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

toddy wang via llvm-dev llvm-dev at lists.llvm.org
Mon Jan 8 08:41:59 PST 2018 

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'

-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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