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9

When using the same code, simply changing the compiler (from a C compiler to a C++ compiler) will change how much memory is allocated. I'm not quite sure why this is and would like to understand it more. So far the best response I've gotten is "probably the IO streams", which isn't very descriptive and makes me wonder about the "you don't pay for what you don't use" aspect of C++.

I'm using the clang and gcc compilers, versions 7.0.1-8 and 8.3.0-6 respectively. My system is running on Debian 10 (Buster), latest. The benchmarks are done via Valgrind Massif.

#include <stdio.h>

int main() 
 printf("Hello, world!n");
 return 0;

The code used does not change, but whether I compile as C or as C++ changes the results of the Valgrind benchmark. The values remain consistent across compilers, however.
The runtime allocations (peak) for the program go as follows:

• GCC (C): 1,032 bytes (1KB)


• G++ (C++): 73,744 bytes, (~74kb)


• Clang (C): 1,032 bytes (1KB)


• Clang++ (C++): 73,744 bytes (~74kb)

For compiling, I use the following commands:

clang -O3 -o c-clang ./main.c
gcc -O3 -o c-gcc ./main.c

clang++ -O3 -o cpp-clang ./main.cpp
g++ -O3 -o cpp-gcc ./main.cpp

For Valgrind, I run valgrind --tool=massif --massif-out-file=m_compiler_lang ./compiler-lang on each compiler and language, then ms_print for displaying the peaks.

Am I doing something wrong here?

c++ c benchmarking

share|improve this question

edited 9 hours ago

Rerumu

asked 9 hours ago

RerumuRerumu

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Rerumu is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  To begin with, how are you building? What options do you use? And how do you measure? How do you run Valgrind?
  
  – Some programmer dude
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  This is C, not C++. C that compiles with a C++ compiler is just malformed C++. Regardless, C++ has more memory overhead than C as you can already tell.
  
  – bigwillydos
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Hypothesis: this allocation comes from the standard library. There's more in the C++ standard library. I don't know where 73kb extra comes from, but maybe loading a shared library counts as an allocation.
  
  – Justin
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  If I remember correctly, modern C++ compilers have to an exception model where there is no performance hit to entering a try block at the expense of a larger memory footprint, maybe with a jump table or something. Maybe try compiling without exceptions and see what impact that has. Edit : In fact, iteratively try disabling various c++ features to see what impact that has on the memory footprint.
  
  – François Andrieux
  9 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  When compiling with clang++ -xc instead of clang, the same allocation was there, which strongly suggests its due to linked libraries
  
  – Justin
  9 hours ago
  

  
  
  
  

 | 
show 4 more comments

9

When using the same code, simply changing the compiler (from a C compiler to a C++ compiler) will change how much memory is allocated. I'm not quite sure why this is and would like to understand it more. So far the best response I've gotten is "probably the IO streams", which isn't very descriptive and makes me wonder about the "you don't pay for what you don't use" aspect of C++.

I'm using the clang and gcc compilers, versions 7.0.1-8 and 8.3.0-6 respectively. My system is running on Debian 10 (Buster), latest. The benchmarks are done via Valgrind Massif.

#include <stdio.h>

int main() 
 printf("Hello, world!n");
 return 0;

The code used does not change, but whether I compile as C or as C++ changes the results of the Valgrind benchmark. The values remain consistent across compilers, however.
The runtime allocations (peak) for the program go as follows:

• GCC (C): 1,032 bytes (1KB)


• G++ (C++): 73,744 bytes, (~74kb)


• Clang (C): 1,032 bytes (1KB)


• Clang++ (C++): 73,744 bytes (~74kb)

For compiling, I use the following commands:

clang -O3 -o c-clang ./main.c
gcc -O3 -o c-gcc ./main.c

clang++ -O3 -o cpp-clang ./main.cpp
g++ -O3 -o cpp-gcc ./main.cpp

For Valgrind, I run valgrind --tool=massif --massif-out-file=m_compiler_lang ./compiler-lang on each compiler and language, then ms_print for displaying the peaks.

Am I doing something wrong here?

c++ c benchmarking

share|improve this question

edited 9 hours ago

Rerumu

asked 9 hours ago

RerumuRerumu

4855 bronze badges

New contributor

Rerumu is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  To begin with, how are you building? What options do you use? And how do you measure? How do you run Valgrind?
  
  – Some programmer dude
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  This is C, not C++. C that compiles with a C++ compiler is just malformed C++. Regardless, C++ has more memory overhead than C as you can already tell.
  
  – bigwillydos
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Hypothesis: this allocation comes from the standard library. There's more in the C++ standard library. I don't know where 73kb extra comes from, but maybe loading a shared library counts as an allocation.
  
  – Justin
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  If I remember correctly, modern C++ compilers have to an exception model where there is no performance hit to entering a try block at the expense of a larger memory footprint, maybe with a jump table or something. Maybe try compiling without exceptions and see what impact that has. Edit : In fact, iteratively try disabling various c++ features to see what impact that has on the memory footprint.
  
  – François Andrieux
  9 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  When compiling with clang++ -xc instead of clang, the same allocation was there, which strongly suggests its due to linked libraries
  
  – Justin
  9 hours ago
  

  
  
  
  

 | 
show 4 more comments

When using the same code, simply changing the compiler (from a C compiler to a C++ compiler) will change how much memory is allocated. I'm not quite sure why this is and would like to understand it more. So far the best response I've gotten is "probably the IO streams", which isn't very descriptive and makes me wonder about the "you don't pay for what you don't use" aspect of C++.

I'm using the clang and gcc compilers, versions 7.0.1-8 and 8.3.0-6 respectively. My system is running on Debian 10 (Buster), latest. The benchmarks are done via Valgrind Massif.

#include <stdio.h>

int main() 
 printf("Hello, world!n");
 return 0;

The code used does not change, but whether I compile as C or as C++ changes the results of the Valgrind benchmark. The values remain consistent across compilers, however.
The runtime allocations (peak) for the program go as follows:

• GCC (C): 1,032 bytes (1KB)


• G++ (C++): 73,744 bytes, (~74kb)


• Clang (C): 1,032 bytes (1KB)


• Clang++ (C++): 73,744 bytes (~74kb)

For compiling, I use the following commands:

clang -O3 -o c-clang ./main.c
gcc -O3 -o c-gcc ./main.c

clang++ -O3 -o cpp-clang ./main.cpp
g++ -O3 -o cpp-gcc ./main.cpp

For Valgrind, I run valgrind --tool=massif --massif-out-file=m_compiler_lang ./compiler-lang on each compiler and language, then ms_print for displaying the peaks.

Am I doing something wrong here?

c++ c benchmarking

share|improve this question

edited 9 hours ago

Rerumu

asked 9 hours ago

RerumuRerumu

4855 bronze badges

New contributor

Rerumu is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

#include <stdio.h>

int main() 
 printf("Hello, world!n");
 return 0;

clang -O3 -o c-clang ./main.c
gcc -O3 -o c-gcc ./main.c

clang++ -O3 -o cpp-clang ./main.cpp
g++ -O3 -o cpp-gcc ./main.cpp

share|improve this question

edited 9 hours ago

Rerumu

asked 9 hours ago

RerumuRerumu

4855 bronze badges

New contributor

Rerumu is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share|improve this question

edited 9 hours ago

Rerumu

asked 9 hours ago

RerumuRerumu

4855 bronze badges

New contributor

Rerumu is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 9 hours ago

RerumuRerumu

4855 bronze badges

New contributor

Rerumu is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 9 hours ago

RerumuRerumu

4855 bronze badges

1 Answer
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11

The heap usage comes from the C++ standard library. It allocates memory for internal library use on startup. If you don't link against it, there should be zero difference between the C and C++ version. With GCC and Clang, you can compile the file with:

g++ -Wl,--as-needed main.cpp

This will instruct the linker to not link against unused libraries. In your example code, the C++ library is not used, so it should not link against the C++ standard library.

You can also test this with the C file. If you compile with:

gcc main.c -lstdc++

The heap usage will reappear, even though you've built a C program.

share|improve this answer

edited 8 hours ago

answered 8 hours ago

Nikos C.Nikos C.

38.6k55 gold badges4343 silver badges7474 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  On seeing this answer, my first thought is, "If this flag helps reduce unneeded overhead, why isn't it on by default?". Is there a good answer to that?
  
  – Nat
  13 mins ago
  

  
  
  
  

add a comment | 

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11

The heap usage comes from the C++ standard library. It allocates memory for internal library use on startup. If you don't link against it, there should be zero difference between the C and C++ version. With GCC and Clang, you can compile the file with:

g++ -Wl,--as-needed main.cpp

This will instruct the linker to not link against unused libraries. In your example code, the C++ library is not used, so it should not link against the C++ standard library.

You can also test this with the C file. If you compile with:

gcc main.c -lstdc++

The heap usage will reappear, even though you've built a C program.

share|improve this answer

edited 8 hours ago

answered 8 hours ago

Nikos C.Nikos C.

38.6k55 gold badges4343 silver badges7474 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  On seeing this answer, my first thought is, "If this flag helps reduce unneeded overhead, why isn't it on by default?". Is there a good answer to that?
  
  – Nat
  13 mins ago
  

  
  
  
  

add a comment | 

11

The heap usage comes from the C++ standard library. It allocates memory for internal library use on startup. If you don't link against it, there should be zero difference between the C and C++ version. With GCC and Clang, you can compile the file with:

g++ -Wl,--as-needed main.cpp

This will instruct the linker to not link against unused libraries. In your example code, the C++ library is not used, so it should not link against the C++ standard library.

You can also test this with the C file. If you compile with:

gcc main.c -lstdc++

The heap usage will reappear, even though you've built a C program.

share|improve this answer

edited 8 hours ago

answered 8 hours ago

Nikos C.Nikos C.

38.6k55 gold badges4343 silver badges7474 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  On seeing this answer, my first thought is, "If this flag helps reduce unneeded overhead, why isn't it on by default?". Is there a good answer to that?
  
  – Nat
  13 mins ago
  

  
  
  
  

add a comment | 

The heap usage comes from the C++ standard library. It allocates memory for internal library use on startup. If you don't link against it, there should be zero difference between the C and C++ version. With GCC and Clang, you can compile the file with:

g++ -Wl,--as-needed main.cpp

This will instruct the linker to not link against unused libraries. In your example code, the C++ library is not used, so it should not link against the C++ standard library.

You can also test this with the C file. If you compile with:

gcc main.c -lstdc++

The heap usage will reappear, even though you've built a C program.

share|improve this answer

edited 8 hours ago

answered 8 hours ago

Nikos C.Nikos C.

38.6k55 gold badges4343 silver badges7474 bronze badges

g++ -Wl,--as-needed main.cpp

gcc main.c -lstdc++

share|improve this answer

edited 8 hours ago

answered 8 hours ago

Nikos C.Nikos C.

38.6k55 gold badges4343 silver badges7474 bronze badges

answered 8 hours ago

Nikos C.Nikos C.

38.6k55 gold badges4343 silver badges7474 bronze badges

answered 8 hours ago

Nikos C.Nikos C.

38.6k55 gold badges4343 silver badges7474 bronze badges