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

Sean Silva via llvm-dev llvm-dev at lists.llvm.org
Sat Jan 6 21:40:05 PST 2018 

No, I meant LLD, the LLVM linker. This option for LLD is relevant for
exploring different pass pipelines for link time optimization.

It is essentially equivalent to the -passes flag for 'opt'.

Such a flag doesn't make much sense for 'llc' because llc mostly runs
backend passes, which are much more difficult to construct custom pipelines
for (backend passes are often required for correctness or have complex
ordering requirements).

-- Sean Silva

On Jan 6, 2018 7:35 PM, "toddy wang" <wenwangtoddy at gmail.com> wrote:

@Sean, do you mean llc ?
For llc 4.0 and llc 5.0, I cannot find -lto-newpm-passes option, is it a
hidden one?

On Sat, Jan 6, 2018 at 7:37 PM, Sean Silva <chisophugis at gmail.com> wrote:

>
>
> On Jan 5, 2018 11:30 PM, "toddy wang via llvm-dev" <
> llvm-dev at lists.llvm.org> 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.
>
>
> LLD has -lto-newpm-passes (and the corresponding -lto-newpm-aa-pipeline)
> which allows you to pass a custom pass pipeline with full control. At one
> point I was using a similar modification to clang (see
> https://reviews.llvm.org/D21954) that never landed.
>
> -- Sean Silva
>
>
> 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
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180106/7af39251/attachment.html>

More information about the llvm-dev mailing list