Pointers in C, Part V: The ‘restrict’ Qualifier

Pointers in C

“Le vrai est trop simple, il faut y arriver toujours par le compliqué.”
(“The truth is too simple: one must always get there by a complicated route.”)
― George Sand, Letter to Armand Barbès, 12 May 1867”

Exactly one year ago, I started this series on pointers, but what I really wanted to blog about originally was a rather arcane and rarely used keyword that first appeared in the C99 language standard: the ‘restrict’ qualifier. But after trying to digest the formal definition in chapter I decided that taking a little detour would make my and my reader’s life much easier.

Let me set the stage for ‘restrict’ by summarizing what I wrote in episode 3 about the “strict aliasing rule”:

1. The compiler might optimize code involving multiple pointers, provided the pointers are not aliased; that is, they don’t point to the same object or memory.

2. The compiler assumes that pointers to incompatible types never alias.

3. The compiler assumes that pointers to compatible types (same types, apart from CV-qualification and signedness) potentially alias.

Therefore, a function with this signature is eligible for compiler optimization:

whereas this one is not:

This is unfortunate, because most likely, the arrays passed to the second version of ‘transform’ are in completely different, non-overlapping memory regions. But the compiler doesn’t know and hence stubbornly adheres to the strict aliasing rule.

The ‘restrict’ qualifier, which — contrary to the ‘const’ and ‘volatile’ qualifiers — can only be applied to pointers, is a promise given by the programmer to the compiler that pointers don’t alias even though they point to objects of the same type. Therefore, this version of ‘transform’ can be optimized by the compiler:

Let’s put this to the test with the ‘silly’ example from episode 3:

Before knowing about the strict aliasing rule, we were surprised to see that the memory access to ‘x’ in the return statement was not replaced with a simple ‘return 0’. After having learned about the strict alias rule, it’s clear: since ‘x and ‘y’ point to the same type, the compiler must assume that they may point to the same memory location and hence it loads the value pointed to by ‘x’ from memory afresh:

Now, if we tell the compiler that ‘x’ and ‘y’ never point to the same memory location, optimization is possible:

Nice, isn’t it?

If you use the ‘restrict’ qualifier on a pointer, you promise that — at least for the lifetime of the restricted pointer — the object pointed to is only accessed through this pointer. Break that promise and you get undefined behavior. (In the ‘silly3’ example, the lifetime of the pointers ‘x’ and ‘y’ end once the call to ‘silly3’ returns.)

In the C99 language standard, many functions from the standard library have been revised and now make use of the ‘restrict’ keyword. Take ‘memcpy’, for instance:

As everybody knows, ‘memcpy’ can only copy non-overlapping blocks of memory and this fact is nicely highlighted by the use of the ‘restrict’ keyword: during the call to ‘memcpy’ the memory regions src[0] to src[n] as well as dst[0] to dst[n] are exclusively owned and may not be accessed by other pointers. Since ‘memmove’ can copy overlapping blocks of memory (with a little speed penalty, of course), ‘memmove’ consequently doesn’t declare restricted pointers:

Please be aware that ‘restrict’ is not supported by the C++ language standard and it’s unclear whether it ever will be. If you mix C99 and C++ code, you might have to strip the ‘restrict’ keyword from C99 headers to avoid compilation errors:

In general, I’m not a big fan of optimization features that the compiler is free to ignore. If utmost performance is important, you want dependable performance. Most likely, your routine is not on the performance critical path, anyway. If you think it is, carefully profile your code and after you proved that it is, you’d better code that part in assembly language. Without such evidence, sprinkling your code with ‘restrict’ is little short of premature optimization. (I complained here about the unnecessarily overused ‘inline’ keyword for the same reason.)

What I do like about the ‘restrict’ keyword, though, is that by unraveling it, we’ve made a beautiful journey through important everyday programming topics like “pointers vs. arrays”, “type qualifiers”, “pointer conversion rules”, and the “strict aliasing rule”. The journey was the destination.

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