Does C++ optimisation level affect Swig Python module performance - c++

Question

I have a large Swig Python module. The C++ wrapper ends up being about 320,000 LoC (including headers I guess). I currently compile this with -O1, and g++ produces a binary that is 44MiB in size, and takes about 3 minutes to compile it.
If I turn off optimisation (-O0), the binary comes out at 40MiB, and it takes 44s to compile.
Is compiling the wrapper with -O0 going to hurt the performance of the python module significantly? Before I go and profile the performance of the module at different optimisation levels, has anyone done this sort of analysis before or have any insight into whether it matters?

Answer 1

-O0 deactivates all the optimizations performed by gcc. And optimizations matter.
So, without a lot of knowledge in your application, I could suggest that this will hurt the performance of your application.
A usually safe optimization level to use is -O2.
You can check the kind of optimizations performed by GCC in:
http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html.
But at the end, if you want to know exactly you should compile at different levels and profile.

Answer 2

This is bad irrespective of SWIG modules or not. There are many optimisations that happen even with gcc -O1 which you will miss if you prevent them happening.
You can check the difference by inspecting the asm generated by your compiler of choice. Of these the ones I trivially know will be detrimental to SWIG's generated wrapper:
Dead code elimination:
void foo() {
int a = 1;
a = 0;
}
With -O1 this entirely pointless code gets totally removed:
foo:
pushl %ebp
movl %esp, %ebp
popl %ebp
ret
whereas with -O0 it becomes:
foo:
pushl %ebp
movl %esp, %ebp
subl $16, %esp
movl $1, -4(%ebp)
movl $0, -4(%ebp)
leave
ret
Register allocation will be detrimentally impacted in functions with lots of local variables - most SWIG wrapper functions will see a hit from this. It's hard to show a concise example of this though.
Another example, the output from gcc compiling the SWIG wrapper for the prototype:
int foo(unsigned int a, unsigned int b, unsigned int c, unsigned int d);
Generates with -O0:
Java_testJNI_foo:
pushl %ebp
movl %esp, %ebp
subl $88, %esp
movl 16(%ebp), %eax
movl %eax, -48(%ebp)
movl 20(%ebp), %eax
movl %eax, -44(%ebp)
movl 24(%ebp), %eax
movl %eax, -56(%ebp)
movl 28(%ebp), %eax
movl %eax, -52(%ebp)
movl 32(%ebp), %eax
movl %eax, -64(%ebp)
movl 36(%ebp), %eax
movl %eax, -60(%ebp)
movl 40(%ebp), %eax
movl %eax, -72(%ebp)
movl 44(%ebp), %eax
movl %eax, -68(%ebp)
movl $0, -32(%ebp)
movl -48(%ebp), %eax
movl %eax, -28(%ebp)
movl -56(%ebp), %eax
movl %eax, -24(%ebp)
movl -64(%ebp), %eax
movl %eax, -20(%ebp)
movl -72(%ebp), %eax
movl %eax, -16(%ebp)
movl -16(%ebp), %eax
movl %eax, 12(%esp)
movl -20(%ebp), %eax
movl %eax, 8(%esp)
movl -24(%ebp), %eax
movl %eax, 4(%esp)
movl -28(%ebp), %eax
movl %eax, (%esp)
call foo
movl %eax, -12(%ebp)
movl -12(%ebp), %eax
movl %eax, -32(%ebp)
movl -32(%ebp), %eax
leave
ret
Compared to -O1 which generates just:
Java_testJNI_foo:
pushl %ebp
movl %esp, %ebp
subl $24, %esp
movl 40(%ebp), %eax
movl %eax, 12(%esp)
movl 32(%ebp), %eax
movl %eax, 8(%esp)
movl 24(%ebp), %eax
movl %eax, 4(%esp)
movl 16(%ebp), %eax
movl %eax, (%esp)
call foo
leave
ret
With -O1 g++ can generate far smarter code for:
%module test
%{
int value() { return 100; }
%}
%feature("compactdefaultargs") foo;
%inline %{
int foo(int a=value(), int b=value(), int c=value()) {
return 0;
}
%}
The short answer is with optimisations disabled completely GCC generates extremely naive code - this is true of SWIG wrappers as much as any other program, if not more given the style of the automatically generated code.