[llvm-dev] How to produce the same result of clang++ -Oz through opt -Oz

[Mircea Trofin via llvm-dev]

Hello Jiayu,

could you detail your scenario a bit more - do you need to run opt + llc,
or would this be acceptable:

(assuming non thinlto build)

1) from your original clang call, e.g. clang <blah>, do clang -v <blah>.
copy the command line (starts with -cc1)
2) clang -cc1 <....> -fembed-bitcode=all

ok, now you can take the resulting .o, let's call it 'file.o', and do this:

3)
llvm-objcopy --dump-section=.llvmbc=/some/path/file.bc  file.o /dev/null
llvm-objcopy --dump-section=.llvmcmd=/some/path/file.cmd   file.o /dev/null

You get 2 files, /some/path/file.bc and /some/path/file.cmd. The .cmd file
contains the command line flags separated by '\0'. The .bc file is the IR.

If you re-issue now clang with those options (in the .cmd file*) over the
.bc, you should get a bit-identical file.o

Note that clang will skip over compiler directives like "-I" or -D or -U,
or paths to pch files or modules (since it now picks up the compilation
from IR). You can basically take the IR file and recompile it with the .cmd
command line arguments on a different machine, away from the build
directory structure where it came from.


* you need to split the .cmd file 'line' by \0, of course.

On Mon, Mar 22, 2021 at 4:43 PM Jiayu Zhao via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Thank you for your reply. The default optimization level of llc is -O2.
> What's more, I found the executable file (.out) produced by clang
> directly and llvm opt will be the same in most cases, but the object file
> (.o) will be different, both for -O3 and -Oz.
>
> Use the following commands to produce object files (.o) :
> 1. Produce the object file of llvm opt -Oz
>
>
> *clang++ -Oz -Xclang -disable-llvm-optzns -emit-llvm -c raytracer.cpp -o
> raytracer.bc *
>
> *opt -Oz raytracer.bc -o tmp.bc *
>
> *llc -filetype=obj tmp.bc -o tmp.o *
>
> *2. Produce the object file of clang++ directly*
>
> *clang++ -Oz -c r**​**aytracer.cpp -o raytracer.o*
>
> 3. compare
>
> *diff tmp.o raytracer.o*
>
> It always shows 'Binary files tmp.o and raytracer.o differ'.
>
> Use the following commands to produce executable files (.out) :
> 1. Produce theexecutable file of llvm opt -Oz
>
>
> *clang++ -Oz -Xclang -disable-llvm-optzns -emit-llvm -c raytracer.cpp -o
> raytracer.bc *
>
> *opt -Oz raytracer.bc -o tmp.bc *
>
> *llc tmp.bc -o tmp.s *
> *clang++ tmp.s -o tmp.out*
>
> *2. Produce theexecutable file of clang++ directly*
>
> *clang++ -Oz raytracer.cpp -o raytracer.out*
>
> 3. compare
>
> *diff tmp.outraytracer.out*
>
> We can find the two files are the same in most cases. However, sometimes
> they are also different. I guess it is because clang has its
> own optimizations, and these optimizations are not always before the llvm
> optimizations. If we use ' *clang -Oz -Xclang -disable-llvm-optzns' *to
> disable llvm passes and then use opt to apply llvm passes, the order of
> all optimizations will be different.
>
>
> Jiayu Zhao
>
> ------------------------------
> *From:* Mehdi AMINI <joker.eph at gmail.com>
> *Sent:* 21 March 2021 4:13
> *To:* Jiayu Zhao <scjzh at leeds.ac.uk>
> *Cc:* llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>
> *Subject:* Re: [llvm-dev] How to produce the same result of clang++ -Oz
> through opt -Oz
>
> In general you'll find many cases where O3 does not reproduce either.
>
> Have you tried `llc -O2` in your case? Clang sets the backend optimization
> level that way for Oz/Os/O2:
> https://github.com/llvm/llvm-project/blob/main/clang/lib/CodeGen/BackendUtil.cpp#L430
>
> --
> Mehdi
>
> On Sat, Mar 20, 2021 at 9:06 PM Jiayu Zhao <scjzh at leeds.ac.uk> wrote:
>
> Hi,
>
> I can reproduce the results of clang++ -O3 by opt -O3. But I cannot
> reproduce the results of clang++ -Oz by opt -Oz.
>
> Just see the previous commands I used to produce the result of opt -O3.
> ------------------------------
> *From:* Mehdi AMINI <joker.eph at gmail.com>
> *Sent:* 21 March 2021 3:23
> *To:* Jiayu Zhao <scjzh at leeds.ac.uk>
> *Cc:* llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>
> *Subject:* Re: [llvm-dev] How to produce the same result of clang++ -Oz
> through opt -Oz
>
> Hi,
>
> This is a known issue: clang -O3 is slightly different from `opt -O3` and
> it is hard to reproduce exactly.  It'd be great to refactor it all so that
> LLVM exposes a common way for frontend to run the exact same thing.
>
> --
> Mehdi
>
>
> On Sat, Mar 20, 2021 at 9:17 AM Jiayu Zhao via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Dear developers,
>
> I am trying to use 'opt' command with different Optlevels -O3 and -Oz to
> an IR file to reproduce the results of directly applying clang++ -O3 and
> -Oz to a source c++ file.
>
> 1. Firstly I use the following commands to produce the result of opt -O3:
> *clang++ -O3 -Xclang -disable-llvm-optzns -emit-llvm -c raytracer.cpp -o
> raytracer.bc*
> *opt -O3 raytracer.bc -o tmp.bc *
> *llc -O3 tmp.bc -o tmp.s*
> *clang++ tmp.s -o tmp.out*
>
> 2. Then I use clang -O3 directly:
> *clang++ -O3 raytracer.cpp -o raytracer.out*
>
> 3. Finally I compare the two files tmp.o and raytracer.o:
> *diff tmp.out raytracer.out*
>
> We can find the two files are exactly the same.
>
> However, things are different in -Oz level
>
> 1. I use the following commands to produce the result of opt -Oz:
> *clang++ -Oz -Xclang -disable-llvm-optzns -emit-llvm -c raytracer.cpp -o
> raytracer.bc*
> *opt -Oz raytracer.bc -o tmp.bc *
> *llc -filetype=obj tmp.bc -o tmp.o (there is no -Oz option for llc)*
>
> 2. Then I use clang -Oz directly:
> *clang++ -Oz -c raytracer.cpp -o raytracer.o*
>
> 3. Finally I compare the two files tmp.o and raytracer.o:
> *diff tmp.o raytracer.o*
>
> It shows 'Binary files tmp.o and raytracer.o differ'
>
> why 'opt -Oz' cannot produce the same result as 'clang++ -Oz' and how to
> solve it ? I use LLVM 10.0.1 and CentOS 7.6.
>
> I'd very much appreciate if you could help me with this. Thank you.
>
> Kind Regards,
> Jiayu Zhao
>
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