From 09d352042fa91d296079e449c4d7fd6f42d5db05 Mon Sep 17 00:00:00 2001
From: Andreas Grapentin <andreas@grapentin.org>
Date: Tue, 14 Mar 2017 17:59:53 +0100
Subject: [PATCH] use _Static_assert in QEMU_BUILD_BUG_ON
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QEMU_BUILD_BUG_ON should use C11's _Static_assert, if the compiler supports it,
to provide more readable messages on failure.

We check for _Static_assert in configure, and set CONFIG_STATIC_ASSERT
accordingly. QEMU_BUILD_BUG_ON invokes _Static_assert if CONFIG_STATIC_ASSERT
is defined, and reverts to the old way otherwise.

That way, systems without C11 conforming compiler will still have the old
messages, as verified by intentionally breaking the configure check.

the following example output was generated by inverting the condition in
QEMU_BUILD_BUG_ON:

without _Static_assert:

> In file included from /qemu/include/qemu/osdep.h:36:0,
>                  from /qemu/qga/commands.c:13:
> /qemu/qga/commands.c: In function ‘qmp_guest_exec_status’:
> /qemu/include/qemu/compiler.h:89:12: error: negative width in bit-field ‘<anonymous>’
>      struct { \
>             ^
> /qemu/include/qemu/compiler.h:96:38: note: in expansion of macro  QEMU_BUILD_BUG_ON_STRUCT’
>  #define QEMU_BUILD_BUG_ON(x) typedef QEMU_BUILD_BUG_ON_STRUCT(x) \
>                                       ^~~~~~~~~~~~~~~~~~~~~~~~
> /qemu/include/qemu/atomic.h:146:5: note: in expansion of macro ‘QEMU_BUILD_BUG_ON’
>      QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *));   \
>      ^~~~~~~~~~~~~~~~~
> /qemu/include/qemu/atomic.h:417:5: note: in expansion of macro ‘atomic_load_acquire’
>      atomic_load_acquire(ptr)
>      ^~~~~~~~~~~~~~~~~~~
> /qemu/qga/commands.c:160:21: note: in expansion of macro ‘atomic_mb_read’
>      bool finished = atomic_mb_read(&gei->finished);
>                      ^~~~~~~~~~~~~~

with _Static_assert:

> In file included from /qemu/include/qemu/osdep.h:36:0,
>                  from /qemu/qga/commands.c:13:
> /qemu/qga/commands.c: In function ‘qmp_guest_exec_status’:
> /qemu/include/qemu/compiler.h:94:30: error: static assertion failed: "not expecting: sizeof(*&gei->finished) > sizeof(void *)"
>  #define QEMU_BUILD_BUG_ON(x) _Static_assert(!(x), #x)
>                               ^
> /qemu/include/qemu/atomic.h:146:5: note: in expansion of macro ‘QEMU_BUILD_BUG_ON’
>      QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *));   \
>      ^~~~~~~~~~~~~~~~~
> /qemu/include/qemu/atomic.h:417:5: note: in expansion of macro ‘atomic_load_acquire’
>      atomic_load_acquire(ptr)
>      ^~~~~~~~~~~~~~~~~~~
> /qemu/qga/commands.c:160:21: note: in expansion of macro ‘atomic_mb_read’
>      bool finished = atomic_mb_read(&gei->finished);
>                      ^~~~~~~~~~~~~~

Signed-off-by: Andreas Grapentin <andreas@grapentin.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
---
 configure               | 18 ++++++++++++++++++
 include/qemu/compiler.h |  4 +++-
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/configure b/configure
index 48a9370cc6..39aadba813 100755
--- a/configure
+++ b/configure
@@ -4853,6 +4853,20 @@ EOF
 fi
 
 ##########################################
+# check for _Static_assert()
+
+have_static_assert=no
+cat > $TMPC << EOF
+_Static_assert(1, "success");
+int main(void) {
+    return 0;
+}
+EOF
+if compile_prog "" "" ; then
+    have_static_assert=yes
+fi
+
+##########################################
 # End of CC checks
 # After here, no more $cc or $ld runs
 
@@ -5848,6 +5862,10 @@ if test "$have_sysmacros" = "yes" ; then
   echo "CONFIG_SYSMACROS=y" >> $config_host_mak
 fi
 
+if test "$have_static_assert" = "yes" ; then
+  echo "CONFIG_STATIC_ASSERT=y" >> $config_host_mak
+fi
+
 # Hold two types of flag:
 #   CONFIG_THREAD_SETNAME_BYTHREAD  - we've got a way of setting the name on
 #                                     a thread we have a handle to
diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
index 18e610083a..340e5fdc09 100644
--- a/include/qemu/compiler.h
+++ b/include/qemu/compiler.h
@@ -82,7 +82,9 @@
         int:(x) ? -1 : 1; \
     }
 
-#ifdef __COUNTER__
+#if defined(CONFIG_STATIC_ASSERT)
+#define QEMU_BUILD_BUG_ON(x) _Static_assert(!(x), "not expecting: " #x)
+#elif defined(__COUNTER__)
 #define QEMU_BUILD_BUG_ON(x) typedef QEMU_BUILD_BUG_ON_STRUCT(x) \
     glue(qemu_build_bug_on__, __COUNTER__) __attribute__((unused))
 #else
-- 
2.11.4.GIT