[llvm-dev] LLVM log file

Mon Jan 25 18:53:34 PST 2021

Hi  Sudakshina,

Glad it helped :)

> I did not find any 'print-changed' option for llvm.
Hmm.. I can't reproduce it myself right now either.. Anyway, let's go with
what works for sure. That's `-print-after-all`. This prints the IR after
every (middle-end) pass, no matter whether the pass made any changes or not.

Alright, now to use that: This is _not_ an option of Clang (or the Clang
driver; i.e., the command: clang test.c -print-after-all won't work), but
an option of opt. opt, in case you're not familiar with it, is basically
the middle-end optimizer of LLVM
i.e. it's supposed to be doing target-independent optimizations, which from
what I understand is that you're interested in. If you want to also print
back-end passes (i.e., register allocation etc.), that's another story.

Anyway, when you type e.g., clang test.c -o test, 3 high-level steps happen:
1) Clang parses, type-checks etc. the C source and generates (very trivial)
LLVM IR
2) This is then passed to opt, which issues a bunch of passes (again,
depending on whether you used -O1, -O2 etc.). Each pass takes IR and
outputs IR
3) When it's done, it passes it to the back-end, which is another story and
uses another IR.

Now, what you want is to get the IR after the first step, so that you can
pass it _yourself_ to opt, with any options _you_ want (one of them being
`print-after-all`). To do that, you type e.g.,: clang test.c -o test.ll -S
-emit-llvm
-emit-llvm tells Clang to stop at step 1) and output the IR in a text file
(note that we used no -O1, -O2 options because we want the fully
unoptimized, trivial IR at this step)
-S tells clang to print in textual format (let's not go now into what is
the other format)

If your file can't be linked to an executable (e.g., it doesn't have a
main()), you should add a -c there.

Alright, now we have our IR but there's a problem. Our functions have the
`optnone` attribute, which tells the optimizer to not touch them (that's
because we used no optimization options).
We don't want that, so we add another option to clang, -Xclang
-disable-O0-optnone

So, all in all, it looks something like this: clang test.c -o test.ll -c
-emit-llvm -S -Xclang -disable-O0-optnone

Now, we have the LLVM IR in test.ll and we can pass it to opt. You can now
say: opt test.ll -O3 -print-after-all

Let me show you the steps in Godbolt:
- Generate (unoptimized) IR from Clang: https://godbolt.org/z/rY3Thx (note
that I also added -g0 to avoid printing debug info, which are probably not
helpful)
- Copy this exact IR and pass it to opt: https://godbolt.org/z/7cjdcf (you
can see on the right window that SROA changed the code a lot)

As you can understand, you can automate all that with a script.

Final comment: After step 1), it's useful to pass your IR through opt with
the option: -metarenamer or -instnamer. This message has already become way
too big so let me not explain now why it's useful, but trust me, it is.

Best,
Stefanos

Στις Τρί, 26 Ιαν 2021 στις 4:18 π.μ., ο/η Sudakshina Dutta <
sudakshina at iitgoa.ac.in> έγραψε:

> Dear Stefanos,
>
> Thank you for your reply. It helped me to understand the optimization
> phase of LLVM. However, I did not find any 'print-changed' option for llvm.
> Can you kindly help me in this regard ? I want to generate the IRs after
> each optimization pass.
>
> Regards,
> Sudakshina
>
>
> On Sun, Jan 24, 2021 at 7:13 PM Stefanos Baziotis <
> stefanos.baziotis at gmail.com> wrote:
>
>> Hi  Sudakshina,
>>
>> > The optimization applied in the optimization pass depends on the source
>> program; hence, the number of optimizations applied differs from source
>> program to source program.
>>
>> "applied" is still ambiguous, at least to me. If by "applied" you mean
>> "attempted", then no, that does not depend on the source program. It
>> depends on the optimization level (e.g., O1, O2, ...) or the individual
>> passes that you may request yourself.
>> That is, for -O1 for example, there is a predetermined sequence of passes
>> that _attempt_ to optimize the program and you can see that with the
>> options I mentioned above (e.g., `-mllvm -opt-bisect-limit=-1`)
>>
>> If by applied you mean "actually changed the code", then yes, this
>> differs from program to program. You can see that with `print-changed`,
>> it'll show you the IR after every transformation that changed your program.
>>
>> Finally, if you want to see why a transformation could or not change the
>> code, you can use the related comments about remarks.
>>
>> Best,
>> Stefanos
>>
>> Στις Κυρ, 24 Ιαν 2021 στις 7:24 π.μ., ο/η Sudakshina Dutta <
>> sudakshina at iitgoa.ac.in> έγραψε:
>>
>>> Dear all,
>>>
>>> In the optimization phase, the compiler applies some optimization to
>>> generate an optimized program. The optimization applied in the optimization
>>> pass depends on the source program; hence, the number of optimizations
>>> applied differs from source program to source program. By mentioning
>>> "applied" transformation, I wanted to know what all transformations are
>>> applied for a specific input program when subjected to the LLVM optimizer.
>>>
>>> Thanks,
>>> Sudakshina
>>>
>>>
>>> On Sun, 24 Jan 2021, 09:27 Stefanos Baziotis, <
>>> stefanos.baziotis at gmail.com> wrote:
>>>
>>>> Hi Sudakshina,
>>>>
>>>> Not really sure what you mean by "applied", so, let me offer some more
>>>> ideas other than Brian's and Adrian's great suggestions. First, there are
>>>> some
>>>> diagnostics / remarks flags in Clang like the -R family [1] or some -f
>>>> flags about printing optimization reports [2] from Clang. They can be
>>>> useful or useless depending
>>>> on your case. They can also be parsed relatively easily.
>>>>
>>>> If you just want to see a list of passes that were attempted in your
>>>> code, you can do it with: `-mllvm -opt-bisect-limit=-1`
>>>> You can also use `-mllvm-debug-pass=Arguments` to see the arguments
>>>> that were passed.
>>>>
>>>> Moving into opt, you can use something like `print-after-all`, which
>>>> was already mentioned. If you don't know what these flags do, is they show
>>>> you
>>>> the IR in different stages in the pipeline (e.g., `print-after-all`
>>>> shows you each pass attempted and how the IR is after it).
>>>>
>>>> Hope it helps,
>>>> Stefanos
>>>>
>>>> [1]
>>>> https://clang.llvm.org/docs/ClangCommandLineReference.html#diagnostic-flags
>>>> [2]
>>>> https://clang.llvm.org/docs/UsersManual.html#cmdoption-f-no-save-optimization-record
>>>>
>>>> Στις Κυρ, 24 Ιαν 2021 στις 5:47 π.μ., ο/η Adrian Vogelsgesang via
>>>> llvm-dev <llvm-dev at lists.llvm.org> έγραψε:
>>>>
>>>>> I used “-print-changed”, “-print-before-all”, “print-after-all” last
>>>>> time I wanted to see the passes together with their inout/output IR modules.
>>>>>
>>>>> In my case, I used them through “clang++”, i.e. I had to prefix them
>>>>> with “-mllvm”
>>>>> > clang++ test_file.cpp -mllvm -print-after-all
>>>>>
>>>>>
>>>>>
>>>>> *From: *llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Brian
>>>>> Cain via llvm-dev <llvm-dev at lists.llvm.org>
>>>>> *Date: *Sunday, 24. January 2021 at 04:40
>>>>> *To: *Sudakshina Dutta <sudakshina at iitgoa.ac.in>
>>>>> *Cc: *LLVM Development List <llvm-dev at lists.llvm.org>
>>>>> *Subject: *Re: [llvm-dev] LLVM log file
>>>>>
>>>>> I don't know if it's exhaustive but there's the "remarks" feature:
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> https://llvm.org/docs/Remarks.html#introduction-to-the-llvm-remark-diagnostics
>>>>>
>>>>>
>>>>>
>>>>> On Sat, Jan 23, 2021 at 9:20 PM Sudakshina Dutta via llvm-dev <
>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>
>>>>> Dear all,
>>>>>
>>>>>
>>>>>
>>>>> Good morning. I want to know whether LLVM creates any log file
>>>>> consisting of applied optimizations in the optimization phase. It will be
>>>>> really useful for the researchers who work on compilers, formal methods,
>>>>> etc.
>>>>>
>>>>>
>>>>>
>>>>> Thanks,
>>>>>
>>>>> Sudakshina
>>>>>
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>>
>>>>> -Brian
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>>
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