Re: [Xen-devel] [xen-4.0-testing test] 7147: regressions - FAIL

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Keir Fraser writes ("Re: [Xen-devel] [xen-4.0-testing test] 7147: regressions - 
FAIL"):
> I've been fiddling with a similar principle but much smaller patch (see
> below). But it's failing for me in the same way as yours -- *all* optional
> flags disappear from my CFLAGS (e.g., -Wno-unused-but-set-variable, which my
> gcc-4.5.1 definitely does support).

You have to check the exit status, not the stderr output.  It might be
right do do that even for ordinary (non -Wno-*) options.

But TBH I really prefer my script because it's actually
comprehensible.  You can even run it by hand from the command line:

mariner:xen-unstable-tools.hg> config/test-cc-option.sh gcc -fno-strict-aliasing
-fno-strict-aliasing
mariner:xen-unstable-tools.hg> config/test-cc-option.sh gcc -fno-rapture

mariner:xen-unstable-tools.hg> config/test-cc-option.sh gcc 
-Wdeclaration-after-statement
-Wdeclaration-after-statement
mariner:xen-unstable-tools.hg> config/test-cc-option.sh gcc 
-Wno-declaration-after-statement
-Wno-declaration-after-statement
mariner:xen-unstable-tools.hg> config/test-cc-option.sh gcc -Wno-rapture

mariner:xen-unstable-tools.hg>

Below is a final version of my fix, ready to push to unstable.

Ian.

# HG changeset patch
# User Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
# Date 1306164314 -3600
# Node ID 512f0b1937b40e3ecc79dea6768646206e0f69f3
# Parent  0f670f5146c858ffdc743176d4e22aef4bfe12da
gcc compile fix (reprise): deal with fallout from GCC PR 28322

21492:19eefd764f6f breaks the build on certain GCC 4.3 systems which
have a broken version of the fix to upstream GCC PR 28322.

Specifically, those gcc's (which include current Debian lenny's)
ignore unknown -Wno-foo options if there are no warnings, but treat
them as an error otherwise.  If you compile with -Wno-error the effect
is to defeat straightforward attempts (like ours) to filter out
unknown -Wno-foo options and instead to bomb out with a complaint the
first time any file is compiled which causes any other, unrelated,
warning.

In this patch I introduce a more sophisticated way of trying to filter
out unknown -Wno-foo options.  The machinery is moved into a helper
script.  For -Wno-foo options, we test whether compiling a file with
another warning, with -Wno-error, and with the option to be tested,
succeeds or fails.  On compilers with no fix for PR 28322, or with the
broken fix, this will fail.  On compilers with a correct fix for PR
28322 this will succeed but the warning option is then harmless.

For normal compiler options, we simply use the same test we did
before: does compiling /dev/null with this option produce any stderr
output.

Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Committed-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>

diff -r 0f670f5146c8 -r 512f0b1937b4 Config.mk
--- a/Config.mk Sat May 21 07:55:46 2011 +0100
+++ b/Config.mk Mon May 23 16:25:14 2011 +0100
@@ -72,8 +72,7 @@ PYTHON_PREFIX_ARG ?= --prefix="$(PREFIX)
 
 # cc-option: Check if compiler supports first option, else fall back to second.
 # 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 $(XEN_ROOT)/config/test-cc-option.sh "$(1)" "$(2)" "$(3)")
 
 # cc-option-add: Add an option to compilation flags, but only if supported.
 # Usage: $(call cc-option-add CFLAGS,CC,-march=winchip-c6)
diff -r 0f670f5146c8 -r 512f0b1937b4 config/test-cc-option.sh
--- /dev/null   Thu Jan 01 00:00:00 1970 +0000
+++ b/config/test-cc-option.sh  Mon May 23 16:25:14 2011 +0100
@@ -0,0 +1,31 @@
+#!/bin/sh
+set -e
+
+cc="$1"
+opt="$2"
+alt="$3"
+
+case "$opt" in
+-Wno-*)
+   # Sadly a broken implementation of the fix to GCC PR 28322
+   # (actually shipped eg in Debian lenny) makes it hard to spot
+   # whether the compiler recognises a -Wno-foo option without
+   # generating a warning for some other reason.
+
+   input="${0%-cc-option.sh}-cc-warning.c"
+   if $cc $opt -Wreturn-type -Wno-error -S -o /dev/null "$input" \
+           >/dev/null 2>&1; then
+       res="$opt"
+   else
+       res="$alt"
+   fi
+   ;;
+*)
+    if test -z "`$cc $opt -S -o /dev/null -xc $input 2>&1`"; then
+        res="$opt"
+    else
+        res="$alt"
+    fi
+    ;;
+esac
+printf "%s\n" "$res"
diff -r 0f670f5146c8 -r 512f0b1937b4 config/test-cc-warning.c
--- /dev/null   Thu Jan 01 00:00:00 1970 +0000
+++ b/config/test-cc-warning.c  Mon May 23 16:25:14 2011 +0100
@@ -0,0 +1,2 @@
+extern int bogus(void);
+int bogus(void) { }

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