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carmfunction-pointers
M.Armoun
M.Armoun

Reputation: 1165

optimize function's parameter in c (GCCarm)

i have limited RAM in my microController.(using arm gcc) i should write my code as efficient as possible. consider function blew :

int foo(uint32_t a , float b)
{
    .....
    return 0;
}

so I have two arguments a and b. now if I change the arguments to *a and *b is the function takes less ram than the first function during execution :

int foo(uint32_t *a,float *b)
{
    ....
    return 0;
}

what if the arguments become String or array ( of int, float ....)?? any references would be great. thanks

Upvotes: 0

Views: 170

Answers (2)

user694733
user694733

Reputation: 16047

You can actually waste memory when using pointers. This is for 2 reasons.

1. Register optimization lost

Code which calls foo must take address of the variable to pass as parameter. If passed variable is local, it could be in register but because you take its address it must be placed on stack. In general, using variable in register is faster than using variable in stack.

2. Value of variable unknown after call

When you give address of variable function, compiler no longer knows if variable is modified, and must refresh it if it's read again. 

 uint32_t u = 1;
 float f = 2.0f;
 foo(&u, &f); // 1. Address taken, u and f cannot be register variables
 // 2. What is value of u now? It must refreshed from memory before addition happens
 u++;

Bottom line: do not take address of primitive types unless you have to.

Strings and arrays are already passed using address, so there is no other option.

Upvotes: 2

auselen
auselen

Reputation: 28087

ARM's Procedure Call Standard defined in Application Binary Interface (ABI) states that the first four word-sized parameters passed to a function will be transferred in registers R0-R3.

Since pointers are also 32-bit in size, there is not that much difference between these two signatures.

int foo(uint32_t a, float b)
int foo(uint32_t *a, float *b)

Both will have a in r0, b in r1 and return value in r0 again.

ARM is a RISC architecture it has many registers and with trivial functions you may even get away from touching any memory at all.

If you are working with micro controllers it is a better idea to check if you should really use floating points. Since they might not be natively supported by the core you are targeting for.

I would check ARM Cortex-A Series Programmer's Guide (most of it applies to micro controllers as well), especially chapters 15 and 17 to learn more about tricks for ARM application development.

Upvotes: 2

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