linux-kernel - [PATCH] [2/98] kbuild: Fix passing -Wno-* options to gcc 4.4+

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Date:	Tue, 26 Jul 2011 17:34:54 -0700 (PDT)
From:	Andi Kleen <andi@...stfloor.org>
To:	mmarek@...e.cz, ak@...ux.intel.com, linux-kernel@...r.kernel.org,
	stable@...nel.org, tim.bird@...sony.com,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: [PATCH] [2/98] kbuild: Fix passing -Wno-* options to gcc 4.4+

2.6.35-longterm review patch.  If anyone has any objections, please let me know.

------------------
From: Michal Marek <mmarek@...e.cz>

[ upstream commit 8417da6f2128008c431c7d130af6cd3d9079922e ]

Starting with 4.4, gcc will happily accept -Wno-<anything> in the
cc-option test and complain later when compiling a file that has some
other warning. This rather unexpected behavior is intentional as per
http://gcc.gnu.org/PR28322, so work around it by testing for support of
the opposite option (without the no-). Introduce a new Makefile function
cc-disable-warning that does this and update two uses of cc-option in
the toplevel Makefile.

Reported-and-tested-by: Stephen Rothwell <sfr@...b.auug.org.au>
Signed-off-by: Michal Marek <mmarek@...e.cz>
Signed-off-by: Andi Kleen <ak@...ux.intel.com>

Index: linux-2.6.35.y/Documentation/kbuild/makefiles.txt
===================================================================
--- linux-2.6.35.y.orig/Documentation/kbuild/makefiles.txt
+++ linux-2.6.35.y/Documentation/kbuild/makefiles.txt
@@ -502,6 +502,18 @@ more details, with real examples.
 	gcc >= 3.00. For gcc < 3.00, -malign-functions=4 is used.
 	Note: cc-option-align uses KBUILD_CFLAGS for $(CC) options
 
+    cc-disable-warning
+	cc-disable-warning checks if gcc supports a given warning and returns
+	the commandline switch to disable it. This special function is needed,
+	because gcc 4.4 and later accept any unknown -Wno-* option and only
+	warn about it if there is another warning in the source file.
+
+	Example:
+		KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
+
+	In the above example, -Wno-unused-but-set-variable will be added to
+	KBUILD_CFLAGS only if gcc really accepts it.
+
     cc-version
 	cc-version returns a numerical version of the $(CC) compiler version.
 	The format is <major><minor> where both are two digits. So for example
Index: linux-2.6.35.y/Makefile
===================================================================
--- linux-2.6.35.y.orig/Makefile
+++ linux-2.6.35.y/Makefile
@@ -547,7 +547,7 @@ KBUILD_CFLAGS += $(call cc-option, -fno-
 endif
 
 # This warning generated too much noise in a regular build.
-KBUILD_CFLAGS += $(call cc-option, -Wno-unused-but-set-variable)
+KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
 
 ifdef CONFIG_FRAME_POINTER
 KBUILD_CFLAGS	+= -fno-omit-frame-pointer -fno-optimize-sibling-calls
@@ -577,7 +577,7 @@ CHECKFLAGS     += $(NOSTDINC_FLAGS)
 KBUILD_CFLAGS += $(call cc-option,-Wdeclaration-after-statement,)
 
 # disable pointer signed / unsigned warnings in gcc 4.0
-KBUILD_CFLAGS += $(call cc-option,-Wno-pointer-sign,)
+KBUILD_CFLAGS += $(call cc-disable-warning, pointer-sign)
 
 # disable invalid "can't wrap" optimizations for signed / pointers
 KBUILD_CFLAGS	+= $(call cc-option,-fno-strict-overflow)
Index: linux-2.6.35.y/scripts/Kbuild.include
===================================================================
--- linux-2.6.35.y.orig/scripts/Kbuild.include
+++ linux-2.6.35.y/scripts/Kbuild.include
@@ -118,6 +118,11 @@ cc-option-yn = $(call try-run,\
 cc-option-align = $(subst -functions=0,,\
 	$(call cc-option,-falign-functions=0,-malign-functions=0))
 
+# cc-disable-warning
+# Usage: cflags-y += $(call cc-disable-warning,unused-but-set-variable)
+cc-disable-warning = $(call try-run,\
+	$(CC) $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS) -W$(strip $(1)) -c -xc /dev/null -o "$$TMP",-Wno-$(strip $(1)))
+
 # cc-version
 # Usage gcc-ver := $(call cc-version)
 cc-version = $(shell $(CONFIG_SHELL) $(srctree)/scripts/gcc-version.sh $(CC))
--
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