llvm dev - Aug 2018 - Replication -O3 optimizations manually

Hello llvm-dev,

my name is Emanuele and I am an intern in ARM. As part of the project I am doing
here, I would like to manually replicate the optimizations that LLVM applies
when I type -O3. In other words, I would like to know what are the compilation
flags/passes that -O3 triggers.

I noticed that GCC reports, on its website, all the flags that are enforced by
-O3 (https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html), but I wasn't
able to find something similar within LLVM documentation. On the other hand, I
found that this command displays all the optimization passes applied by opt when
-O3 flag is on:

llvm-as < /dev/null | opt -O3 -disable-output -debug-pass=Arguments

I tried to apply the same optimization passes through opt, but, even though the
performance are similar, the resulting binary is slower than the one generated
using -O3 (also the binaries differ, of course).

Again, I found this other command that does something similar (it lists the
sequence of optimization passes applied):

clang -O3 -mllvm -debug-pass=Arguments file.c

In this case, the performance are still different and some of the optimization
passes listed in the last block of passes (e.g. -machinemoduleinfo,
-stack-protector, etc.) are unknown to opt.


Said that, my question is: how can I find out what optimization passes/flags -O3
enforces in order to manually apply the same optimizations and have, hopefully,
the same binary and performance?


I am currently using LLVM version 5.0.2.


Thank you for both your help and your time!


Best regards

Emanuele


IMPORTANT NOTICE: The contents of this email and any attachments are
confidential and may also be privileged. If you are not the intended recipient,
please notify the sender immediately and do not disclose the contents to any
other person, use it for any purpose, or store or copy the information in any
medium. Thank you.
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20180816/737bb95c/attachment.html>

Hello Emanuele,
When you provide the optimization level -O3 to the clang driver, it does not
simply schedule a sequence of passes to be run on the intermediate
representation.Indeed, it schedules target-independent and target-dependent
passes.Moreover, IIRC, the optimization level is also used in the later stages
of the code generation to apply target-dependent optimizations (i.e.
vectorizer).
The most common use case when someone wants to test its own pass/work within the
LLVM toolchain is the following
- use clang to generate a LLVM-IR file- use opt to run your desired pass / pass
sequence and output another LLVM-IR file- use clang -O3 to compile to executable
machine code
However, with this approach you will run the passes on the LLVM-IR twice.There
are use cases when this could invalidate your results.As opt stops at LLVM-IR
level, I would suggest you to use also other LLVM tools to run individually the
backend stages / sequence of passes which cannot be run by opt (such as llc /
llvm-mc).An extensive list of tools/commands you can use is available at [0].For
your specific case, I would suggest you to have a look at this restricted schema
[1].
Yet there is another way to get into even fine grain detail.You can check which
are the clang DriverActions you are running with a given command line. See
[2].From that point you can rebuild the exact whole sequence of commands that
the clang driver triggers.
If you can provide more details about what is your use case (measure
performance, pass development and testing, flag selection, phase ordering), we
can suggest the most suitable approach.
Kind regards,
Stefano Cherubin
[0] http://llvm.org/docs/CommandGuide/[1]
https://github.com/skeru/LLVM-intro/blob/master/img/03/toolchain.pdf[2]
https://clang.llvm.org/docs/DriverInternals.html#driver-stages




    On Thursday, 16 August 2018, 12:46:04 CEST, Emanuele Del Sozzo via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
 
  


Hello llvm-dev,

my name is Emanuele and I am an intern in ARM. As part of the project I am doing
here, I would like to manually replicate the optimizations that LLVM applies
when I type -O3. In other words, I would like to know what are the compilation
flags/passes that -O3 triggers. 

I noticed that GCC reports, on its website, all the flags that are enforced by
-O3 (https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html), but I wasn't
able to find something similar within LLVM documentation. On the other hand, I
found that this command displays all the optimization passes applied by opt when
-O3 flag is on:

llvm-as < /dev/null | opt -O3 -disable-output -debug-pass=Arguments

I tried to apply the same optimization passes through opt, but, even though the
performance are similar, the resulting binary is slower than the one generated
using -O3 (also the binaries differ, of course).

Again, I found this other command that does something similar (it lists the
sequence of optimization passes applied):

clang -O3 -mllvm -debug-pass=Arguments file.c 

In this case, the performance are still different and some of the optimization
passes listed in the last block of passes (e.g. -machinemoduleinfo,
-stack-protector, etc.) are unknown to opt.




Said that, my question is: how can I find out what optimization passes/flags -O3
enforces in order to manually apply the same optimizations and have, hopefully,
the same binary and performance?




I am currently using LLVM version 5.0.2.




Thank you for both your help and your time!




Best regards

Emanuele



IMPORTANT NOTICE: The contents of this email and any attachments are
confidential and may also be privileged. If you are not the intended recipient,
please notify the sender immediately and do not disclose the contents to any
other person, use it for any purpose, or store or copy the information in any
medium. Thank you._______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
  
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20180816/f153881a/attachment.html>

Hi, Stefano
I also have the problem as described by Emanuele. You say that clang schedules
target-independent and target-dependent passes.
However, when I use lli to execute bitcode generated by opt with -O3 or with the
same optimization passes as -O3, the performance are still different.
So, are there some special operations by -O3 option? I read the source code of
opt, but I cannot find the reason.

Best regards
Zide

At 2018-08-16 22:13:14, "Stefano Cherubin via llvm-dev" <llvm-dev
at lists.llvm.org> wrote:

Hello Emanuele,


When you provide the optimization level -O3 to the clang driver, it does not
simply schedule a sequence of passes to be run on the intermediate
representation.
Indeed, it schedules target-independent and target-dependent passes.
Moreover, IIRC, the optimization level is also used in the later stages of the
code generation to apply target-dependent optimizations (i.e. vectorizer).


The most common use case when someone wants to test its own pass/work within the
LLVM toolchain is the following

- use clang to generate a LLVM-IR file
- use opt to run your desired pass / pass sequence and output another LLVM-IR
file
- use clang -O3 to compile to executable machine code


However, with this approach you will run the passes on the LLVM-IR twice.
There are use cases when this could invalidate your results.
As opt stops at LLVM-IR level, I would suggest you to use also other LLVM tools
to run individually the backend stages / sequence of passes which cannot be run
by opt (such as llc / llvm-mc).
An extensive list of tools/commands you can use is available at [0].
For your specific case, I would suggest you to have a look at this restricted
schema [1].


Yet there is another way to get into even fine grain detail.
You can check which are the clang DriverActions you are running with a given
command line. See [2].
From that point you can rebuild the exact whole sequence of commands that the
clang driver triggers.


If you can provide more details about what is your use case (measure
performance, pass development and testing, flag selection, phase ordering), we
can suggest the most suitable approach.


Kind regards,


Stefano Cherubin


[0] http://llvm.org/docs/CommandGuide/
[1] https://github.com/skeru/LLVM-intro/blob/master/img/03/toolchain.pdf
[2] https://clang.llvm.org/docs/DriverInternals.html#driver-stages









On Thursday, 16 August 2018, 12:46:04 CEST, Emanuele Del Sozzo via llvm-dev
<llvm-dev at lists.llvm.org> wrote:





Hello llvm-dev,

my name is Emanuele and I am an intern in ARM. As part of the project I am doing
here, I would like to manually replicate the optimizations that LLVM applies
when I type -O3. In other words, I would like to know what are the compilation
flags/passes that -O3 triggers.

I noticed that GCC reports, on its website, all the flags that are enforced by
-O3 (https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html), but I wasn't
able to find something similar within LLVM documentation. On the other hand, I
found that this command displays all the optimization passes applied by opt when
-O3 flag is on:

llvm-as < /dev/null | opt -O3 -disable-output -debug-pass=Arguments

I tried to apply the same optimization passes through opt, but, even though the
performance are similar, the resulting binary is slower than the one generated
using -O3 (also the binaries differ, of course).

Again, I found this other command that does something similar (it lists the
sequence of optimization passes applied):

clang -O3 -mllvm -debug-pass=Arguments file.c 

In this case, the performance are still different and some of the optimization
passes listed in the last block of passes (e.g. -machinemoduleinfo,
-stack-protector, etc.) are unknown to opt.




Said that, my question is: how can I find out what optimization passes/flags -O3
enforces in order to manually apply the same optimizations and have, hopefully,
the same binary and performance?




I am currently using LLVM version 5.0.2.




Thank you for both your help and your time!




Best regards

Emanuele




IMPORTANT NOTICE: The contents of this email and any attachments are
confidential and may also be privileged. If you are not the intended recipient,
please notify the sender immediately and do not disclose the contents to any
other person, use it for any purpose, or store or copy the information in any
medium. Thank you.
_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20180817/c087c442/attachment.html>