[M3devel] optimizing for size or speed?

[Jay K]

I still find these decisions difficult.
Esp. when the option is to call a function or not.
Esp. for "builtin" stuff like 64bit math and set operations.
 
 - Jay

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> From: jay.krell at cornell.edu
> To: hendrik at topoi.pooq.com; m3devel at elegosoft.com
> Date: Thu, 11 Feb 2010 12:41:12 +0000
> Subject: Re: [M3devel] optimizing for size or speed?
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> cache -- good point I had forgotten, thanks.
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> Still:
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> use the divide instruction, which is smallest, or multiply by reciprocal (which is generally a multiply and a shift)
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> (Given a 32x32=>64 multiply operation. x86 doesn't even have 32x32=>32, only 32x32=>64, I believe.)
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> Any 32bit division by a constant can be optimized this way and every C compiler knows it.
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> multiply by a constant using multiply instruction, or decompose into some adds?
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> The AMD64 optimization guide suggests speed optimizations where they give a sequence for multiplication for every constant up to 32, some are just to use mul. But many are one or two other instructions.
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> Multiply by 5 is:
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> lea eax,[eax+eax*4]
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> Multiply by 10 is:
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> lea eax,[eax+eax*4]
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> add eax,eax
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> The AMD64 manual even advises to inline 64bit shifts by a non-constant.
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> But I can't get Visual C++ to do that. It always calls a function.
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> - Jay
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>> Date: Thu, 11 Feb 2010 07:03:09 -0500
>> From: hendrik at topoi.pooq.com
>> To: m3devel at elegosoft.com
>> Subject: Re: [M3devel] optimizing for size or speed?
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>> On Thu, Feb 11, 2010 at 08:53:08AM +0000, Jay K wrote:
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>>> There are all kinds of equivalent code sequences.
>>> For the maintainer of m3back to chose among.
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>> In case of doubt, go for size; size all by itself costs time in cache
>> misses, paging, etc.
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>> Besides, it's possible to measure space.
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>> -- hendrik