In which cases will the restrict qualifier applied to a return value have an effect?

Question

If I have a member function declared like so:

double* restrict data(){
    return m_data; // array member variable
}

can the restrict keyword do anything?

Apparently, with g++ (x86 architecture) it cannot , but are there other compilers/architectures where this type of construction makes sense, and would allow for optimized machine code generation?

I'm asking because the Blitz library (Blitz++) has a whole slew of functions declared in this manner, and it doesn't make sense that someone would go in and add the restrict keyword unless it actually does something. So before I go in and remove the restrict 's (to get rid of compiler warnings) I'd like to know how I'm abusing the code.

Answer 1

WHAT restrict ARE WE TALKING ABOUT?

restrict is, as it currently stands, non-standard.. which means that it's a compiler extension; it's non-portable in the sense that the C++ Standard doesn't mandate its existance, nor is there any formal text in it that tells us what it is supposed to do.

restrict is currently compiler specific in C++, and one has to resort to the compiler documentation of their choice to see exactly what it is doing.

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SOME THOUGHTS

There are many papers about the usage of restrict , among them:

• Restricted Pointers - Using the GNU Compiler Collection
• restrict - wikipedia.org
• Demystifying The Restrict Keyword - CellPerformance

It's hinted at several places that the purpose of restrict is to qualify pointers so that the compiler knows that two pointers in the same scope doesn't refer to the same memory location.

With this in mind we can easily see that the return-type has no potential collision with other pointers, so using it in such context will generally not gain any optimization opportunities. However; one must refer to the documented behaviour of the used implementation to know for sure.. as stated: restrict is not standard, yet .

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I also found the following thread where the developers of Blitz++ discusses the removal of strict applied to the return-type of a function, since it doesn't do anything:

• Re: [Blitz-devel] type qualifiers ignored on function return type

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A LITTLE NOTE

As a further note, here's what the LLVM Documentation says about noalias vs restrict :

For function return values, C99's restrict is not meaningful, while LLVM's noalias is.

Answer 2

Generaly restrict qualifier can only help to better optimize code. By removing 'restrict' you don't break anything, but when you add it without care you can get some errors. A great example is the difference between memcpy and memmove. You can always use slower memmove, but you can use faster memcpy only if you know that src and dst aren't overlaping.