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

toddy wang via llvm-dev llvm-dev at lists.llvm.org
Sun Jan 7 23:16:26 PST 2018 

-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.

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
>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
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