Rob Stewart wrote:
From: Simon Buchan <simon@hand-multimedia.co.nz>
Marcin Kalicinski wrote:
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!
<snip> I don't get it, isn't the point of assert that it DOES check them? Or
The point is that a violated assertion makes the program die. It isn't supposed to happen. However, since assertions are often used only in debug builds, they only help if you run a debug build. Thus, if you write assertions, but your clients only run release builds, they don't help much.
Given that the asserted condition holds, then telling the compiler about that condition gives the compiler information it might be able to use to optimize better. IOW, putting __assume() in the non-debug assertion means, effectively, "assuming that the asserted condition is true (verified in debug builds by an assertion), you may apply any additional optimizations you might otherwise have skipped."
If the checking of a debug build isn't brought to bear, then the asserted condition may well be violated anyway. So, whether you get bad results due to "overly agressive" optimization or due to assumptions made by the code following the assertion hardly matters.
I think the addition is a great idea. Do other compilers offer anything similar? I'd hate to miss this opportunity on other platforms. OK, cool. I think the closest GCC comes to just assuming is __builtin_expect, but as an optimisation, it should be good enough. (Personnely, I don't think optimization levels should change the semantics of a program, but assertions are obviously an edge-case)
PS: Don't take me too seriously, I have proven myself to be a complete idiot many a time :D