[Xen-changelog] [xen-unstable] Fix Config.mk's cc-option for -Wno-* options.

[Xen patchbot-unstable <patchbot@xxxxxxx>]

# HG changeset patch
# User Keir Fraser <keir@xxxxxxx>
# Date 1306168708 -3600
# Node ID 37c77bacb52aa7795978b994f9d371b979b2cb07
# Parent  0f670f5146c858ffdc743176d4e22aef4bfe12da
Fix Config.mk's cc-option for -Wno-* options.

These disable-warning options are handled specially by GCC:
 (a) they are ignored unless the compiler emits a warning; and
 (b) even then they produce a warning rather than an error

To handle this, modify the test invocation of GCC to compile a
fragment of code that will always provoke a warning (integer assigned
to pointer). This works around (a) above.

Then, we grep the compiler's stdout/stderr for the option-under-test,
the presence of which would indicate an "unrecognized command-line
option" warning/error. This works around (b) above, letting us
distinguish between the "integer assigned to pointer" and
"unrecognized command-line option" warnings.

Signed-off-by: Keir Fraser <keir@xxxxxxx>
---


diff -r 0f670f5146c8 -r 37c77bacb52a Config.mk
--- a/Config.mk Sat May 21 07:55:46 2011 +0100
+++ b/Config.mk Mon May 23 17:38:28 2011 +0100
@@ -71,9 +71,19 @@
 #  https://bugs.launchpad.net/ubuntu/+bug/362570
 
 # cc-option: Check if compiler supports first option, else fall back to second.
+#
+# This is complicated by the fact that unrecognised -Wno-* options:
+#   (a) are ignored unless the compilation emits a warning; and
+#   (b) even then produce a warning rather than an error
+# To handle this we do a test compile, passing the option-under-test, on a code
+# fragment that will always produce a warning (integer assigned to pointer).
+# We then grep for the option-under-test in the compiler's output, the presence
+# of which would indicate an "unrecognized command-line option" warning/error.
+#
 # Usage: cflags-y += $(call cc-option,$(CC),-march=winchip-c6,-march=i586)
-cc-option = $(shell if test -z "`$(1) $(2) -S -o /dev/null -xc \
-              /dev/null 2>&1`"; then echo "$(2)"; else echo "$(3)"; fi ;)
+cc-option = $(shell if test -z "`echo 'void*p=1;' | \
+              $(1) $(2) -S -o /dev/null -xc - 2>&1 | grep -- $(2)`"; \
+              then echo "$(2)"; else echo "$(3)"; fi ;)
 
 # cc-option-add: Add an option to compilation flags, but only if supported.
 # Usage: $(call cc-option-add CFLAGS,CC,-march=winchip-c6)

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