linux-kernel - [PATCH 34/35] tools build: Use the same CC for feature detection and actual build

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Date:   Mon,  6 Mar 2017 16:38:24 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        David Ahern <dsahern@...il.com>, Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Wang Nan <wangnan0@...wei.com>
Subject: [PATCH 34/35] tools build: Use the same CC for feature detection and actual build

From: Arnaldo Carvalho de Melo <acme@...hat.com>

	When build with: 'make CC=clang' we were not using that CC to do
feature detection, which resulted in features being detected with gcc
and then the actual tools being built with clang.

	Most of the time these compilers are compatible enough, so no
problem was being noticed.

	As soon as a system with an old enough clang, one that hasn't
the cpuid.h header is used, and a gcc with it, the "get_cpuid" feature
will be found available but then code that will use can't be compiled.

	Noticed with this combination:

  / $ gcc --version | head -1
  gcc (Alpine 6.3.0) 6.3.0
  / $ clang --version | head -1
  clang version 3.8.1 (tags/RELEASE_381/final)
  / $ cat /etc/alpine-release
  3.5.0
  / $

Cc: Adrian Hunter <adrian.hunter@...el.com>
Cc: David Ahern <dsahern@...il.com>
Cc: Jiri Olsa <jolsa@...nel.org>
Cc: Namhyung Kim <namhyung@...nel.org>
Cc: Wang Nan <wangnan0@...wei.com>
Link: http://lkml.kernel.org/n/tip-60q18nvlvgpyfv7e2qqgx4ou@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@...hat.com>
---
 tools/build/feature/Makefile | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/build/feature/Makefile b/tools/build/feature/Makefile
index ab1e2bbc2e96..09c9626ea666 100644
--- a/tools/build/feature/Makefile
+++ b/tools/build/feature/Makefile
@@ -53,8 +53,8 @@ FILES=                                          \
 
 FILES := $(addprefix $(OUTPUT),$(FILES))
 
-CC := $(CROSS_COMPILE)gcc -MD
-CXX := $(CROSS_COMPILE)g++ -MD
+CC ?= $(CROSS_COMPILE)gcc -MD
+CXX ?= $(CROSS_COMPILE)g++ -MD
 PKG_CONFIG := $(CROSS_COMPILE)pkg-config
 LLVM_CONFIG ?= llvm-config
 
-- 
2.9.3