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

Craig Topper via llvm-dev llvm-dev at lists.llvm.org
Fri Jan 5 19:00:08 PST 2018 

I don't think "clang -help" prints options about optimizations. Clang
itself doesn't have direct support for fine grained optimization control.
Just the flag for levels -O0/-O1/-O2/-O3. This is intended to be simple and
sufficient interface for most users who just want to compile their code. So
I don't think there's a way to pass just -dse to clang.

opt on the other hand is more of a utility for developers of llvm that
provides fine grained control of optimizations for testing purposes.



~Craig

On Fri, Jan 5, 2018 at 5:41 PM, toddy wang <wenwangtoddy at gmail.com> wrote:

> 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/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
>>>>>>>>
>>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> LLVM Developers mailing list
>>>>>>> llvm-dev at lists.llvm.org
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>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>> _______________________________________________
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>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>
>>>>>>
>>>>>
>>>>> _______________________________________________
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>>>>>
>>>>>
>>>>
>>>
>>
>
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