[llvm-dev] LLVM log file

[Sudakshina Dutta via llvm-dev]

Dear Stefanos,

Thanks for all your reply. I have one simple question to ask. I am new to
llvm. I know that the loop to be optimized has to be enclosed between
#pragma scop and #pragma endoscop. I have seen the llvm log. It says about
the attempted optimizations. My question is does the log specify the loop
which is optimized ? More precisely, if there are multiple loops, does the
log specify which loop has been optimized ? I request you to kindly answer
the above question or give me pointers to the answers.

Thanks.
Sudakshina

On Tue, Jan 26, 2021 at 8:10 PM Stefanos Baziotis via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi David,
>
> Sure I agree in part but "very misleading" is a strong statement and I
> think we might want to see a different perspective. People present this
> mental model in conferences [1]
> "If you want to optimize, you use opt, which takes LLVM IR and generates
> LLVM IR". We could say that this is misleading too, as it seems that opt
> does the optimization, but is it really?
> This an "Introduction to LLVM", going into specifics at this point would
> probably confuse people a lot more than it would help them.
>
> Same here [2]. Chandler was using clang -O2, then he used opt -O2
> mentioning it as the "default" pipeline. Wow, that should be super
> misleading, but again is it really? Would it help
> if Chandler stopped and said "Oh by the way... Let me digress here and
> explain a thing about libraries and opt etc."
>
> With the same logic, when I said "target-independent" optimizations, that
> was misleading too.
>
> But my message was like, 1 page long already and I think spending another
> 1 page (or more) to explain such things would not help. And I think this is
> similar for people in conferences and llvm posts.
>
> Anyway, it's obvious that your message came with good intentions and I
> appreciate that. But I think it should be mentioned that for any beginner
> trying to understand the beast called LLVM,
> it's not super important to know it now, at least IMHO. But it's good to
> be mentioned that it's not the grand truth so that they come back to it
> when they're more comfortable with the "approximation".
>
> Best,
> Stefanos
>
> [1] https://youtu.be/J5xExRGaIIY?t=429
> [2] https://youtu.be/s4wnuiCwTGU?t=433
>
> Στις Τρί, 26 Ιαν 2021 στις 11:47 π.μ., ο/η David Chisnall via llvm-dev <
> llvm-dev at lists.llvm.org> έγραψε:
>
>> On 26/01/2021 02:53, Stefanos Baziotis via llvm-dev wrote:
>> > 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 think this is sufficiently close to being true that it ends up being
>> very misleading.  I've seen a lot of posts on the mailing lists from
>> people who have a mental model of LLVM like this.
>>
>> The opt tool is a thin wrapper around the LLVM pass pipeline
>> infrastructure.  Most of the command-line flags for opt are not specific
>> to opt, they are exposed by LLVM libraries.  Opt passes all of its
>> arguments to LLVM, clang passes only the ones prefixed with -mllvm, but
>> they are both handled by the same logic.
>>
>> Opt has some default pipelines with names such as -O1 and -O3 but these
>> are *not* the same as the pipelines of the same names in clang (or other
>> compilers that use LLVM).  This is a common source of confusion from
>> people wondering why clang and opt give different output at -O2 (for
>> example).
>>
>> The opt tool is primarily intended for unit testing.  It is a convenient
>> way of running a single pass or sequence of passes (which is also useful
>> for producing reduced test cases when a long pass pipeline generates a
>> miscompile).  Almost none of the logic, including most of the
>> command-line handling, is actually present in opt.
>>
>> David
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
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