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

Craig Topper via llvm-dev llvm-dev at lists.llvm.org
Sat Jan 6 12:43:13 PST 2018 

-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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