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Date:   Mon, 14 Jun 2021 17:46:39 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Kees Cook <keescook@chromium.org>
Cc:     Marco Elver <elver@google.com>, Bill Wendling <morbo@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Masahiro Yamada <masahiroy@kernel.org>,
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        Andrew Morton <akpm@linux-foundation.org>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        Fangrui Song <maskray@google.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        Andrey Konovalov <andreyknvl@gmail.com>,
        Dmitry Vyukov <dvyukov@google.com>, johannes.berg@intel.com,
        oberpar@linux.vnet.ibm.com, linux-toolchains@vger.kernel.org
Subject: Re: [PATCH v9] pgo: add clang's Profile Guided Optimization
 infrastructure
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On Mon, Jun 14, 2021 at 08:26:01AM -0700, Kees Cook wrote:
> > 2. Like (1) but also keep GCOV, given proper support for attribute
> > no_instrument_function would probably fix it (?).
> > 
> > 3. Keep GCOV (and KCOV of course). Somehow extract PGO profiles from KCOV.
> > 
> > 4. Somehow extract PGO profiles from GCOV, or modify kernel/gcov to do so.
> 
> If there *is* a way to "combine" these, I don't think it makes sense
> to do it now. PGO has users (and is expanding[1]), and trying to
> optimize the design before even landing the first version seems like a
> needless obstruction, and to likely not address currently undiscovered
> requirements.

Even if that were so (and I'm not yet convinced), the current proposal
is wedded to llvm-pgo, there is no way gcc-pgo could reuse any of this
code afaict, which then means they have to create yet another variant.

Sorting this *before* the first version is exactly the right time.

Since when are we merging code when the requirements are not clear?

Just to clarify:

Nacked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

For all this PGO crud.