From 61163efae02040f66a95c8ed17f4407951ba58fa Mon Sep 17 00:00:00 2001
From: Behan Webster <behanw@converseincode.com>
Date: Tue, 3 Sep 2013 22:27:23 -0400
Subject: [PATCH] kbuild: LLVMLinux: Add Kbuild support for building kernel
 with Clang
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Add support to toplevel Makefile for compiling with clang, both for
HOSTCC and CC. Use cc-option to prevent gcc option from breaking clang, and
from clang options from breaking gcc.

Clang 3.4 semantics are the same as gcc semantics for unsupported flags. For
unsupported warnings clang 3.4 returns true but shows a warning and gcc shows
a warning and returns false.

Signed-off-by: Behan Webster <behanw@converseincode.com>
Signed-off-by: Jan-Simon Möller <dl9pf@gmx.de>
Signed-off-by: Mark Charlebois <charlebm@gmail.com>
Cc: PaX Team <pageexec@freemail.hu>
---
 Makefile | 30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index cf3e07516a043..10812e495e8df 100644
--- a/Makefile
+++ b/Makefile
@@ -248,6 +248,11 @@ HOSTCXX      = g++
 HOSTCFLAGS   = -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 -fomit-frame-pointer
 HOSTCXXFLAGS = -O2
 
+ifeq ($(shell $(HOSTCC) -v 2>&1 | grep -c "clang version"), 1)
+HOSTCFLAGS  += -Wno-unused-value -Wno-unused-parameter \
+		-Wno-missing-field-initializers -fno-delete-null-pointer-checks
+endif
+
 # Decide whether to build built-in, modular, or both.
 # Normally, just do built-in.
 
@@ -324,6 +329,14 @@ endif
 
 export quiet Q KBUILD_VERBOSE
 
+ifneq ($(CC),)
+ifeq ($(shell $(CC) -v 2>&1 | grep -c "clang version"), 1)
+COMPILER := clang
+else
+COMPILER := gcc
+endif
+export COMPILER
+endif
 
 # Look for make include files relative to root of kernel src
 MAKEFLAGS += --include-dir=$(srctree)
@@ -383,7 +396,7 @@ KBUILD_CFLAGS   := -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \
 		   -fno-strict-aliasing -fno-common \
 		   -Werror-implicit-function-declaration \
 		   -Wno-format-security \
-		   -fno-delete-null-pointer-checks
+		   $(call cc-option,-fno-delete-null-pointer-checks,)
 KBUILD_AFLAGS_KERNEL :=
 KBUILD_CFLAGS_KERNEL :=
 KBUILD_AFLAGS   := -D__ASSEMBLY__
@@ -623,9 +636,24 @@ endif
 endif
 KBUILD_CFLAGS += $(stackp-flag)
 
+ifeq ($(COMPILER),clang)
+KBUILD_CPPFLAGS += $(call cc-option,-Qunused-arguments,)
+KBUILD_CPPFLAGS += $(call cc-option,-Wno-unknown-warning-option,)
+KBUILD_CFLAGS += $(call cc-disable-warning, unused-variable)
+KBUILD_CFLAGS += $(call cc-disable-warning, format-invalid-specifier)
+KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
+# Quiet clang warning: comparison of unsigned expression < 0 is always false
+KBUILD_CFLAGS += $(call cc-disable-warning, tautological-compare)
+# CLANG uses a _MergedGlobals as optimization, but this breaks modpost, as the
+# source of a reference will be _MergedGlobals and not on of the whitelisted names.
+# See modpost pattern 2
+KBUILD_CFLAGS += $(call cc-option, -mno-global-merge,)
+else
+
 # This warning generated too much noise in a regular build.
 # Use make W=1 to enable this warning (see scripts/Makefile.build)
 KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
+endif
 
 ifdef CONFIG_FRAME_POINTER
 KBUILD_CFLAGS	+= -fno-omit-frame-pointer -fno-optimize-sibling-calls
-- 
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