base: Check for static_assert support and provide fallback

C++11 has support for static_asserts to provide compile-time assertion
checking. This is very useful when testing, for example, structure
sizes to make sure that the compiler got the right alignment or vector
sizes.

SConstruct

@@ -684,17 +684,34 @@ def CheckLeading(context):
     context.Result(ret)
     return ret
 
+# Test for the presence of C++11 static asserts. If the compiler lacks
+# support for static asserts, base/compiler.hh enables a macro that
+# removes any static asserts in the code.
+def CheckStaticAssert(context):
+    context.Message("Checking for C++11 static_assert support...")
+    ret = context.TryCompile('''
+        static_assert(1, "This assert is always true");
+        ''', extension=".cc")
+    context.env.Append(HAVE_STATIC_ASSERT=ret)
+    context.Result(ret)
+    return ret
+
 # Platform-specific configuration.  Note again that we assume that all
 # builds under a given build root run on the same host platform.
 conf = Configure(main,
                  conf_dir = joinpath(build_root, '.scons_config'),
                  log_file = joinpath(build_root, 'scons_config.log'),
-                 custom_tests = { 'CheckLeading' : CheckLeading })
+                 custom_tests = { 'CheckLeading' : CheckLeading,
+                                  'CheckStaticAssert' : CheckStaticAssert,
+                                })
 
 # Check for leading underscores.  Don't really need to worry either
 # way so don't need to check the return code.
 conf.CheckLeading()
 
+# Check for C++11 features we want to use if they exist
+conf.CheckStaticAssert()
+
 # Check if we should compile a 64 bit binary on Mac OS X/Darwin
 try:
     import platform
@@ -923,7 +940,7 @@ sticky_vars.AddVariables(
 # These variables get exported to #defines in config/*.hh (see src/SConscript).
 export_vars += ['USE_FENV', 'SS_COMPATIBLE_FP',
                 'TARGET_ISA', 'CP_ANNOTATE', 'USE_POSIX_CLOCK', 'PROTOCOL',
-               ]
+                'HAVE_STATIC_ASSERT']
 
 ###################################################
 #

src/base/compiler.hh

@@ -31,6 +31,8 @@
 #ifndef __BASE_COMPILER_HH__
 #define __BASE_COMPILER_HH__
 
+#include "config/have_static_assert.hh"
+
 //http://msdn2.microsoft.com/en-us/library/ms937669.aspx
 //http://msdn2.microsoft.com/en-us/library/aa448724.aspx
 //http://docs.sun.com/source/819-3688/sun.specific.html#marker-998278
@@ -56,4 +58,32 @@
 #error "Need to define compiler options in base/compiler.hh"
 #endif
 
+/*
+ * Define a compatibility macro that emulates the behavior of
+ * static_assert using template magic if the compiler doesn't have
+ * native support.
+ */
+#if !HAVE_STATIC_ASSERT
+
+template<bool>
+struct static_assert_failure;
+
+template<>
+struct static_assert_failure<false> {};
+
+/* The following macro causes the compiler to evaluate the size of the
+ * static_assert_failure struct. The templates are designed so that
+ * only static_assert_failure<false> evaluates to a proper size, while
+ * static_assert_failure<true> generates a compile time error.
+ */
+#define static_assert(expr, msg)                                        \
+    namespace ns_static_assert {                                        \
+        enum {                                                          \
+            static_assert_ ## __LINE__ =                                \
+                sizeof(static_assert_failure<!(expr)>)                  \
+        };                                                              \
+    }
+
+#endif
+
 #endif // __BASE_COMPILER_HH__