GNU C and C++ offer the const and pure function attributes.
From the gnu online docs (emphasis mine):
In GNU C and C++, you can use function attributes to specify certain function properties that may help the compiler optimize calls or check code more carefully for correctness. For example, you can use attributes to specify that a function never returns (noreturn), returns a value depending only on the values of its arguments (const), or has printf-style arguments (format).
Where the const attribute seems to be a subset to pure, also taken from the gnu docs:
The const attribute imposes greater restrictions on a function’s definition than the similar pure attribute. Declaring the same function with both the const and the pure attribute is diagnosed.
With C++ 11, the constexpr specifier was added.
When applied to functions, is there a difference between the const attribute and the constexpr specifier? Does GCC apply different optimizations?
A similar sounding question is Difference between `constexpr` and `const` . But I think this is not a duplicate. My question is specifically about the function attribute const, which seems to have overlapping functionality with constexpr.
When applied to functions, is there a difference between the const attribute and the constexpr specifier?
There are differences.
Firstly, C does not have constexpr, so you cannot take advantage of it in that language.
Calls to constexpr function can be constant expressions. As such, their result can be used for example as the size of an array. The GNU attributes cannot be used to achieve the same (ignoring GCC VLA language extension).
Constexpr functions are good for taking advantage of pre-calculation at compile time. The GNU attributes are still useful for allowing the compiler take advantage of runtime constness. For example, let's say there is a function that cannot be constexpr - perhaps because it calls a non-constexpr function. But we may know that every call to the function produces same output with no side-effects. Const attribute allows the compiler to not repeat redundant calls.
Another difference is that constexpr functions must be defined inline. Non-constexpr functions don't need to be defined inline.