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From:   Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Date:   Fri, 16 Apr 2021 14:23:26 +0200
Message-ID: <CANiq72=TDkRzZi7fN5y9QmtRbvoZTzxHXDg_ELjdD+YhmXzgPw@mail.gmail.com>
Subject: Re: [PATCH 04/13] Kbuild: Rust support
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Masahiro Yamada <masahiroy@kernel.org>,
        Miguel Ojeda <ojeda@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        rust-for-linux@vger.kernel.org,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Geoffrey Thomas <geofft@ldpreload.com>,
        Finn Behrens <me@kloenk.de>,
        Adam Bratschi-Kaye <ark.email@gmail.com>,
        Wedson Almeida Filho <wedsonaf@google.com>,
        Michael Ellerman <mpe@ellerman.id.au>
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On Thu, Apr 15, 2021 at 8:03 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> Until then, I don't see why we need to permit developers to express
> such flexibility for just the Rust code, or have it differ from the
> intent of the C code. Does it make sense to set RUST_OPT_LEVEL_3 and
> CC_OPTIMIZE_FOR_SIZE? I doubt it. That doesn't seem like a development
> feature, but a mistake.  YAGNI.  Instead developers should clarify
> what they care about in terms of high level intent; if someone wants
> to micromanage optimization level flags in their forks they don't need
> a Kconfig to do it (they're either going to hack KBUILD_CFLAGS,
> CFLAGS_*.o, or KCFLAGS), and there's probably better mechanisms for
> fine-tooth precision of optimizing actually hot code or not via PGO
> and AutoFDO.

I completely agree when we are talking about higher level optimization
levels. From a user perspective, it does not make much sense to want
slightly different optimizations levels or different size/performance
trade-offs between C and Rust. However, I am thinking from the
debugging side, i.e. mostly low or no optimization; rather than about
micromanaging optimizations for performance.

For instance, last year I used `RUST_OPT_LEVEL_0/1` to quickly rule
out optimizer/codegen/etc. bugs on the Rust side when we had some
memory corruption over Rust data
(https://github.com/Rust-for-Linux/linux/pull/28), which is important
when dealing with compiler nightly versions. It was also nice to be
able to easily follow along when stepping, too.

Having said that, I agree that in those cases one can simply tweak the
flags manually -- so that's why I said it is fine dropping the the
`Kconfig` options. There might be some advantages of having them, such
as making developers aware that those builds should work, to keep them
tested/working, etc.; but we can do that manually too in the CI/docs
too.

Cheers,
Miguel