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

[Peizhao Ou via llvm-dev]

Thanks, Craig! That totally corrects my wrong understanding about the front
end. I tried it and indeed now I can see almost the same results.

Best,
Peizhao

On Mon, Apr 10, 2017 at 5:21 PM, Craig Topper <craig.topper at gmail.com>
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
>
> 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.
>
> 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/relationship-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
>>
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>> llvm-dev at lists.llvm.org
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>>
>>
>
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