[llvm-dev] Definitive list of optimisations at each optimisation level

[Ryan Taylor via llvm-dev]

I agree, it's much clearer, it just takes runs at multiple opt levels and
therefore I don't find it to be a "one stop shop".

On Jun 24, 2017 8:44 PM, "Sean Silva" <chisophugis at gmail.com> wrote:

> Looking at PassManagerBuilder can be useful because there are sometimes
> comments giving some idea of the intent of the particular choice of passes,
> but it can be difficult to see the big picture of all the passes that are
> run because it is very parameterized and split across multiple subroutines.
>
> Running clang at O0 through O3 with -mllvm -debug-pass=Structure is
> generally more enlightening because then you see everything in one place
> for each optimization level.
>
> -- Sean Silva
>
> On Jun 24, 2017 12:54 PM, "Ryan Taylor" <ryta1203 at gmail.com> wrote:
>
> Right but then you'll have to call each opt level. Have u looked in the
> pass manager?
>
> On Jun 24, 2017 3:52 PM, "Sean Silva via llvm-dev" <
> llvm-dev at lists.llvm.org> wrote:
>
>> The most definitive list you can probably hope to get will be obtained by
>> passing -mllvm -debug-pass=Structure to a clang invocation.
>>
>> -- Sean Silva
>>
>> On Jun 24, 2017 7:10 AM, "Martin J. O'Riordan via llvm-dev" <
>> llvm-dev at lists.llvm.org> wrote:
>>
>> I am often asked what optimisations “our” compiler performs at each
>> level.  But “our” compiler is actually CLang/LLVM which we have retargeted
>> to our proprietary target.
>>
>>
>>
>> Most of the work we do is in maintaining our target specific backend.
>> Certainly there are optimisations that we do to take best advantage of our
>> instruction set during lowering and instruction selection, and we have also
>> added a couple of additional passes which manipulate the IR in advance of
>> lowering to better shape it for our target.
>>
>>
>>
>> But in practice the vast majority of optimisations are contributed by the
>> continuously evolving and excellent LLVM target independent passes, and
>> have little or nothing to do with the work we do in our backend - though it
>> is obviously directed/tuned by the various target call-backs and the target
>> cost models.
>>
>>
>>
>> Is there a “one stop shop” list of the optimisation passes that LLVM
>> performs, and identification of which are enabled by default for each of
>> the 4 standard optimisation levels ‘-O0’ thru ‘-O3’?
>>
>>
>>
>> The reason that I ask, is that I really don’t have an honest or informed
>> answer that I can provide to people when they ask, and I haven’t found a
>> definitive statement of this in the LLVM documentation that I could refer
>> them to.
>>
>>
>>
>> Thanks,
>>
>>
>>
>>             MartinO
>>
>>
>>
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>>
>>
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>
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