How long does it take to access stack memory?

Question

Does re-declaring the same variable over and over again affect the performance of the code?

int i,a;
for(i=0;i<10;i++)
{
// Some operations with a
}

V/S

int i;
for(i=0;i<10;i++)
{
int a;
// Some operations with a
}

incorrect assumptions all over. Due to [as-if rule](https://stackoverflow.com/questions/15718262/what-exactly-is-the-as-if-rule) there need not be any difference. — Antti Haapala -- Слава Україні, Nov 02 '19 at 18:57
It is likely that storage for `int a;` in the second example is allocated once only for each call of the function anyway. Defining it within the `for` loop restricts the access to it that the compiler will allow (its scope), even though it physically exists outside of the code block. — Weather Vane, Nov 02 '19 at 19:16
I read it somewhere on SO that by limiting the scope of a variable, one can ensure that more memory is available to the stack memory? — Nephew of Stackoverflow, Nov 02 '19 at 19:20
It more usually done that way to restrict the scope of `a` than to save on stack space. — Weather Vane, Nov 02 '19 at 19:23
So these minor things don't reduce the performance of the code? — Nephew of Stackoverflow, Nov 02 '19 at 19:25
Let the compiler deal with that: don't worry about micro-optimisations. — Weather Vane, Nov 02 '19 at 19:32
So can't I help the compiler help with those micro-optimization :(? — Nephew of Stackoverflow, Nov 02 '19 at 19:35
@NephewofStackoverflow no you can't. The scope does not ensure that the variable is *destroyed* at the end. Did you read the Q/A I linked to?! All programs that have the same external behaviour can be compiled to the same code. If you have got *2* programs with *different behaviour* it is different then... — Antti Haapala -- Слава Україні, Nov 02 '19 at 19:36

score 2 · Accepted Answer · answered Nov 02 '19 at 18:57

2

Normally "stack variables", what are really local variables, are zero-cost. The only price you'd pay is if there's initialization of some sort.

The compiler may or may not reserve memory for that value. In the second case you don't actually use a so it will probably be eliminated by the optimization pass, making it truly zero cost.

Don't think of them in terms of "stack". That's an antiquated concept that pre-dates optimizing compilers.

answered Nov 02 '19 at 18:57

tadman

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So declaring a variable in the block of a loop doesn't affect the performance as local variables destroyed at the end of their scope? – Nephew of Stackoverflow Nov 02 '19 at 19:02
1

Look at the assembly output to be sure. – tadman Nov 02 '19 at 19:08
How do I check that? – Nephew of Stackoverflow Nov 02 '19 at 19:10
1

Each compiler has a different method, but they can all do it. – tadman Nov 02 '19 at 19:11
For Turbo C compiler? – Nephew of Stackoverflow Nov 02 '19 at 19:11
2

Maybe, but that compiler is museum grade. Why not Visual Studio Express? – tadman Nov 02 '19 at 19:12

How long does it take to access stack memory?

1 Answers1