Jens Gustedt's Blog

October 23, 2010

A generic swap implementation

Filed under: C99, integers, preprocessor — Jens Gustedt @ 11:13

Swapping the contents of two variable is an elementary task that is often met in daily programming. There are two generic strategies to do that for general types.

The first, most commonly used, creates a temporary variable and assigns the values cyclically:

T a = ...;
T b = ...;
T tmp = a; a = b; a = tmp;

We will call this strategy P99_SWAP1. Here the compiler must realize the three assignments strictly in order, otherwise the resulting program would not be correct.
The second one, call it P99_SWAP2, tries to do something similar, but to relax the ordering constraints a bit:

T a = ...;
T b = ...;
T tmpa = a; T tmpb = b;
a = tmpb; b = tmpa;

For the tradeof of using more resources (stack space or registers) this can result in more efficient code. The two objects can be loaded and stored in parallel. But the gain will probably only be visible for small objects. So a possible attempt to combine the two could be

#define P99_SWAP(A, B) (sizeof(A) > sizeof(uintmax_t) ? P99_SWAP1(A, B) : P99_SWAP2(A, B))

But then, how to implement the two “submacros” P99_SWAP1 and P99_SWAP2(A, B)? A particular difficulty if we want to do this generically as a C macro or function is that we don’t have access to the type of the expressions that are passed as arguments A and B. So let us first write “crossfingers” function and macro that just forget about the type issues:

void p00_swap2(void* a, void* b, void* tmpa, void* tmpb, size_t len) {
   memcpy(tmpa, a, len);
   memcpy(tmpb, b, len);
   memcpy(b, tmpa, len);
   memcpy(a, tmpb, len);
#define P00X_SWAP2(A, B) p00_swap2(&(A), &(B), (char[sizeof(A)]){ [0] = 0 }, (char[sizeof(A)]){ [0] = 0 }, sizeof(A))

Here the weirdo expressions (char[sizeof(A)]){ [0] = 0 } are so-called compound literals that provide temporary objects for the copy operations.

This has several drawbacks. First we don’t even check if A and B correspond to objects of the same size, but we happily copy into them. So first we have to assert that they at least agree on their sizes such that we don’t provoke undefined behavior. This can be achieved by some expression magic for the two compound literals:

(char[sizeof(A)]){ [(intmax_t)sizeof(A) - sizeof(B)] = 0 }

What is happening here? The right part inside the [], a designated initializer, is used to initialize one element of the char[]. Now here we take a the difference of the two sizes: this will be the element at position 0 if both sizes are the same. If sizeof(A) is smaller than sizeof(B) because of the cast to intmax_t this will produce a negative number at compile time.

If now we apply the symmetric strategy to the second compound literal we get a macro that compiles well when it is called with two objects of the same size, and that produces a compile time error when the sizes disagree:

#define P00_SWAP2(A, B) 
p00_swap2(                                                      \
     &(A),                                                       \
     &(B),                                                       \
     (char[sizeof(A)]){ [(intmax_t)sizeof(A) - sizeof(B)] = 0 }, \
     (char[sizeof(B)]){ [(intmax_t)sizeof(B) - sizeof(A)] = 0 }, \

This now already is much safer, but perhaps not safe enough, since the two objects might have the same size but still not be of the same type. What we can do is an extra check if the two types are assignment compatible. This can be done by something like the following which might look a bit hacky a first glance

(1 ? &(A) : ((A = B), NULL))

This is a conditional that is always true, so it always evaluates to `&(A)`. The second “false” part is never evaluated at run time but only checked if it is correct C code. And it wouldn’t be if A and B would not be assignment compatible.



  1. I just tried the two buffer version in GCC 4.6.3 with the -O3 optimization flag turned on and looked at the disassembly in GDB. Absolutely no buffers get allocated, no references to memcpy can be seen in the resulting code. The compiler just swapped the two register names (ecx and edx in my case) in the code after the point at which the swap was supposed to happen.

    To recap: this swap method does not incur any performance or code size penalties whatsoever, at least in simple cases.

    Comment by Antti Nilakari — May 22, 2012 @ 19:49

    • Thanks for the check. I suppose you did this with int on an i86 architecture?

      Since this post is a bit long ago I can’t be sure completely, but usually I do check the assembler of all such constructs to see if they produce something reasonable, and in particular if the compilers are able to reduce all the additions for the syntactic sugar. Should have added that, somewhere, but it is always good to have an independent verification.


      Comment by Jens Gustedt — May 22, 2012 @ 20:08

      • Yes, it was a simple “allocate two ints on the stack, swap them, print them” case on an x86-64.

        Comment by Antti Nilakari — June 24, 2012 @ 19:38

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