[llvm-dev] Relationship between clang, opt and llc

Mehdi AMINI via llvm-dev llvm-dev at lists.llvm.org
Sun Jan 7 21:56:51 PST 2018 

Hi Toddy,

You can achieve what you're looking for with a pipeline based on `clang
-Ox` + `opt -Ox` + `llc -Ox` (or lld instead of llc), but this won't be
guarantee'd to be well supported across releases of the compiler.

Otherwise, if there are some performance-releated (or not...) command line
options you think clang is missing / would benefit, I invite you to propose
adding them to cfe-dev at lists.llvm.org and submit a patch!

Best,

-- 
Mehdi

2018-01-07 21:03 GMT-08:00 toddy wang <wenwangtoddy at gmail.com>:

> Thanks a lot, Mehdi.
>
> For GCC, there are around 190 optimization flags exposed as command-line
> options.
> For Clang/LLVM, the number is 40, and many important optimization
> parameters are not exposed at all, such as loop unrolling factor, inline
> function size parameters.
>
> I understand there is very different idea for whether or not expose many
> flags to end-user.
> Personally, I believe it is a reasonable to keep end-user controllable
> command-line options minimal for user-friendliness.
> However, for users who care a lot for a tiny bit performance improvement,
> like HPC community, it may be better to expose as many fine-grained
> tunables in the form of command line options as possible. Or, at least
> there should be a way to achieve this fairly easy.
>
> I am curious about which way is the best for my purpose.
> Please see my latest reply for 3 possible fine-grained optimization
> pipeline.
> Looking forward to more discussions.
>
> Thanks a lot!
>
> On Sun, Jan 7, 2018 at 10:11 AM, Mehdi AMINI <joker.eph at gmail.com> wrote:
>
>> Hi,
>>
>> "SetC" options are LLVM cl::opt options, they are intended for LLVM
>> developer and experimentations. If a settings is intended to be used as a
>> public API, there is usually a programmatic way of setting it in LLVM.
>> "SetA" is what clang as a C++ compiler exposes to the end-user.
>> Internally clang will (most of the time) use one or multiple LLVM APIs to
>> propagate a settings.
>>
>> Best,
>>
>> --
>> Mehdi
>>
>> 2018-01-05 17:41 GMT-08:00 toddy wang via llvm-dev <
>> llvm-dev at lists.llvm.org>:
>>
>>> Craig, thanks a lot!
>>>
>>> I'm actually confused by clang optimization flags.
>>>
>>> If I run clang -help, it will show many optimizations (denoted as set A)
>>>  and non-optimization options (denoted as set B).
>>> If I run llvm-as < /dev/null | opt -O0/1/2/3 -disable-output
>>> -debug-pass=Arguments, it also shows many optimization flags (denote as set
>>> C).
>>>
>>> There are many options in set C while not in set A, and also options in
>>> set A but not in set C.
>>>
>>> The general question is:  what is the relationship between set A and set
>>> C, at the same optimization level O0/O1/O2/O3?
>>> Another question is: how to specify an option in set C as a clang
>>> command line option, if it is not in A?
>>>
>>> For example, -dse is in set C but not in set A, how can I specify it as
>>> a clang option? Or simply I cannot do that.
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> On Fri, Jan 5, 2018 at 7:55 PM, Craig Topper <craig.topper at gmail.com>
>>> wrote:
>>>
>>>> O0 didn't start applying optnone until r304127 in May 2017 which is
>>>> after the 4.0 family was branched. So only 5.0, 6.0, and trunk have that
>>>> behavior. Commit message copied below
>>>>
>>>> Author: Mehdi Amini <joker.eph at gmail.com>
>>>>
>>>> Date:   Mon May 29 05:38:20 2017 +0000
>>>>
>>>>
>>>>     IRGen: Add optnone attribute on function during O0
>>>>
>>>>
>>>>
>>>>     Amongst other, this will help LTO to correctly handle/honor files
>>>>
>>>>     compiled with O0, helping debugging failures.
>>>>
>>>>     It also seems in line with how we handle other options, like how
>>>>
>>>>     -fnoinline adds the appropriate attribute as well.
>>>>
>>>>
>>>>
>>>>     Differential Revision: https://reviews.llvm.org/D28404
>>>>
>>>>
>>>>
>>>> ~Craig
>>>>
>>>> On Fri, Jan 5, 2018 at 4:49 PM, toddy wang <wenwangtoddy at gmail.com>
>>>> wrote:
>>>>
>>>>> @Zhaopei, thanks for the clarification.
>>>>>
>>>>> @Craig and @Michael, for clang 4.0.1,  -Xclang -disable-O0-optnone
>>>>> gives the following error message. From which version -disable-O0-optnone
>>>>> gets supported?
>>>>>
>>>>> [twang15 at c89 temp]$ clang++ -O0 -Xclang -disable-O0-optnone -Xclang
>>>>> -disable-llvm-passes -c -emit-llvm -o a.bc LULESH.cc
>>>>> error: unknown argument: '-disable-O0-optnone'
>>>>>
>>>>> [twang15 at c89 temp]$ clang++ --version
>>>>> clang version 4.0.1 (tags/RELEASE_401/final)
>>>>> Target: x86_64-unknown-linux-gnu
>>>>>
>>>>> On Fri, Jan 5, 2018 at 4:45 PM, Craig Topper <craig.topper at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> If you pass -O0 to clang, most functions will be tagged with an
>>>>>> optnone function attribute that will prevent opt and llc even if you pass
>>>>>> -O3 to opt and llc. This is the mostly likely cause for the slow down in 2.
>>>>>>
>>>>>> You can disable the optnone function attribute behavior by passing
>>>>>> "-Xclang -disable-O0-optnone" to clang
>>>>>>
>>>>>> ~Craig
>>>>>>
>>>>>> On Fri, Jan 5, 2018 at 1:19 PM, toddy wang via llvm-dev <
>>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>>
>>>>>>> I tried the following on LULESH1.0 serial version (
>>>>>>> https://codesign.llnl.gov/lulesh/LULESH.cc)
>>>>>>>
>>>>>>> 1. clang++ -O3 LULESH.cc; ./a.out 20
>>>>>>> Runtime: 9.487353 second
>>>>>>>
>>>>>>> 2. clang++ -O0 -Xclang -disable-llvm-passes -c -emit-llvm -o a.bc
>>>>>>> LULESH.cc; opt -O3 a.bc -o b.bc; llc -O3 -filetype=obj b.bc -o b.o ;
>>>>>>> clang++ b.o -o b.out; ./b.out 20
>>>>>>> Runtime: 24.15 seconds
>>>>>>>
>>>>>>> 3. clang++ -O3 -Xclang -disable-llvm-passes -c -emit-llvm -o a.bc
>>>>>>> LULESH.cc; opt -O3 a.bc -o b.bc; llc -O3 -filetype=obj b.bc -o b.o ;
>>>>>>> clang++ b.o -o b.out; ./b.out 20
>>>>>>> Runtime: 9.53 seconds
>>>>>>>
>>>>>>> 1 and 3 have almost the same performance, while 2 is significantly
>>>>>>> worse, while I expect 1, 2 ,3 should have trivial difference.
>>>>>>>
>>>>>>> Is this a wrong expectation?
>>>>>>>
>>>>>>> @Peizhao, what did you try in your last post?
>>>>>>>
>>>>>>> On Tue, Apr 11, 2017 at 12:15 PM, Peizhao Ou via llvm-dev <
>>>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>>>
>>>>>>>> It's really nice of you pointing out the -Xclang option, it makes
>>>>>>>> things much easier. I really appreciate your help!
>>>>>>>>
>>>>>>>> Best,
>>>>>>>> Peizhao
>>>>>>>>
>>>>>>>> On Mon, Apr 10, 2017 at 10:12 PM, Mehdi Amini <
>>>>>>>> mehdi.amini at apple.com> wrote:
>>>>>>>>
>>>>>>>>>
>>>>>>>>> On Apr 10, 2017, at 5:21 PM, Craig Topper via llvm-dev <
>>>>>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>>>>>
>>>>>>>>> clang -O0 does not disable all optimization passes modify the IR.;
>>>>>>>>> In fact it causes most functions to get tagged with noinline to prevent
>>>>>>>>> inlinining
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> It also disable lifetime instrinsics emission and TBAA, etc.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> What you really need to do is
>>>>>>>>>
>>>>>>>>> clang -O3 -c emit-llvm -o source.bc -v
>>>>>>>>>
>>>>>>>>> Find the -cc1 command line from that output. Execute that command
>>>>>>>>> with --disable-llvm-passes. leave the -O3 and everything else.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> That’s a bit complicated: CC1 options can be passed through with
>>>>>>>>> -Xclang, for example here just adding to the regular clang invocation `
>>>>>>>>> -Xclang -disable-llvm-passes`
>>>>>>>>>
>>>>>>>>> Best,
>>>>>>>>>
>>>>>>>>> —
>>>>>>>>> Mehdi
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> You should be able to feed the output from that command to opt/llc
>>>>>>>>> and get consistent results.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> ~Craig
>>>>>>>>>
>>>>>>>>> On Mon, Apr 10, 2017 at 4:57 PM, Peizhao Ou via llvm-dev <
>>>>>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>>>>>
>>>>>>>>>> Hi folks,
>>>>>>>>>>
>>>>>>>>>> I am wondering about the relationship clang, opt and llc. I
>>>>>>>>>> understand that this has been asked, e.g.,
>>>>>>>>>> http://stackoverflow.com/questions/40350990/relationsh
>>>>>>>>>> ip-between-clang-opt-llc-and-llvm-linker. Sorry for posting a
>>>>>>>>>> similar question again, but I still have something that hasn't been
>>>>>>>>>> resolved yet.
>>>>>>>>>>
>>>>>>>>>> More specifically I am wondering about the following two
>>>>>>>>>> approaches compiling optimized executable:
>>>>>>>>>>
>>>>>>>>>> 1. clang -O3 -c source.c -o source.o
>>>>>>>>>>     ...
>>>>>>>>>>     clang a.o b.o c.o ... -o executable
>>>>>>>>>>
>>>>>>>>>> 2. clang -O0 -c -emit-llvm -o source.bc
>>>>>>>>>>     opt -O3 source.bc -o source.bc
>>>>>>>>>>     llc -O3 -filetype=obj source.bc -o source.o
>>>>>>>>>>     ...
>>>>>>>>>>     clang a.o b.o c.o ... -o executable
>>>>>>>>>>
>>>>>>>>>> I took a look at the source code of the clang tool and the opt
>>>>>>>>>> tool, they both seem to use the PassManagerBuilder::populateModulePassManager()
>>>>>>>>>> and PassManagerBuilder::populateFunctionPassManager() functions
>>>>>>>>>> to add passes to their optimization pipeline; and for the backend, the
>>>>>>>>>> clang and llc both use the addPassesToEmitFile() function to generate
>>>>>>>>>> object code.
>>>>>>>>>>
>>>>>>>>>> So presumably the above two approaches to generating optimized
>>>>>>>>>> executable file should do the same thing. However, I am seeing that the
>>>>>>>>>> second approach is around 2% slower than the first approach (which is the
>>>>>>>>>> way developers usually use) pretty consistently.
>>>>>>>>>>
>>>>>>>>>> Can anyone point me to the reasons why this happens? Or even
>>>>>>>>>> correct my wrong understanding of the relationship between these two
>>>>>>>>>> approaches?
>>>>>>>>>>
>>>>>>>>>> PS: I used the -debug-pass=Structure option to print out the
>>>>>>>>>> passes, they seem the same except that the first approach has an extra pass
>>>>>>>>>> called "-add-discriminator", but I don't think that's the reason.
>>>>>>>>>>
>>>>>>>>>> Peizhao
>>>>>>>>>>
>>>>>>>>>> _______________________________________________
>>>>>>>>>> LLVM Developers mailing list
>>>>>>>>>> llvm-dev at lists.llvm.org
>>>>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>> _______________________________________________
>>>>>>>>> LLVM Developers mailing list
>>>>>>>>> llvm-dev at lists.llvm.org
>>>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>> _______________________________________________
>>>>>>>> LLVM Developers mailing list
>>>>>>>> llvm-dev at lists.llvm.org
>>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> LLVM Developers mailing list
>>>>>>> llvm-dev at lists.llvm.org
>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>> _______________________________________________
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>>
>
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