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Re: RFC: Non-fragile ivars


From: Saso Kiselkov
Subject: Re: RFC: Non-fragile ivars
Date: Tue, 03 Jun 2008 11:35:10 +0200
User-agent: Thunderbird 2.0.0.14 (X11/20080502)

>From what I understood, it's going to be in a classname-mangled global
variable.

--
Saso

Fred Kiefer wrote:
> I like this whole idea a lot, but we will need a bit more performance
> testing to make sure we understand the consequences.
>
> What I did not quite get is where are we going to store the size of
> the different classes? Perhaps a more detailed outline of the
> algorithm would help here.
>
> Fred
>
> Saso Kiselkov wrote:
>> I've tried to measure the differences by using a simple C program which
>> simulates both behaviors. I'm no compiler writer and I didn't
>> disassemble the compiled code, so I'm not sure that this really proves
>> anything, but in case somebody is interrested, here are my results:
>>
>> - Both test cases iterated over numbers from 2 to 100000 trying to find
>> primes using a stupid, simple algorithm.
>>
>> - Both test cases hold no temporary variables on the stack, but instead
>> use a malloc'd structure for that.
>>
>> - Test A uses direct access to the struct's fields.
>>
>> - Test B uses indirect access by adding to the struct's pointer a static
>> global variable, which holds the offset to the fields.
>>
>> - Both tests were tried with no compiler optimization (-O0) and with
>> maximum compiler optimization (-O3 -fomit-frame-pointer).
>>
>> The results are:
>>
>> Machine 1: Intel Celeron M @ 1.5GHz, x86-32, GCC 4.1.3 20070929
>> (prerelease)
>> Test A (unoptimized): 5466612us
>> Test B (unoptimized): 11096380us
>> Slowdown: 102.9800%
>> Test A (optimized): 5280693us
>> Test B (optimized): 5704486us
>> Slowdown:  8.0200%
>>
>> Machine 2: Intel Pentium 4 @ 1.7GHz, x86-32, GCC 4.1.2 20061115
>> (prerelease)
>> Test A (unoptimized): 11228890us
>> Test B (unoptimized): 17972084us
>> Slowdown: 60.0500%
>> Test A (optimized): 15903029us
>> Test B (optimized): 16032732us
>> Slowdown:  .8100%
>>
>> Machine 3: Intel Pentium Dual-Core E2180 @ 2.0GHz, x86-64, GCC 4.2.3
>> Test A (unoptimized): 3646244us
>> Test B (unoptimized): 5469573us
>> Slowdown: 50.0000%
>> Test A (optimized): 3680493us
>> Test B (optimized): 3630129us
>> Slowdown:  -1.3700%
>>
>> Machine 4: Intel Core 2 Duo E6600 @ 2.4GHz, x86-64, GCC 4.2.3
>> Test A (unoptimized): 3066969us
>> Test B (unoptimized): 4653554us
>> Slowdown: 51.7300%
>> Test A (optimized): 3132884us
>> Test B (optimized): 3070829us
>> Slowdown:  -1.9900%
>>
>> I've attached a tarball holding the program and test script. Hope it's
>> of any use.
>>
>> -- 
>> Saso
>>
>> Richard Frith-Macdonald wrote:
>>> On 31 May 2008, at 16:21, David Chisnall wrote:
>>>> The advantages of this would be:
>>>>
>>>> - No code using GNUstep or other frameworks compiled with clang/LLVM
>>>> (which we are almost in a position to do) would break if it inherited
>>>> from a class whose layout changed.
>>>>
>>>> - No ABI breakage would be needed - code compiled with GCC would
>>>> still work on the modified runtime, although the existing constraints
>>>> on modification would still apply.
>>>>
>>>> The disadvantages are:
>>>>
>>>> - Currently ivar accesses on most platforms will be a single load /
>>>> store instruction in an indirect addressing mode with a constant
>>>> offset embedded in the instruction.  This would add another load and
>>>> addition to every ivar access.
>>>>
>>>> - The extra work that the runtime would do would increase load times
>>>> slightly.
>>>>
>>>> So, my questions is, is this worth doing?
>>> IMO ... yes.  It's a good feature to have, and the overheads get more
>>> insignificant as processor seeds increase.
>>>
>>>
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