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

[Craig Topper via llvm-dev]

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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