On 06/10/2017 11:00, Henri Sivonen wrote:
> Do we already have a C++ analog of Rust's test::black_box() function?
> I.e. a function that just passes through a value but taints it in such
> a way that the optimizer can't figure out that there are no side
> effects. (For the purpose of ensuring that the compiler can't
> eliminate computation that's being benchmarked.)
>
> If we don't have one, how should one be written so that it works in
> GCC, clang and MSVC?
>
> It's surprisingly hard to find an answer to this on Google or
> StackOverflow, and experimentation on godbolt.org suggests that easy
> answers that are found are also wrong.
>
> Specifically, this isn't the answer for GCC:
> void* black_box(void* foo) {
> asm ("":"=r" (foo): "r" (foo):"memory");
> return foo;
> }
IIUC what you are looking for is the '+' constraint which implies the
parameter is both read and written in the asm statement, e.g.:
void* black_box(void* foo) {
asm ("":"+r" (foo): "r" (foo):"memory");
return foo;
}
Gabriele
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