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

[Mehdi Amini via llvm-dev]

> 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 <mailto: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 <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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