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

[Jiayu Zhao via llvm-dev]

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<mailto: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<mailto:joker.eph at gmail.com>>
Sent: 21 March 2021 3:23
To: Jiayu Zhao <scjzh at leeds.ac.uk<mailto:scjzh at leeds.ac.uk>>
Cc: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org> <llvm-dev at lists.llvm.org<mailto: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<mailto: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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