[boost] BOOST_ASSERT and __assume on MSVC

From: Marcin Kalicinski (kalita_at_[hidden])
Date: 2005-09-13 17:22:29

Hi All,

Have you considered modifying BOOST_ASSERT to use __assume on MSVC?

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!

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;
}

produces:

  xor eax, eax
  cmp DWORD PTR _n$[esp-4], 1
  sete al
  ret 0

with __assume uncommented:

  xor eax, eax
  ret 0

And the comparison is gone.

Microsoft docs for __assume are here:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vccelng/htm/msmod_36.asp

cheers,
Marcin