[Qemu-devel] [PATCH v3 4/4] configure: Add workaround for ccache and clang

[John Snow <jsnow@redhat.com>]

Test if ccache is interfering with our life, and
disable its habit of trying to compile already pre-processed
versions of code if so.

In particular, clang has different semantic warnings based on
if the warning arose from a macro or not. By trying to build
preprocessed versions of code, we get more errors than we should.

ccache allows us to disable this feature, opting instead to
compile the original version instead of its preprocessed version.

This makes ccache much slower for cache misses, but at least it
becomes usable with QEMU/clang.

Thanks to Peter Eisentraut for his writeup on the issue:
http://peter.eisentraut.org/blog/2014/12/01/ccache-and-clang-part-3/

Signed-off-by: John Snow <jsnow@redhat.com>
---
 configure | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/configure b/configure
index fbcbf50..75219a9 100755
--- a/configure
+++ b/configure
@@ -4172,6 +4172,33 @@ if compile_prog "" "" ; then
     getauxval=yes
 fi
 
+########################################
+# check if ccache is interfering with
+# semantic analysis of macros
+
+ccache=no
+cat > $TMPC << EOF
+static const int Z = 1;
+#define fn() ({ Z; })
+#define TAUT(X) ((X) == Z)
+#define PAREN(X, Y) (X == Y)
+#define ID(X) (X)
+int main(int argc, char *argv[])
+{
+    int x = 0, y = 0;
+    x = ID(x);
+    x = fn();
+    fn();
+    if (PAREN(x, y)) return 0;
+    if (TAUT(Z)) return 0;
+    return 0;
+}
+EOF
+
+if ! compile_object; then
+    ccache=yes
+fi
+
 ##########################################
 # End of CC checks
 # After here, no more $cc or $ld runs
@@ -5462,6 +5489,10 @@ if test "$numa" = "yes"; then
   echo "CONFIG_NUMA=y" >> $config_host_mak
 fi
 
+if test "$ccache" = "yes"; then
+  echo "export CCACHE_CPP2=y" >> $config_host_mak
+fi
+
 # build tree in object directory in case the source is not in the current directory
 DIRS="tests tests/tcg tests/tcg/cris tests/tcg/lm32 tests/libqos tests/qapi-schema tests/tcg/xtensa tests/qemu-iotests"
 DIRS="$DIRS fsdev"
-- 
2.1.0