linux-kernel - [PATCH v2 7/9] LoongArch: Tweak CFLAGS for Clang compatibility

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Date:   Sun, 25 Jun 2023 02:40:53 +0800
From:   WANG Xuerui <kernel@...0n.name>
To:     Huacai Chen <chenhuacai@...nel.org>
Cc:     WANG Rui <wangrui@...ngson.cn>, Xi Ruoyao <xry111@...111.site>,
        loongarch@...ts.linux.dev, linux-kbuild@...r.kernel.org,
        llvm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        WANG Xuerui <git@...0n.name>
Subject: [PATCH v2 7/9] LoongArch: Tweak CFLAGS for Clang compatibility

From: WANG Xuerui <git@...0n.name>

Now the arch code is mostly ready for LLVM/Clang consumption, it is time
to re-organize the CFLAGS a little to actually enable the LLVM build.

In particular, -mexplicit-relocs and -mdirect-extern-access are not
necessary nor supported on Clang; feature detection via cc-option would
not work, because that way the broken combo of "new GNU as + old GCC"
would seem to get "fixed", but actually produce broken kernels.
Explicitly depending on CONFIG_CC_IS_CLANG is thus necessary to not
regress UX for those building their own kernels.

A build with !RELOCATABLE && !MODULE is confirmed working within a QEMU
environment; support for the two features are currently blocked on
LLVM/Clang, and will come later.

Signed-off-by: WANG Xuerui <git@...0n.name>
---
 arch/loongarch/Makefile | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
index 366771016b99..82c619791a63 100644
--- a/arch/loongarch/Makefile
+++ b/arch/loongarch/Makefile
@@ -51,7 +51,9 @@ LDFLAGS_vmlinux			+= -static -n -nostdlib
 
 # When the assembler supports explicit relocation hint, we must use it.
 # GCC may have -mexplicit-relocs off by default if it was built with an old
-# assembler, so we force it via an option.
+# assembler, so we force it via an option. For LLVM/Clang the desired behavior
+# is the default, and the flag is not supported, so don't pass it if Clang is
+# being used.
 #
 # When the assembler does not supports explicit relocation hint, we can't use
 # it.  Disable it if the compiler supports it.
@@ -61,8 +63,10 @@ LDFLAGS_vmlinux			+= -static -n -nostdlib
 # combination of a "new" assembler and "old" compiler is not supported.  Either
 # upgrade the compiler or downgrade the assembler.
 ifdef CONFIG_AS_HAS_EXPLICIT_RELOCS
+ifndef CONFIG_CC_IS_CLANG
 cflags-y			+= -mexplicit-relocs
 KBUILD_CFLAGS_KERNEL		+= -mdirect-extern-access
+endif
 else
 cflags-y			+= $(call cc-option,-mno-explicit-relocs)
 KBUILD_AFLAGS_KERNEL		+= -Wa,-mla-global-with-pcrel
-- 
2.40.0