“Escape” and “Clobber” equivalent in MSVC

Question

In Chandler Carruth\'s CppCon 2015 talk he introduces two magical functions for defeating the optimizer without any extra performance penalties.

For reference, here

Answer 1

While I don't know of an equivalent assembly trick for MSVC, Facebook uses the following in their Folly benchmark library:

/**
 * Call doNotOptimizeAway(var) against variables that you use for
 * benchmarking but otherwise are useless. The compiler tends to do a
 * good job at eliminating unused variables, and this function fools
 * it into thinking var is in fact needed.
 */
#ifdef _MSC_VER

#pragma optimize("", off)

template 
void doNotOptimizeAway(T&& datum) {
  datum = datum;
}

#pragma optimize("", on)

#elif defined(__clang__)

template 
__attribute__((__optnone__)) void doNotOptimizeAway(T&& /* datum */) {}

#else

template 
void doNotOptimizeAway(T&& datum) {
  asm volatile("" : "+r" (datum));
}

#endif

Here is a link to code on GitHub.