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

Craig Topper via llvm-dev llvm-dev at lists.llvm.org
Sat Jan 6 13:19:22 PST 2018 

Why are you using build directions from "flang" which is a fortran compiler
and maintained by different people than the LLVM/clang community? But then
compiling C/C++ code? Their bug database should be used for filing bugs
against the fortran compiler not a C/C++ compiler issue.

~Craig

On Sat, Jan 6, 2018 at 1:04 PM, toddy wang <wenwangtoddy at gmail.com> wrote:

> 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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