Add configure test to see if the C compiler has gcc-style computed gotos.

We'll need this for the upcoming patch to speed up expression evaluation.
Might as well push it now to see if it behaves sanely in the buildfarm.

Andres Freund

Discussion: https://postgr.es/m/20170320062511.hp5qeurtxrwsvfxr@alap3.anarazel.de

config/c-compiler.m4

@@ -273,6 +273,30 @@ fi])# PGAC_C_BUILTIN_UNREACHABLE
 
 
 
+# PGAC_C_COMPUTED_GOTO
+# -----------------------
+# Check if the C compiler knows computed gotos (gcc extension, also
+# available in at least clang).  If so, define HAVE_COMPUTED_GOTO.
+#
+# Checking whether computed gotos are supported syntax-wise ought to
+# be enough, as the syntax is otherwise illegal.
+AC_DEFUN([PGAC_C_COMPUTED_GOTO],
+[AC_CACHE_CHECK(for computed goto support, pgac_cv_computed_goto,
+[AC_COMPILE_IFELSE([AC_LANG_PROGRAM([],
+[[void *labeladdrs[] = {&&my_label};
+  goto *labeladdrs[0];
+  my_label:
+  return 1;
+]])],
+[pgac_cv_computed_goto=yes],
+[pgac_cv_computed_goto=no])])
+if test x"$pgac_cv_computed_goto" = xyes ; then
+AC_DEFINE(HAVE_COMPUTED_GOTO, 1,
+          [Define to 1 if your compiler handles computed gotos.])
+fi])# PGAC_C_COMPUTED_GOTO
+
+
+
 # PGAC_C_VA_ARGS
 # --------------
 # Check if the C compiler understands C99-style variadic macros,

configure

@@ -11531,6 +11531,40 @@ if test x"$pgac_cv__builtin_unreachable" = xyes ; then
 
 $as_echo "#define HAVE__BUILTIN_UNREACHABLE 1" >>confdefs.h
 
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for computed goto support" >&5
+$as_echo_n "checking for computed goto support... " >&6; }
+if ${pgac_cv_computed_goto+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+void *labeladdrs[] = {&&my_label};
+  goto *labeladdrs[0];
+  my_label:
+  return 1;
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  pgac_cv_computed_goto=yes
+else
+  pgac_cv_computed_goto=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv_computed_goto" >&5
+$as_echo "$pgac_cv_computed_goto" >&6; }
+if test x"$pgac_cv_computed_goto" = xyes ; then
+
+$as_echo "#define HAVE_COMPUTED_GOTO 1" >>confdefs.h
+
 fi
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for __VA_ARGS__" >&5
 $as_echo_n "checking for __VA_ARGS__... " >&6; }

configure.in

@@ -1322,6 +1322,7 @@ PGAC_C_BUILTIN_BSWAP32
 PGAC_C_BUILTIN_BSWAP64
 PGAC_C_BUILTIN_CONSTANT_P
 PGAC_C_BUILTIN_UNREACHABLE
+PGAC_C_COMPUTED_GOTO
 PGAC_C_VA_ARGS
 PGAC_STRUCT_TIMEZONE
 PGAC_UNION_SEMUN

src/include/pg_config.h.in

@@ -108,6 +108,9 @@
 /* Define to 1 if you have the `clock_gettime' function. */
 #undef HAVE_CLOCK_GETTIME
 
+/* Define to 1 if your compiler handles computed gotos. */
+#undef HAVE_COMPUTED_GOTO
+
 /* Define to 1 if you have the <crtdefs.h> header file. */
 #undef HAVE_CRTDEFS_H
 

src/include/pg_config.h.win32

@@ -78,6 +78,9 @@
 /* Define to 1 if you have the `clock_gettime' function. */
 /* #undef HAVE_CLOCK_GETTIME */
 
+/* Define to 1 if your compiler handles computed gotos. */
+/* #undef HAVE_COMPUTED_GOTO */
+
 /* Define to 1 if you have the `crypt' function. */
 /* #undef HAVE_CRYPT */