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

[Jiayu Zhao via llvm-dev]

Q: Is the only difference between the two method is that in the second case you're using clang to invoke the assembler instead of letting llc do the assembly?

A: No, we will get different .bc IR files with clang and opt, as following:
clang++ -Oz -Xclang -disable-llvm-optzns -emit-llvm -c raytracer.cpp -o raytracer.bc
opt -Oz raytracer.bc -o tmp.bc
clang++ -Oz -emit-llvm -c raytracer.cpp -o raytracer.bc
The tmp.bc will be different with raytracer.bc, and in some cases it will lead to different code sizes.
________________________________
From: Mehdi AMINI <joker.eph at gmail.com>
Sent: 23 March 2021 1:18
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



On Mon, Mar 22, 2021 at 4:43 PM Jiayu Zhao <scjzh at leeds.ac.uk<mailto:scjzh at leeds.ac.uk>> 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 raytracer.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.

Is the only difference between the two method is that in the second case you're using clang to invoke the assembler instead of letting llc do the assembly? If so this seems fishy that the second case would match but not the first one: assembling the file is very mechanical. I'm interested in seeing a reproducer for this.


However, sometimes they are also different. I guess it is because clang has its own optimizations,

Not really: clang does not have separate optimizations from LLVM.

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.

It isn't as much the order as it is how it is set up: passes take options in their constructor, and some analyses inserted in the pipeline have knowledge of the platform (like TargetLibraryInformation).

--
Mehdi



________________________________
From: Mehdi AMINI <joker.eph at gmail.com<mailto:joker.eph at gmail.com>>
Sent: 21 March 2021 4:13
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

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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