Subject: Re: [boost] [function] function wrapping with no exceptionsafetyguarantee
From: Daniel Walker (daniel.j.walker_at_[hidden])
Date: 2010-10-30 12:43:51
On Fri, Oct 29, 2010 at 4:13 PM, Jeff Flinn
<TriumphSprint2000_at_[hidden]> wrote:
> Daniel Walker wrote:
>>
>> On Thu, Oct 21, 2010 at 2:25 AM, Doug Gregor <doug.gregor_at_[hidden]>
>> wrote:
>>>
>>> On Wed, Oct 20, 2010 at 1:51 PM, Daniel Walker
>>> <daniel.j.walker_at_[hidden]> wrote:
>>>>
>>>> So,
>>>> adding "empty" vtable objects would increase the space requirements of
>>>> boost::function; each template instantiation would need two
>>>> corresponding vtable objects, one for actual targets and one as a
>>>> fallback that throws bad_function_call. (The "empty" vtable objects
>>>> could be stored statically, but static storage is also a very precious
>>>> resource on many platforms.) Not all users would appreciate this
>>>> trade-off; i.e. an increase in the space overhead for a small decrease
>>>> in time overhead (with compiler optimization the time savings are
>>>> minuscule, in my experience).
>>>
>>> Without quantifying the trade-off, we don't know which way is better.
>>>
>>> In any case, this issue is fairly easy to settle; it just takes
>>> effort. Someone implements this alternative scheme and determines the
>>> cost/benefit in space and time, and with any luck the choice is
>>> obvious.
>>
>> OK, I implemented the alternative scheme and ran some benchmarks to
>> determine the cost/benefit in space/time. The code is in a Trac ticket
>> with a patch that allows boost::function to be configured to represent
>> its empty state using a static object that contains an "empty"
>> function which calls boost::throw_exception when invoked. The user may
>> select the alternative scheme by defining the macro
>> BOOST_FUNCTION_USE_STATIC_EMPTY.
>>
>> https://svn.boost.org/trac/boost/ticket/4803
>>
>> I also attached a tarball to the ticket with a Jamfile and source code
>> to compile and run benchmarks of a function pointer, boost::function,
>> and boost::function using the static empty scheme. The time overhead
>> per call and space overhead per object are measured by the executables
>> at runtime. The space overhead per type is the size of the static
>> initialized date section in the executable's data segment as reported
>> by /usr/bin/size.
>>
>> The following results were obtained on a x86 machine running an Unix
>> variant using the manufacture's build of gcc 4.2. The machine was not
>> in a labratory environment, so I am not controlling for changes in
>> operating tempature that could impact performance at these time
>> scales. The following tables present the raw numbers from bjam's
>> release and debug builds. (See the Jamfile for the complete arguments
>> to bjam.)
>>
>> Data (Release):
>> | funcion ptr | function | function (static empty)
>> time/call | 6.28e-10s | 6.68e-09s | 6.40e-09s
>> space/type | 32B | 48B | 64B
>> space/obj | 8B | 32B | 32B
>>
>> Result (Release): Defining BOOST_FUNCTION_USE_STATIC_EMPTY yields a 4%
>> decrease in time overhead per call but doubles the space overhead per
>> type. (On msvc 10 the decrease in time overhead is closer to 10%.)
>>
>> Data (Debug):
>> | funcion ptr | function | function (static empty)
>> time/call | 6.33e-09s | 2.62e-08s | 2.26e-08s
>> space/type | 32B | 48B | 64B
>> space/obj | 8B | 32B | 32B
>>
>> Result (Debug): Defining BOOST_FUNCTION_USE_STATIC_EMPTY yields a 18%
>> decrease in time overhead per call but doubles the space overhead per
>> type.
>>
>> So, I think the current boost::function implementation is certainly
>> the right default, since many users would not appreciate doubling the
>> static space overhead for a time savings of less than 10% per call.
>> However, I think it is a good idea to offer users the opertunity to
>> tinker and experiment with this trade-off, so that they can choose
>> what works best for their application.
>
> I've followed this thread only peripherally, but the important info to me
> would be does removing the check for empty function affect the optimized
> performance/code-size for the non-empty scenarios? In my experience, these
> are the scenarios most often encountered.
Yes, if you look at the code in the benchmark, you will see that it is
measuring the cost of a call to a non-empty boost::function. In
optimized object code, the call is 4% faster without the check, but
removing the check means that it is necessary to store a special,
internal static object per instantiation to hold an "empty" function
that must be available if boost::function becomes empty. This static
object doubles boost::function's space overhead in the initialized
static data section of the executable's data segment.
Daniel Walker