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

toddy wang via llvm-dev llvm-dev at lists.llvm.org
Sat Jan 6 13:04:55 PST 2018 

Thanks a lot, it is clear to me now.

BTW, for Clang's slowdown, I submit an issue here:
https://github.com/flang-compiler/flang/issues/356

I have no idea about the root cause.
Maybe due to debug symbols. But, I already use -DCMAKE_BUILD_TYPE=Release.
Anyway, I believe there is a bug somewhere.


On Sat, Jan 6, 2018 at 3:43 PM, Craig Topper <craig.topper at gmail.com> wrote:

> -disable-O0-optnone has no effect with anything other than -O0.
>
> -O0 being passed to clang also causes all functions to be marked noinline.
> I don't know if there is a command line option to turn that off.
>
> I recommend passing "-O1 -Xclang -disable-llvm-passes" to clang. Passing
> -O0 very specifically means disable optimizations.
>
> ~Craig
>
> On Sat, Jan 6, 2018 at 12:25 PM, toddy wang <wenwangtoddy at gmail.com>
> wrote:
>
>> @Craig and @Michael
>>
>> After installing clang-5.0 (download from http://releases.llvm.org, does
>> not have Flang build's slowdown mention above),
>>
>> 1. clang++ -O0 -Xclang -disable-O0-optnone -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: 2.354069e+01
>>
>> 2. clang++ -O1 -Xclang -disable-O0-optnone -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: 9.046271e+00
>>
>> 3. clang++ -O3 LULESH.cc
>> runtime: 9.118835e+00
>>
>> 4. clang++ -O2 -Xclang -disable-O0-optnone -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: 9.091278e+00
>>
>> 5. clang++ -O3 -Xclang -disable-O0-optnone -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: 9.096919e+00
>>
>> Apparently, clang++ -O0 -Xclang -disable-O0-optnone does not work as
>> expected.
>>
>> The conclusion seems to be  -Xclang -disable-O0-optnone works when clang
>> optimization level is O1/O2/O3, not O0.
>>
>> Any comments?
>>
>>
>>
>>
>> On Sat, Jan 6, 2018 at 2:30 AM, toddy wang <wenwangtoddy at gmail.com>
>> wrote:
>>
>>> What I am trying is to compile a program with different sets of
>>> optimization flags.
>>> If there is no fine-grained control over clang optimization flags, it
>>> would be impossible to achieve what I intend.
>>>
>>> Although there is fine-grained control via opt, for a large-scale
>>> projects, clang-opt-llc pipeline may not be a drop-in solution.
>>>
>>> On Fri, Jan 5, 2018 at 10:00 PM, Craig Topper <craig.topper at gmail.com>
>>> wrote:
>>>
>>>> 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/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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