CODEWITHSUNDEEP
ccompiler-optimization
Elif Ozbay
Elif Ozbay

Reputation: 41

Do compilers read the code and optimize the random memory allocations?

I have a code where I declare a pointer (named "removed") in type of a struct called DLINKED_LIST_NODE.

DLINKED_LIST_NODE curr = dlinked_list_goto_idx(list, idx, false);
DLINKED_LIST_NODE removed;

if(curr->next != NULL)
    removed = curr->next;
else {
    printf("Error @ dlinked_list_remove: No such index.\n");
    exit(1);
}

dlinked_list_goto_idx just returns the starting point of the double linked list. So removed will be the head of the list.

When I debug the code, dlinked_list_goto_idx returns the list itself (which is the starting node) and I can see it's address. Then removed is declared and again I see the address of it.

Before I execute the removed = curr->next; block I see that the address of removed is actually identical to curr->next.

Does compiler do this to optimize the code or I cannot see an obvious stuff?

This is how it looks when I debug to the point where removed is declared. https://pasteboard.co/HHe6eQE.png

And this is the debug screen where you can see the curr->next. https://pasteboard.co/HHe7dF8.png

Upvotes: 1

Views: 58

Answers (1)

Bob Jarvis - Слава Україні
Bob Jarvis - Слава Україні

Reputation: 50017

If you're compiling with anything except -O0 or its equivalent - i.e. "no optimizations" - the compiler may very well be rewriting your code as:

register DLINKED_LIST_NODE curr = dlinked_list_goto_idx(list, idx, false);
register DLINKED_LIST_NODE removed = curr->next;

if(removed == 0) {
    printf("Error @ dlinked_list_remove: No such index.\n");
    exit(1);
}

In other words, it may have chosen to pull removed = curr->next up to the point of declaration and then test for NULL (0) against the register var removed because testing the contents of a register for zero is generally a pretty cheap test, and may in fact not even require a test if the zero flag is set by the assignment, which it might be depending on the architecture.

The point here is - don't debug optimized code, unless the non-optimized version works and the optimized version doesn't (which can happen due to various issues).

Best of luck.

Upvotes: 2

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