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

toddy wang via llvm-dev llvm-dev at lists.llvm.org
Sat Jan 6 13:32:54 PST 2018 

I have a code written in Fortran but with C/C++ kernels.
So I have to use both Flang and Clang to compile, maybe I should use
LLVM5.0 release for c/c++ and only their Flang for fortran code.
It should work but I have not tried yet.

On Sat, Jan 6, 2018 at 4:19 PM, Craig Topper <craig.topper at gmail.com> wrote:

> 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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