Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 20 Jul 2020 11:12:22 -0700
Message-Id: <20200720181237.3015826-1-maskray@google.com>
Mime-Version: 1.0
Subject: [PATCH] Makefile: Fix GCC_TOOLCHAIN_DIR prefix for Clang cross compilation
From:   Fangrui Song <maskray@google.com>
To:     Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
        clang-built-linux@googlegroups.com
Cc:     Fangrui Song <maskray@google.com>,
        Nathan Chancellor <natechancellor@gmail.com>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

When CROSS_COMPILE is set (e.g. aarch64-linux-gnu-), if
$(CROSS_COMPILE)elfedit is found at /usr/bin/aarch64-linux-gnu-,
GCC_TOOLCHAIN_DIR will be set to /usr/bin/.  --prefix= will be set to
/usr/bin/ and Clang as of 11 will search for both
$(prefix)aarch64-linux-gnu-$needle and $(prefix)$needle.

GCC searchs for $(prefix)aarch64-linux-gnu/$version/$needle,
$(prefix)aarch64-linux-gnu/$needle and $(prefix)$needle. In practice,
$(prefix)aarch64-linux-gnu/$needle rarely contains executables.

To better model how GCC's -B/--prefix takes in effect in practice, newer
Clang only searches for $(prefix)$needle and for example it will find
/usr/bin/as instead of /usr/bin/aarch64-linux-gnu-as.

Set --prefix= to $(GCC_TOOLCHAIN_DIR)$(CROSS_COMPILE)
(/usr/bin/aarch64-linux-gnu-) so that newer Clang can find the
appropriate cross compiling GNU as (when -no-integrated-as is in
effect).

Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Fangrui Song <maskray@google.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/1099
---
 Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 0b5f8538bde5..3ac83e375b61 100644
--- a/Makefile
+++ b/Makefile
@@ -567,7 +567,7 @@ ifneq ($(shell $(CC) --version 2>&1 | head -n 1 | grep clang),)
 ifneq ($(CROSS_COMPILE),)
 CLANG_FLAGS	+= --target=$(notdir $(CROSS_COMPILE:%-=%))
 GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))
-CLANG_FLAGS	+= --prefix=$(GCC_TOOLCHAIN_DIR)
+CLANG_FLAGS	+= --prefix=$(GCC_TOOLCHAIN_DIR)$(CROSS_COMPILE)
 GCC_TOOLCHAIN	:= $(realpath $(GCC_TOOLCHAIN_DIR)/..)
 endif
 ifneq ($(GCC_TOOLCHAIN),)
-- 
2.28.0.rc0.105.gf9edc3c819-goog