Re: [boost] BOOST_ASSERT and __assume on MSVC

From: Simon Buchan (simon_at_[hidden])
Date: 2005-09-15 20:52:19

Rob Stewart wrote:
> From: Simon Buchan <simon_at_[hidden]>
> 
>>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