linux-kernel - Re: [PATCH v2 2/7] Revert "kbuild: disable clang's default use of -fmerge-all-constants"

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Message-ID: <20200901165558.sgzbui6khedvwy3o@google.com>
Date:   Tue, 1 Sep 2020 09:55:58 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Marco Elver <elver@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        clang-built-linux@...glegroups.com,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Will Deacon <will@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/7] Revert "kbuild: disable clang's default use of
 -fmerge-all-constants"



On 2020-08-31, Nathan Chancellor wrote:
>On Mon, Aug 31, 2020 at 05:23:21PM -0700, Nick Desaulniers wrote:
>> This reverts commit 87e0d4f0f37fb0c8c4aeeac46fff5e957738df79.
>>
>> This was fixed in clang-6; the minimum supported version of clang in the
>> kernel is clang-10 (10.0.1).
>>
>> Link: https://reviews.llvm.org/rL329300.
>> Link: https://github.com/ClangBuiltLinux/linux/issues/9
>> Suggested-by: Nathan Chancellor <natechancellor@...il.com>
>> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
>
>Reviewed-by: Nathan Chancellor <natechancellor@...il.com>

How about expanding "This was fixed in clang-6" to be
-fno-merge-all-constants has been the default since clang-6?

(Both gcc|clang -fmerge-all-constants can cause an assertion failure for
the example on https://bugs.llvm.org/show_bug.cgi?id=18538 )

Reviewed-by: Fangrui Song <maskray@...gle.com>

>> ---
>>  Makefile | 9 ---------
>>  1 file changed, 9 deletions(-)
>>
>> diff --git a/Makefile b/Makefile
>> index 37739ee53f27..144ac6a073ff 100644
>> --- a/Makefile
>> +++ b/Makefile
>> @@ -932,15 +932,6 @@ KBUILD_CFLAGS += $(call cc-disable-warning, maybe-uninitialized)
>>  # disable invalid "can't wrap" optimizations for signed / pointers
>>  KBUILD_CFLAGS	+= $(call cc-option,-fno-strict-overflow)
>>
>> -# clang sets -fmerge-all-constants by default as optimization, but this
>> -# is non-conforming behavior for C and in fact breaks the kernel, so we
>> -# need to disable it here generally.
>> -KBUILD_CFLAGS	+= $(call cc-option,-fno-merge-all-constants)
>> -
>> -# for gcc -fno-merge-all-constants disables everything, but it is fine
>> -# to have actual conforming behavior enabled.
>> -KBUILD_CFLAGS	+= $(call cc-option,-fmerge-constants)
>> -
>>  # Make sure -fstack-check isn't enabled (like gentoo apparently did)
>>  KBUILD_CFLAGS  += $(call cc-option,-fno-stack-check,)
>>
>> --
>> 2.28.0.402.g5ffc5be6b7-goog
>>
>
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