Re: [Boost-users] [boost.numeric] Poor Performance of numeric_cast

Subject: Re: [Boost-users] [boost.numeric] Poor Performance of numeric_cast
From: Tang Jiang Jun (tangjiangjun_at_[hidden])
Date: 2012-10-16 04:35:03

Very incisive! I checked the assembly code and verified your hypothesis.
The integer-to-floating cast of numeric_cast was really inlined by the
compiler, meanwhile the other two were using function call to invoke
numeric_cast. So maybe the better solution is to force inline for
numeric_cast in all cases.

On Tue, Oct 16, 2012 at 4:12 PM, John Maddock <boost.regex_at_[hidden]>wrote:

>  results:
>>
>> Native Integer Cast: 26729 nanoseconds
>> Boost Integer Cast: 26449 nanoseconds
>> Native Integer-Floating Cast: 105479 nanoseconds
>> Boost Integer-Floating Cast: 105455 nanoseconds
>> Native Floating-Integer Cast: 168933 nanoseconds
>> Boost Floating-Integer Cast: 453505 nanoseconds
>>
>>
>> so no overhead in Integer-Integer or Integer-Floating. But
>> Floating-Integer has bad performance.
>>
>
> That's sort of what I would expect - think about it - if the cast is to
> the same or a wider type, then there is no check, and numeric_cast and
> static_cast do the same thing.  However for a narrowing cast (float to
> integer), then at the very least there has to be an extra if statement to
> test whether the value being cast is in range - that would normally roughly
> double the runtime cost.  But there *may* be another hidden cost: depending
> on the loop the compiler may decide not to inline the numeric_cast in order
> to give a tighter loop, and the cost of the function call would add a big
> chunk of time.  Plus the extra code associated with the error handling if
> the value is out of range adds a certain amount of code bloat to the loop,
> reducing code locality.  But the thing is you can't avoid this if you want
> the runtime check.  There's no such thing as a free lunch sadly.
>
> John.
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