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From:   Nathan Chancellor <nathan@kernel.org>
To:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, Masahiro Yamada <masahiroy@kernel.org>
Cc:     "H. Peter Anvin" <hpa@zytor.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        linux-kernel@vger.kernel.org, linux-kbuild@vger.kernel.org,
        clang-built-linux@googlegroups.com, llvm@lists.linux.dev,
        Nathan Chancellor <nathan@kernel.org>
Subject: [PATCH 2/2] kbuild: Add -Werror=ignored-optimization-argument to CLANG_FLAGS
Date:   Mon, 23 Aug 2021 19:26:40 -0700
Message-Id: <20210824022640.2170859-3-nathan@kernel.org>
In-Reply-To: <20210824022640.2170859-1-nathan@kernel.org>
References: <20210824022640.2170859-1-nathan@kernel.org>
MIME-Version: 1.0
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Precedence: bulk

Similar to commit 589834b3a009 ("kbuild: Add
-Werror=unknown-warning-option to CLANG_FLAGS").

Clang ignores certain GCC flags that it has not implemented, only
emitting a warning:

$ echo | clang -fsyntax-only -falign-jumps -x c -
clang-14: warning: optimization flag '-falign-jumps' is not supported
[-Wignored-optimization-argument]

When one of these flags gets added to KBUILD_CFLAGS unconditionally, all
subsequent cc-{disable-warning,option} calls fail because -Werror was
added to these invocations to turn the above warning and the equivalent
-W flag warning into errors.

To catch the presence of these flags earlier, turn
-Wignored-optimization-argument into an error so that the flags can
either be implemented or ignored via cc-option and there are no more
weird errors.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 scripts/Makefile.clang | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
index 4cce8fd0779c..2fe38a9fdc11 100644
--- a/scripts/Makefile.clang
+++ b/scripts/Makefile.clang
@@ -29,7 +29,11 @@ CLANG_FLAGS	+= --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
 else
 CLANG_FLAGS	+= -fintegrated-as
 endif
+# By default, clang only warns on unknown warning or optimization flags
+# Make it behave more like gcc by erroring when these flags are encountered
+# so they can be implemented or wrapped in cc-option.
 CLANG_FLAGS	+= -Werror=unknown-warning-option
+CLANG_FLAGS	+= -Werror=ignored-optimization-argument
 KBUILD_CFLAGS	+= $(CLANG_FLAGS)
 KBUILD_AFLAGS	+= $(CLANG_FLAGS)
 export CLANG_FLAGS
-- 
2.33.0