fid = fopen("out2","w ");
fwrite(ph,sizeo f(float),128*10 24*2,fid);


When you're writing binary data you must open the file with "wb" rather
than "w".

--

Pete Becker
Dinkumware, Ltd. (http://www.dinkumware.com)
Dec 29 '05 #6
mjbackues at yahoo wrote:
Wow. Since you fancy yourself clever and obviously have time on your
hands....follow ing is a test program that works with gcc but not with
visual studio (either with or without optimization). The output using
'fprintf' is correct, whereas the output with 'fwrite' is not, and the
resulting file doesn't even have the right size. The 'fwrite' does
work however if the '3.14' in the preceeding calculation is replaced
with another pi approximation with more digits.

Useful responses from other people are also welcome.

#include <stdio.h>
#include <math.h>
#include <malloc.h>
#define PI 3.1415926535897 9

main(){
float* ph;
int i;
FILE *fid;

ph = (float*)malloc( sizeof(float)*1 28*1024*2);

for(i=0;i<128*1 024*2;i+=2){
ph[i]=(float)cos(3.1 4*(double)i*0.2 );
ph[i+1]=1.0;
}

fid=fopen("out1 ","w");
for(i=0;i<128*1 024*2;i++)
fprintf(fid,"%l f\n",ph[i]);
fclose(fid);

fid = fopen("out2","w ");
fwrite(ph,sizeo f(float),128*10 24*2,fid);
fclose(fid);

free(ph);
}


In what way is the output incorrect?

BTW, a more C++-style way to do it might look something like:

#include <vector>
#include <cmath>
#include <fstream>
#include <iostream>
#include <iterator>
#include <algorithm>

using namespace std;

#define PI 3.1415926535897 9

int main()
{
vector<double> v( 128*1024*2 );

for(unsigned int i=0; i < v.size(); i+=2 )
{
v[i]=cos(PI*i*0.2);
v[i+1]=1.0;
}

ofstream f1("out1");
if( !f1 )
{
cerr << "error!" << endl;
return -1;
}
copy( v.begin(), v.end(),
ostream_iterato r<double>( f1, "\n" ) );

ofstream f2( "out2", ios_base::binar y );
if( !f2
|| !f2.write( reinterpret_cas t<const char*>( &v[0] ),
v.size()*sizeof (double) ) )
{
cerr << "error!" << endl;
return -1;
}

return 0;
}

Dec 29 '05 #7
yeah, that's it, thanks very much.

Dec 29 '05 #8
Thanks. This was a problem a coworker had, and he wound up using
ofstream and write to get around it.

Dec 29 '05 #9
>yeah, that's it, thanks very much.

oops, intended as reply to Pete Becker

Dec 29 '05 #10

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