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

[toddy wang via llvm-dev]

On Mon, Jan 8, 2018 at 11:53 AM, Mehdi AMINI <joker.eph at gmail.com> wrote:

>
>
> 2018-01-08 8:41 GMT-08:00 toddy wang <wenwangtoddy at gmail.com>:
>
>> Hi Medhi,
>>
>> It seems -mllvm does not work as expected. Anything wrong?
>>
>> [twang15 at c92 temp]$ clang++ -O3 -mllvm *-deadargelim* LULESH.cc
>> clang (LLVM option parsing): Unknown command line argument
>> '-deadargelim'.  Try: 'clang (LLVM option parsing) -help'
>> clang (LLVM option parsing): Did you mean '-regalloc'?
>>
>> [twang15 at c92 temp]$ clang++ -O3 -mllvm *deadargelim* LULESH.cc
>> clang (LLVM option parsing): Unknown command line argument
>> 'deadargelim'.  Try: 'clang (LLVM option parsing) -help'
>>
>
> You can't schedule passes this way, only set parameters
> like -unroll-threshold=<uint> etc.
>
> Where can I find options like  -unroll-threshold=<uint>? I cannot find it
in either opt -help or clang -help.

> --
> Mehdi
>
>
>>
>> -Tao
>>
>> On Mon, Jan 8, 2018 at 11:12 AM, Mehdi AMINI <joker.eph at gmail.com> wrote:
>>
>>>
>>>
>>> 2018-01-07 23:16 GMT-08:00 toddy wang <wenwangtoddy at gmail.com>:
>>>
>>>> -mllvm <value>          Additional arguments to forward to LLVM's
>>>> option processing
>>>>
>>>> This is dumped by clang. I am not sure what I am supposed to put as
>>>> value in order to tune unrolling/inlining threshold.
>>>>
>>>
>>>
>>> As the help says, this is used to pass argument to LLVM itself. If you
>>> remember you earlier question about setA (clang options) and setC (opt
>>>  options), this allows to reach setC from the clang command line.
>>> Any option that you see in the output of `opt --help` can be set from
>>> clang using `-mllvm`. Same caveat as I mentioned before: these aren't
>>> supposed to be end-user options.
>>>
>>> --
>>> Mehdi
>>>
>>>
>>>
>>>>
>>>> On Mon, Jan 8, 2018 at 2:02 AM, Sean Silva <chisophugis at gmail.com>
>>>> wrote:
>>>>
>>>>> For the types of things that you are looking for, you may just want to
>>>>> try a bunch of -mllvm options. You can tune inlining and unrolling
>>>>> threshold like that, for example.
>>>>>
>>>>> On Jan 7, 2018 10:33 PM, "toddy wang via llvm-dev" <
>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>
>>>>>> Hi Mehdi,
>>>>>>
>>>>>> Now we have 5 pipelines. (In addition to the first 3, which I have
>>>>>> described in detail above, please refer my latest reply for details)
>>>>>> 1. clang + opt + gold
>>>>>> 2. clang + opt + lld
>>>>>> 3. clang + GNU ld/ gold /lld
>>>>>>
>>>>>> 4. clang + opt + llc + clang
>>>>>> clang -emit-llvm -O1 -Xclang -disable-llvm-passes for c/c++ to .bc
>>>>>> generation and minimal  front-end optimization
>>>>>> opt for single bc file optimization
>>>>>> llc single bc file to obj file generation and back-end optimization
>>>>>> (no link-time optimization is possible, since llc works on 1 bc file at a
>>>>>> time)
>>>>>> clang again for linking all obj file to generate final executable. (although
>>>>>> in principle there can be a link-time optimization even with all obj files,
>>>>>> it requires a lot of work and is machine-dependent. This may also be the
>>>>>> reason why modern compilers like LLVM/GCC/ICC, etc performs LTO not at obj
>>>>>> level. But, obj level may yield extra benefit even LTO at intermediate
>>>>>> level has been applied by compilers, because obj level can see more
>>>>>> information.)
>>>>>>
>>>>>> `clang -Ox` + `opt -Ox` + `llc -Ox` is too coarse-grain.
>>>>>>
>>>>>> 5. Modify clang to align with GCC/ICC so that many tunables are
>>>>>> exposed at clang command line. Not sure how much work is needed, but at
>>>>>> least requires an overall understanding of compiler internals, which can be
>>>>>> gradually figured out.
>>>>>>
>>>>>> I believe 5 is interesting, but 2 may be good enough. More
>>>>>> experiments are needed before decision is made.
>>>>>>
>>>>>> On Mon, Jan 8, 2018 at 12:56 AM, Mehdi AMINI <joker.eph at gmail.com>
>>>>>> wrote:
>>>>>>
>>>>>>> Hi Toddy,
>>>>>>>
>>>>>>> You can achieve what you're looking for with a pipeline based on
>>>>>>> `clang -Ox` + `opt -Ox` + `llc -Ox` (or lld instead of llc), but this won't
>>>>>>> be guarantee'd to be well supported across releases of the compiler.
>>>>>>>
>>>>>>> Otherwise, if there are some performance-releated (or not...)
>>>>>>> command line options you think clang is missing / would benefit, I invite
>>>>>>> you to propose adding them to cfe-dev at lists.llvm.org and submit a
>>>>>>> patch!
>>>>>>>
>>>>>>> Best,
>>>>>>>
>>>>>>> --
>>>>>>> Mehdi
>>>>>>>
>>>>>>> 2018-01-07 21:03 GMT-08:00 toddy wang <wenwangtoddy at gmail.com>:
>>>>>>>
>>>>>>>> Thanks a lot, Mehdi.
>>>>>>>>
>>>>>>>> For GCC, there are around 190 optimization flags exposed as
>>>>>>>> command-line options.
>>>>>>>> For Clang/LLVM, the number is 40, and many important optimization
>>>>>>>> parameters are not exposed at all, such as loop unrolling factor, inline
>>>>>>>> function size parameters.
>>>>>>>>
>>>>>>>> I understand there is very different idea for whether or not expose
>>>>>>>> many flags to end-user.
>>>>>>>> Personally, I believe it is a reasonable to keep end-user
>>>>>>>> controllable command-line options minimal for user-friendliness.
>>>>>>>> However, for users who care a lot for a tiny bit performance
>>>>>>>> improvement, like HPC community, it may be better to expose as many
>>>>>>>> fine-grained tunables in the form of command line options as possible. Or,
>>>>>>>> at least there should be a way to achieve this fairly easy.
>>>>>>>>
>>>>>>>> I am curious about which way is the best for my purpose.
>>>>>>>> Please see my latest reply for 3 possible fine-grained optimization
>>>>>>>> pipeline.
>>>>>>>> Looking forward to more discussions.
>>>>>>>>
>>>>>>>> Thanks a lot!
>>>>>>>>
>>>>>>>> On Sun, Jan 7, 2018 at 10:11 AM, Mehdi AMINI <joker.eph at gmail.com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>> Hi,
>>>>>>>>>
>>>>>>>>> "SetC" options are LLVM cl::opt options, they are intended for
>>>>>>>>> LLVM developer and experimentations. If a settings is intended to be used
>>>>>>>>> as a public API, there is usually a programmatic way of setting it in LLVM.
>>>>>>>>> "SetA" is what clang as a C++ compiler exposes to the end-user.
>>>>>>>>> Internally clang will (most of the time) use one or multiple LLVM APIs to
>>>>>>>>> propagate a settings.
>>>>>>>>>
>>>>>>>>> Best,
>>>>>>>>>
>>>>>>>>> --
>>>>>>>>> Mehdi
>>>>>>>>>
>>>>>>>>> 2018-01-05 17:41 GMT-08:00 toddy wang via llvm-dev <
>>>>>>>>> llvm-dev at lists.llvm.org>:
>>>>>>>>>
>>>>>>>>>> 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
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> _______________________________________________
>>>>>>>>>> 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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