Gprof and shared libraries

[Christian Sturz]

Hi,

I've a larger project that consists of several shared libraries.
The compilation is done by autoconf and automake. One of the libraries
seems to be slow so I would like to profile it. My idea was to use gprof
(I'm using gcc 3.4.6). First, I modified the Makefiles so that each library
is built with the flags "-g -pg". Next, I've set the variable LD_PROFILE to
the library I want to analyze: "LD_PROFILE=/path/to/lirary.so". Then, I ran the 
program and got the file "gmon.out". However, when I execute
"gpof MYEXECUTABLE gmon.out" all the displayed functions have a time of
0 seconds (number of calls is available) and I can't find and references
to the function from the library I specified by LD_PROFILE.

What am I doing wrong? I was looking in the internet for a solution
but could not find any helpful hints.

Thank you.

Regards,
Christian
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Compiler loop optimizations

[Christian Sturz]

Hi,

I was curious if there are any gcc compiler optimizations that can 
improve this code:

void foo10( )
{
  for ( int i = 0; i < 10; ++i )
  {
[...]
if( i == 15 ) { [BLOCK1] }
  }
}

void foo100( )
{
  for ( int i = 0; i < 100; ++i )
  {
[...]
if( i == 15 ) { [BLOCK2] }
  }
}

int main( void )
{
  foo10(  );
  foo100( );
  return 0;
}

1) For the function foo10:
The if-block following "if( i == 15 )" will be never executed since 
'i' will never become 15 here. So, this entire block could be removed
without changing the semantics. This would improve the program execution
since the if-condition does not need to be evaluated in each loop iteration. 
Can this code transformation be automatically performed by a compiler? If so, 
which techniques/analyses and optimizations must be applied? Would gcc simplify 
this loop?

2) For the function foo100:
This code is not optimal as well. The if-condition is just once met but
has to be evaluated for all of the 100 loop iterations. An idea
I had in mind was to split the loop in two parts:
for ( int i = 0; i <= 15; ++i ) {...}
BLOCK2
for ( int 16 = 0; i < 100; ++i ) {...}
So, here the evaluation of "i==15" is not required any more and we save 
100 comparisions. Is this a good idea and always applicable? Or are there
better compiler optimzations?

Thank you very much for your help. 

Regards,
Chris
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