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

Craig Topper via llvm-dev llvm-dev at lists.llvm.org
Fri Jan 5 16:55:10 PST 2018 

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/relationship-between-clang-opt-llc-and-l
>>>>>> lvm-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
>>>>>>
>>>>>>
>>>>> _______________________________________________
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>>>>>
>>>>>
>>>>>
>>>>
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>>>
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>>
>
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