linux-kernel - [PATCH v3 2/7] Revert "kbuild: disable clang's default use of -fmerge-all-constants"

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Message-Id: <20200902225911.209899-3-ndesaulniers@google.com>
Date:   Wed,  2 Sep 2020 15:59:06 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Nathan Chancellor <natechancellor@...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,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Fangrui Song <maskray@...gle.com>
Subject: [PATCH v3 2/7] Revert "kbuild: disable clang's default use of -fmerge-all-constants"

This reverts commit 87e0d4f0f37fb0c8c4aeeac46fff5e957738df79.

-fno-merge-all-constants has been the default since clang-6; the minimum
supported version of clang in the kernel is clang-10 (10.0.1).

Suggested-by: Nathan Chancellor <natechancellor@...il.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
Tested-by: Sedat Dilek <sedat.dilek@...il.com>
Reviewed-by: Fangrui Song <maskray@...gle.com>
Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
Reviewed-by: Sedat Dilek <sedat.dilek@...il.com>
Reviewed-by: Kees Cook <keescook@...omium.org>
Link: https://reviews.llvm.org/rL329300.
Link: https://github.com/ClangBuiltLinux/linux/issues/9
---
 Makefile | 9 ---------
 1 file changed, 9 deletions(-)

diff --git a/Makefile b/Makefile
index 1cfa5029fd2f..e475650c9599 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