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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Fri, 29 Jan 2021 13:48:10 -0800
Message-ID: <CAKwvOdmyerPPJo=m+13jtWR_kPwrmOeb89jh=qgGzEaPhKDOLA@mail.gmail.com>
Subject: Re: [PATCH v7] pgo: add clang's Profile Guided Optimization infrastructure
To:     Sedat Dilek <sedat.dilek@gmail.com>
Cc:     Bill Wendling <morbo@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Sami Tolvanen <samitolvanen@google.com>
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On Thu, Jan 28, 2021 at 11:43 PM Sedat Dilek <sedat.dilek@gmail.com> wrote:
>
> Some comments:
>
> [ hash mismatches ]
>
> Observed identical warnings when doing a rebuild with GAS or Clang-IAS.
>
> [ Importance of LLVM_IAS=1 working ]
>
> Clang-LTO and Clang-CFI depend both on LLVM_IAS=1 (see for example
> "kbuild: add support for Clang LTO").
> Sooner or later we will deal with this issue (hope it is not a local problem).

If you're switching back and forth between GAS and IAS, then I would
expect a hash error if you're trying to reuse profiling data from one
with the other.  The profiling data is not portable when switching
toolchains between when the profile was collected, and when it was
used.
-- 
Thanks,
~Nick Desaulniers