[llvm-dev] Programmatically Toggle Specific LLVM Optimizations

[Qiongsi Wu via llvm-dev]

Thanks for the fast reply! I will submit a patch and find out! 

Sincerely,
Qiongsi

> On Jul 10, 2018, at 6:49 PM, Michael Kruse <llvmdev at meinersbur.de> wrote:
> 
> Unless you intend to upstream you change (which I encourage you to),
> there is no need to follow any best practices. If you intend to submit
> the patch, the code reviews exist to ensure that the best practices
> are followed. You can avoid the most obvious change requests by
> considering https://llvm.org/docs/CodingStandards.html .
> 
> Please note that there is a newer pass manager. You might get feedback
> to update the that pass manager as well/instead or that your use case
> is too 'niche' to be useful for others. However, you won't find out
> before submitting the patch. And the experience might be useful if you
> intend collaborate to LLVM more often.
> 
> Michael
> 
> 
> 
> 2018-07-10 17:38 GMT-05:00 Qiongsi Wu <qiongsi.wu at mail.utoronto.ca>:
>> Hi Michael!
>> 
>> Thanks for your reply. I managed to add a small amount of code to PassManager to have this feature implemented.
>> 
>> One can now turn off a pass by doing
>> 
>> opt -disablepass=licm or clang -mllvm -disablepass=licm
>> 
>> to turn off, for example, loop invariant code motion.
>> 
>> I programmed the command line option to be really hidden. This maybe a useful thing to have in the trunk for people who want to fine tune the optimizations.
>> 
>> Should I submit a patch? I am not sure if my code follows all the best practices of the community, but it is working from my own testing.
>> 
>> Thanks again for your help!
>> 
>> Qiongsi
>> 
>>> On Jun 1, 2018, at 1:16 PM, Michael Kruse <llvmdev at meinersbur.de> wrote:
>>> 
>>> Some passes have supported options to disable them, e.g.
>>> -fno-vectorize and -fno-unroll-loops, but there is no general option.
>>> Since it's not useful in general to disable arbitrary options, some
>>> handywork is required. Using the legacy (=default) pass manager here.
>>> 
>>> To get the passes that are run:
>>> $ clang -O3 -mllvm -debug-pass=Arguments
>>> 
>>> To optimize separately:
>>> $ clang -Xclang -disable-llvm-passes -S -emit-llvm -o - | opt <your
>>> list of passes> | llc
>>> (not tested, so don't expect to work straight out of the box)
>>> 
>>> This is not strictly the same as optimizing using clang (pass options
>>> are missing), so there will be performance differences. You could
>>> instead customize PassManagerBuilder::populateModulePassManager() and
>>> PassManagerBuilder::populateFunctionPassManager().
>>> 
>>> Michael
>>