How does the c++ compiler organize the variables that are initialized in a function to store them so the computer will find them the fastest way?
I understand that the compiler puts them one after an other on the stack but there has to be some logic behind it, I was searching Google for hours on end but I could not find anything.
For example:
int main()
{
float a;
int b;
char c;
double d;
}
This should occupy more memory than the one below because of the way the c++ compiler is storing in the memory.
The exact bits used are the same, of course, but they should be stored in a more efficient order in the example below. Where in memory would these variables be stored by the compiler in the next example? As far as I understood a variable is always stored on a block such that (logical number) % (number of bytes the datatype) = 0
int main()
{
char c;
int b;
float a;
double d;
}
This should occupy more memory than the one below because of the way the c++ compiler is storing in the memory.
Not really, the stack memory consumed by both functions should be the same for any sane optimizing compiler... Modern C++ Compilers are really aggressive at certain optimizations.
Other than suitable alignments, C++ does not impose memory address ordering for automatic variables in functions. While that is true, the observable behavior of the program must not be changed by the compiler.
I guess you are trying to talk about structs and classes where the memory layout and address ordering of variables are as declared.
How does the c++ compiler organize the variables that are initialized in a function to store them so the computer will find them the fastest way?
In practice, every access to an automatic variable in C++ is a simple pointer offset with respect to the stack pointer1 (except variables the compiler placed directly in a register). Additionally, to speed things up to such automatic variables (in no order):
The Compiler eliminates dead variables
The compiler will find the best order to store each of them to meet suitable alignment
The compiler may use the CPU register directly depending on what it's Register Allocation algorithm decides
The compiler may lump certain variables together into a vector register and use vector instructions provided it will yield correct results.
...and so much more.
1: Even the stack pointer is aligned by most compilers.