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@xen.org>

diff --git a/Config.mk b/Config.mk
index d57a6e4478473738cae558f6c223af276de04dda..aa681aebd04d0061f1dbcd8cf101aa1c0eda6a88 100644 (file)
--- a/Config.mk
+++ b/Config.mk
@@ -71,9 +71,19 @@ PYTHON_PREFIX_ARG ?= --prefix="$(PREFIX)"
 #  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)