[cfe-dev] [LLVMdev] RFC: Codifying (but not formalizing) the optimization levels in LLVM and Clang

[James Molloy]

Would it be possible to have a parameterised attribute? opt(min),
opt(size), opt(max) ?


On 14 January 2013 12:46, Justin Holewinski <justin.holewinski at gmail.com>wrote:

> If I understand the attributes correctly, they would be function-level
> attributes applied to IR functions, correct?  I'm curious what the
> semantics would be for cross-function optimization.  For example, consider
> a function "foo" defined with maxopt and a function "bar" defined with
> optsize.  If foo() calls bar() and the inliner wants to inline bar() into
> foo(), is that legal?  If so, that may cause odd effects as you may perform
> expensive optimizations later on the inlined version of bar(), even though
> the original function is marked optsize.
>
> Also, a nit-pick:  can we make the naming consistent?  It feels a bit
> weird to have maxOPT and OPTsize.  Perhaps use sizeopt and minsizeopt, or
> optmax and optquick?
>
>
> On Mon, Jan 14, 2013 at 4:09 AM, Chandler Carruth <chandlerc at gmail.com>wrote:
>
>> This has been an idea floating around in my head for a while and after
>> several discussions with others it continues to hold up so I thought I
>> would mail it out. Sorry for cross posting to both lists, but this is an
>> issue that would significantly impact both LLVM and Clang.
>>
>> Essentially, LLVM provides canned optimization "levels" for frontends to
>> re-use. This is nothing new. However, we don't have good names for them, we
>> don't expose them at the IR level, and we have a hard time figuring out
>> which optimizations belong in which levels. I'd like to try addressing that
>> by coming up with names and a description of the basic intend goal of each
>> level. I would like, if folks are happy with these ideas, to add these
>> types of descriptions along side these attributes to the langref. Ideas on
>> other (better?) places to document this would be welcome. Certainly,
>> Clang's documentation would need to be updated to reflect this.
>>
>> Hopefully we can minimally debate this until the bikeshed is a tolerable
>> shade. Note that I'm absolutely biased based on the behavior of Clang and
>> GCC with these optimization levels, and the relevant history there.
>> However, I'm adding and deviating from the purely historical differences to
>> try and better model the latest developments in LLVM's optimizer... So here
>> goes:
>>
>>
>> 1) The easiest: 'Minimize Size' or '-Oz'
>> - Attribute: minsize (we already have it, nothing to do here)
>> - Goal: minimize the size of the resulting binary, at (nearly) any cost.
>>
>>
>> 2) Optimize for size or '-Os'
>> - Attribute: optsize (we already have it, nothing to do here)
>> - Goal: Optimize the execution of the binary without unreasonably[1]
>> increasing the binary size.
>> This one is a bit fuzzy, but usually people don't have a hard time
>> figuring out where the line is. The primary difference between minsize and
>> optsize is that with minsize a pass is free to *hurt* performance to shrink
>> the size.
>>
>> [1] The definition of 'unreasonable' is of course subjective, but here is
>> at least one strong indicator: any code size growth which is inherently
>> *speculative* (that is, there isn't a known, demonstrable performance
>> benefit, but rather it is "often" or "maybe" a benefit) is unlikely to be a
>> good fit in optsize. The canonical example IMO is a vectorizer -- while it
>> is reasonable to vectorize a loop, if the vector version might not be
>> executed, and thus the scalar loop remains as well, then it is a poor fit
>> for optsize.
>>
>>
>> 3) Optimize quickly or '-O1'
>> - Attribute: quickopt (this would be a new attribute)
>> - Goal: Perform basic optimizations to improve both performance and
>> simplicity of the code, but perform them *quickly*.
>> This level is all about compile time, but in a holistic sense. It tries
>> to perform basic optimizations to get reasonably efficient code, and get it
>> very quickly.
>>
>>
>> 4) Good, well-balanced optimizations, or '-O2'
>> - Attribute: opt (new attribute)
>> - Goal: produce a well optimized binary trading off compile time, space,
>> and runtime efficiency.
>> This should be an excellent default for general purpose programs. The
>> idea is to do as much optimization as we can, in as reasonable of a time
>> frame, and with as reasonable code size impact as possible. This level
>> should always produce binaries at least as fast as optsize, but they might
>> be both bigger and faster. This level should always produce binaries at
>> least as fast as quickopt, but they might be both slower to compile.
>>
>>
>> 5) Optimize to the max or '-O3'
>> - Attribute: maxopt (new attribute)
>> - Goal: produce the fastest binary possible.
>> This level has historically been almost exclusively about trading off
>> more binary size for speed than '-O2', but I would propose we change it to
>> be more about trading off either binary size or compilation time to achieve
>> a better performing binary. This level should always produce binaries at
>> least as fast as opt, but they might be faster at the cost of them being
>> larger and taking more time to compile. This would in some cases be a
>> change for LLVM and is definitely a deviation from GCC where O3 will in
>> many cases produce *slower* binaries due to code size increases that are
>> not accompanied by corresponding performance increases.
>>
>>
>> To go with these LLVM attributes I'd like to add support for adding
>> attributes in Clang, both compatible with GCC and with the names above for
>> clarity. The goal being to allow a specific function to have its
>> optimization level overridden from the command line based level.
>>
>>
>> A final note: I would like to remove all other variations on the '-O'
>> flag. That includes the really strange '-O4' behavior. Whether the
>> compilation is LTO should be an orthogonal decision to the particular level
>> of optimization, and we have -flto to achieve this.
>>
>> -Chandler
>>
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>>
>
>
> --
>
> Thanks,
>
> Justin Holewinski
>
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