[LLVMdev] [RFC] Stripping unusable intrinsics

[Sean Silva]

On Thu, Dec 11, 2014 at 4:46 PM, Chris Bieneman <cbieneman at apple.com> wrote:
>
> That library isn’t actually representative of what we ship in terms of
> it’s overall size. For that test I only stripped unused backends. We also
> strip unused functionality, but we also have out-of-tree functionality that
> we add in.
>
> I our shipping library for WebKit is under 15MB, which would put this
> closer to ~3% savings. As far as low-hanging fruit go, that’s a pretty big
> one.
>
>
Ah, that makes more sense.

-- Sean Silva


> -Chris
>
> On Dec 11, 2014, at 4:43 PM, Sean Silva <chisophugis at gmail.com> wrote:
>
>
>
> On Thu, Dec 11, 2014 at 4:23 PM, Chris Bieneman <cbieneman at apple.com>
> wrote:
>>
>> I’ve got some new patches and some numbers.
>>
>> The patches are all the changes required to strip intrinsics using the
>> preprocessor defines I showed in my earlier patches. It actually isn’t that
>> much change to LLVM. Clang will need similar changes too. There were only 9
>> files that referenced target intrinsics outside the corresponding target.
>>
>> These patches are still WIP, and there is some nastiness, but they work.
>>
>> Using these patches I see about 500k reduction in binary size when
>> building libLLVM.dylib with just the ARM backend enabled.
>>
>> > du -k ../*.dylib
>> 34236 ../libLLVM-ARM-only-after.dylib
>> 34732 ../libLLVM-ARM-only-before.dylib
>>
>> These savings are substantial for our use case.
>>
>
> I'm sort of curious that ~1.5% savings is "substantial". It smells of
> diminishing returns/microoptimization. If there is really such a dire need
> for size, surely there are whole parts of the API that aren't used by your
> application and could be stripped for much greater benefit?
>
> -- Sean Silva
>
>
>>
>> -Chris
>>
>>
>> On Dec 10, 2014, at 5:39 PM, Eric Christopher <echristo at gmail.com> wrote:
>>
>>
>>
>> On Wed Dec 10 2014 at 5:37:06 PM Chris Bieneman <beanz at apple.com> wrote:
>>
>>> On Dec 10, 2014, at 4:22 PM, Eric Christopher <echristo at gmail.com>
>>> wrote:
>>>
>>>
>>>
>>> On Wed Dec 10 2014 at 3:57:25 PM Chris Bieneman <beanz at apple.com> wrote:
>>>
>>>> llvm-dev,
>>>>
>>>> In my ongoing saga to improve LLVM for embedded use, we would like to
>>>> support stripping out unused intrinsics based on the LLVM targets actually
>>>> being built.
>>>>
>>>> I’ve attached two patches.
>>>>
>>>> The first is a new flag for tablegen to take a list of targets. If
>>>> passed tablegen will only emit intrinsics that either have empty target
>>>> prefixes, or target prefixes matching one of the targets in the list. If
>>>> the flag is not passed the behavior is unchanged. This patch can land today
>>>> (subject to review).
>>>>
>>>> The second patch is a WIP, and adds support to the CMake build system
>>>> for using the new tablegen flag, and for generating a new
>>>> llvm/Config/llvm-targets.h header which contains defines for each target
>>>> specified with LLVM_TARGETS_TO_BUILD.
>>>>
>>>> This new header will allow us to #ifdef code using target-specific
>>>> intrinsics outside the targets, thus allowing us to strip out all the
>>>> unused intrinsics.
>>>>
>>>>
>>> I like the general idea and, as you asked on irc, will happily help with
>>> the autoconf changes. Do you have a small (even pseudo) code example of
>>> what the changes to the middle end machinery will look like?
>>>
>>>
>>> The big change required outside the patches on my first email is that
>>> any use of a target intrinsic will need to be #ifdef’d. I haven’t tracked
>>> all of those down yet.
>>>
>>>
>> That's a little gross. I wonder if there's a better abstraction for this
>> - I think the "check if a preprocessor define" is valid would be painful
>> for maintenance and testing.
>>
>> -eric
>>
>>
>>> -Chris
>>>
>>>
>>> -eric
>>>
>>>
>>>> -Chris
>>>>
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>>>
>>
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>
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