[Xen-changelog] [xen-4.1-testing] Fix Config.mk's cc-option for -Wno-* options.

[Xen patchbot-4.1-testing <patchbot@xxxxxxx>]

# HG changeset patch
# User Keir Fraser <keir@xxxxxxx>
# Date 1306221522 -3600
# Node ID a0ed85c8ddcfeb0da7134d177d2ec6389fef95c7
# Parent  17a91d705badece4806cde4a3ee5daf216c62dd3
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>
xen-unstable changeset:   23369:37c77bacb52a
xen-unstable date:        Mon May 23 17:38:28 2011 +0100
---


diff -r 17a91d705bad -r a0ed85c8ddcf Config.mk
--- a/Config.mk Sat May 21 07:59:43 2011 +0100
+++ b/Config.mk Tue May 24 08:18:42 2011 +0100
@@ -62,9 +62,19 @@
 #  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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