[libvirt] [RFC PATCH] maint: let MIN/MAX evaluate only once, on modern compilers

[Eric Blake]

We might as well take advantage of gcc's extensions for a safer
MIN()/MAX() macro.

Signed-off-by: Eric Blake <eblake at redhat.com>
---

RFC because we could also try to fall back to older gcc's typeof(a)_a=(a)
when the newer __auto_type _a=(a) is not present, and because I don't
know how to properly probe for __auto_type and/or typeof support in
clang (I know that clang added __auto_type in 2015, per
https://reviews.llvm.org/D12686); I also don't know if we have officially
stated that we require gcc/clang (because of attribute((cleanup)), which
really cannot be emulated without extensions), or if we still try to
allow compilation with other compilers; if we have, I don't know if we
have declared a minimum compiler version that we demand (qemu has recently
demanded gcc 4.8 or clang 3.4 [branded 5.1 on Apple]).

 src/util/virutil.h | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/src/util/virutil.h b/src/util/virutil.h
index 908d8920ec..364e354bcb 100644
--- a/src/util/virutil.h
+++ b/src/util/virutil.h
@@ -28,10 +28,26 @@
 # include <sys/types.h>

 # ifndef MIN
-#  define MIN(a, b) ((a) < (b) ? (a) : (b))
+#  if __GNUC_PREREQ(4, 9)
+#   define MIN(a, b) ({                         \
+            __auto_type _a = (a);               \
+            __auto_type _b = (b);               \
+            _a < _b ? _a : _b;                  \
+        })
+#  else
+#   define MIN(a, b) ((a) < (b) ? (a) : (b))
+#  endif
 # endif
 # ifndef MAX
-#  define MAX(a, b) ((a) > (b) ? (a) : (b))
+#  if __GNUC_PREREQ(4, 9)
+#   define MAX(a, b) ({                         \
+            __auto_type _a = (a);               \
+            __auto_type _b = (b);               \
+            _a > _b ? _a : _b;                  \
+        })
+#  else
+#   define MAX(a, b) ((a) > (b) ? (a) : (b))
+#  endif
 # endif


-- 
2.20.1