Marcin Kalicinski <kalita <at> poczta.onet.pl> writes:
Hi All,
Have you considered modifying BOOST_ASSERT to use __assume on MSVC?
I thought about optimizing assert in this way some time ago but I never looked for implementation.
Briefly speaking __assume(expr) is a hint for optimizer that 'expr' evaluates to nonzero. It can help optimize the code. Standalone use of __assume is not recommended because compiler does not check if the assumption is correct, and if it isn't the program is ill formed. But it fits perfectly in ASSERT macro. This way putting many ASSERTs will not only help to verify the code - it will also help the optimizer. Wow!
Wow!
To be sure I did a small test with MSVC:
int f(int n) { // __assume(n == 0); if (n == 1) return 1; else return 0; }
I recall now how I came to that idea. I was reading http://www.hackersdelight.org/basics.pdf section 2.20 "Alternating among Two or More Values" when I realized that this kind of optimization is possible without bits magic. Marcin, could you try an example from the book void f(int& n) { // __assume(n == 0 || n == 1); if (n == 0) n = 1; else n = 0; } with and without __assume? -- Alexander Nasonov