Fix for bug #42965: isinf() on 32bit x86 with gcc 4.3 can

                    produce incorrect results for ROUND() 

Added a workaround and a configure check to test whether 
isinf() is affected by the GCC bug #39228. 
       
Since no code in MySQL server is currently affected by that 
bug, the patch is actually a safeguard for possible future 
code modifications. No test cases or changelog entries are 
needed. 


configure.in:
  Added a configure check to test whether isinf() is safe to use
  in C code.
include/my_global.h:
  Added a workaround for GCC bug #39228.

configure.in

@@ -2094,6 +2094,27 @@ esac
 AC_MSG_CHECKING(for isinf in <math.h>)
 AC_TRY_LINK([#include <math.h>], [float f = 0.0; int r = isinf(f); return r],
   AC_MSG_RESULT(yes)
+  AC_MSG_CHECKING(whether isinf() is safe to use in C code)
+  AC_TRY_RUN([
+#include <math.h>
+int main()
+{
+  double a= 10.0;
+  double b= 1e308;
+
+  return !isinf(a * b);
+}
+],
+  [AC_MSG_RESULT(yes)],
+  [AC_MSG_RESULT(no)
+   AC_DEFINE([HAVE_BROKEN_ISINF], [1],
+             [Define to 1 if isinf() uses 80-bit register for intermediate values])
+  ],
+  [
+# Let's be optimistic when cross-compiling, since the only compiler known
+# to be affected by this isinf() bug is GCC 4.3 on 32-bit x86.
+   AC_MSG_RESULT([[cross-compiling, assuming 'yes']])
+  ])
   AC_MSG_CHECKING(whether isinf() can be used in C++ code)
   AC_LANG_SAVE
   AC_LANG_CPLUSPLUS

include/my_global.h

@@ -797,9 +797,19 @@ typedef SOCKET_SIZE_TYPE size_socket;
 #endif
 
 #ifdef HAVE_ISINF
-/* isinf() can be used in both C and C++ code */
-#define my_isinf(X) isinf(X)
+/* Check if C compiler is affected by GCC bug #39228 */
+#if !defined(__cplusplus) && defined(HAVE_BROKEN_ISINF)
+/* Force store/reload of the argument to/from a 64-bit double */
+static inline double my_isinf(double x)
+{
+  volatile double t= x;
+  return isinf(t);
+}
 #else
+/* System-provided isinf() is available and safe to use */
+#define my_isinf(X) isinf(X)
+#endif
+#else /* !HAVE_ISINF */
 #define my_isinf(X) (!finite(X) && !isnan(X))
 #endif