Improve handling of ereport(ERROR) and elog(ERROR).

In commit 71450d7, we added code to inform
suitably-intelligent compilers that ereport() doesn't return if the elevel
is ERROR or higher.  This patch extends that to elog(), and also fixes a
double-evaluation hazard that the previous commit created in ereport(),
as well as reducing the emitted code size.

The elog() improvement requires the compiler to support __VA_ARGS__, which
should be available in just about anything nowadays since it's required by
C99.  But our minimum language baseline is still C89, so add a configure
test for that.

The previous commit assumed that ereport's elevel could be evaluated twice,
which isn't terribly safe --- there are already counterexamples in xlog.c.
On compilers that have __builtin_constant_p, we can use that to protect the
second test, since there's no possible optimization gain if the compiler
doesn't know the value of elevel.  Otherwise, use a local variable inside
the macros to prevent double evaluation.  The local-variable solution is
inferior because (a) it leads to useless code being emitted when elevel
isn't constant, and (b) it increases the optimization level needed for the
compiler to recognize that subsequent code is unreachable.  But it seems
better than not teaching non-gcc compilers about unreachability at all.

Lastly, if the compiler has __builtin_unreachable(), we can use that
instead of abort(), resulting in a noticeable code savings since no
function call is actually emitted.  However, it seems wise to do this only
in non-assert builds.  In an assert build, continue to use abort(), so that
the behavior will be predictable and debuggable if the "impossible"
happens.

These changes involve making the ereport and elog macros emit do-while
statement blocks not just expressions, which forces small changes in
a few call sites.

Andres Freund, Tom Lane, Heikki Linnakangas

Author: tglsfdc

config/c-compiler.m4

@@ -122,7 +122,7 @@ fi])# PGAC_C_FUNCNAME_SUPPORT
 
 
 # PGAC_C_STATIC_ASSERT
-# -----------------------
+# --------------------
 # Check if the C compiler understands _Static_assert(),
 # and define HAVE__STATIC_ASSERT if so.
 #
@@ -161,6 +161,63 @@ fi])# PGAC_C_TYPES_COMPATIBLE
 
 
 
+# PGAC_C_BUILTIN_CONSTANT_P
+# -------------------------
+# Check if the C compiler understands __builtin_constant_p(),
+# and define HAVE__BUILTIN_CONSTANT_P if so.
+AC_DEFUN([PGAC_C_BUILTIN_CONSTANT_P],
+[AC_CACHE_CHECK(for __builtin_constant_p, pgac_cv__builtin_constant_p,
+[AC_TRY_COMPILE([static int x; static int y[__builtin_constant_p(x) ? x : 1];],
+[],
+[pgac_cv__builtin_constant_p=yes],
+[pgac_cv__builtin_constant_p=no])])
+if test x"$pgac_cv__builtin_constant_p" = xyes ; then
+AC_DEFINE(HAVE__BUILTIN_CONSTANT_P, 1,
+          [Define to 1 if your compiler understands __builtin_constant_p.])
+fi])# PGAC_C_BUILTIN_CONSTANT_P
+
+
+
+# PGAC_C_BUILTIN_UNREACHABLE
+# --------------------------
+# Check if the C compiler understands __builtin_unreachable(),
+# and define HAVE__BUILTIN_UNREACHABLE if so.
+#
+# NB: Don't get the idea of putting a for(;;); or such before the
+# __builtin_unreachable() call.  Some compilers would remove it before linking
+# and only a warning instead of an error would be produced.
+AC_DEFUN([PGAC_C_BUILTIN_UNREACHABLE],
+[AC_CACHE_CHECK(for __builtin_unreachable, pgac_cv__builtin_unreachable,
+[AC_TRY_LINK([],
+[__builtin_unreachable();],
+[pgac_cv__builtin_unreachable=yes],
+[pgac_cv__builtin_unreachable=no])])
+if test x"$pgac_cv__builtin_unreachable" = xyes ; then
+AC_DEFINE(HAVE__BUILTIN_UNREACHABLE, 1,
+          [Define to 1 if your compiler understands __builtin_unreachable.])
+fi])# PGAC_C_BUILTIN_UNREACHABLE
+
+
+
+# PGAC_C_VA_ARGS
+# --------------
+# Check if the C compiler understands C99-style variadic macros,
+# and define HAVE__VA_ARGS if so.
+AC_DEFUN([PGAC_C_VA_ARGS],
+[AC_CACHE_CHECK(for __VA_ARGS__, pgac_cv__va_args,
+[AC_TRY_COMPILE([#include <stdio.h>],
+[#define debug(...) fprintf(stderr, __VA_ARGS__)
+debug("%s", "blarg");
+],
+[pgac_cv__va_args=yes],
+[pgac_cv__va_args=no])])
+if test x"$pgac_cv__va_args" = xyes ; then
+AC_DEFINE(HAVE__VA_ARGS, 1,
+          [Define to 1 if your compiler understands __VA_ARGS__ in macros.])
+fi])# PGAC_C_VA_ARGS
+
+
+
 # PGAC_PROG_CC_CFLAGS_OPT
 # -----------------------
 # Given a string, check if the compiler supports the string as a

configure

@@ -15659,6 +15659,184 @@ cat >>confdefs.h <<\_ACEOF
 #define HAVE__BUILTIN_TYPES_COMPATIBLE_P 1
 _ACEOF
 
+fi
+{ $as_echo "$as_me:$LINENO: checking for __builtin_constant_p" >&5
+$as_echo_n "checking for __builtin_constant_p... " >&6; }
+if test "${pgac_cv__builtin_constant_p+set}" = set; then
+  $as_echo_n "(cached) " >&6
+else
+  cat >conftest.$ac_ext <<_ACEOF
+/* confdefs.h.  */
+_ACEOF
+cat confdefs.h >>conftest.$ac_ext
+cat >>conftest.$ac_ext <<_ACEOF
+/* end confdefs.h.  */
+static int x; static int y[__builtin_constant_p(x) ? x : 1];
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+rm -f conftest.$ac_objext
+if { (ac_try="$ac_compile"
+case "(($ac_try" in
+  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
+  *) ac_try_echo=$ac_try;;
+esac
+eval ac_try_echo="\"\$as_me:$LINENO: $ac_try_echo\""
+$as_echo "$ac_try_echo") >&5
+  (eval "$ac_compile") 2>conftest.er1
+  ac_status=$?
+  grep -v '^ *+' conftest.er1 >conftest.err
+  rm -f conftest.er1
+  cat conftest.err >&5
+  $as_echo "$as_me:$LINENO: \$? = $ac_status" >&5
+  (exit $ac_status); } && {
+	 test -z "$ac_c_werror_flag" ||
+	 test ! -s conftest.err
+       } && test -s conftest.$ac_objext; then
+  pgac_cv__builtin_constant_p=yes
+else
+  $as_echo "$as_me: failed program was:" >&5
+sed 's/^/| /' conftest.$ac_ext >&5
+
+	pgac_cv__builtin_constant_p=no
+fi
+
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+fi
+{ $as_echo "$as_me:$LINENO: result: $pgac_cv__builtin_constant_p" >&5
+$as_echo "$pgac_cv__builtin_constant_p" >&6; }
+if test x"$pgac_cv__builtin_constant_p" = xyes ; then
+
+cat >>confdefs.h <<\_ACEOF
+#define HAVE__BUILTIN_CONSTANT_P 1
+_ACEOF
+
+fi
+{ $as_echo "$as_me:$LINENO: checking for __builtin_unreachable" >&5
+$as_echo_n "checking for __builtin_unreachable... " >&6; }
+if test "${pgac_cv__builtin_unreachable+set}" = set; then
+  $as_echo_n "(cached) " >&6
+else
+  cat >conftest.$ac_ext <<_ACEOF
+/* confdefs.h.  */
+_ACEOF
+cat confdefs.h >>conftest.$ac_ext
+cat >>conftest.$ac_ext <<_ACEOF
+/* end confdefs.h.  */
+
+int
+main ()
+{
+__builtin_unreachable();
+  ;
+  return 0;
+}
+_ACEOF
+rm -f conftest.$ac_objext conftest$ac_exeext
+if { (ac_try="$ac_link"
+case "(($ac_try" in
+  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
+  *) ac_try_echo=$ac_try;;
+esac
+eval ac_try_echo="\"\$as_me:$LINENO: $ac_try_echo\""
+$as_echo "$ac_try_echo") >&5
+  (eval "$ac_link") 2>conftest.er1
+  ac_status=$?
+  grep -v '^ *+' conftest.er1 >conftest.err
+  rm -f conftest.er1
+  cat conftest.err >&5
+  $as_echo "$as_me:$LINENO: \$? = $ac_status" >&5
+  (exit $ac_status); } && {
+	 test -z "$ac_c_werror_flag" ||
+	 test ! -s conftest.err
+       } && test -s conftest$ac_exeext && {
+	 test "$cross_compiling" = yes ||
+	 $as_test_x conftest$ac_exeext
+       }; then
+  pgac_cv__builtin_unreachable=yes
+else
+  $as_echo "$as_me: failed program was:" >&5
+sed 's/^/| /' conftest.$ac_ext >&5
+
+	pgac_cv__builtin_unreachable=no
+fi
+
+rm -rf conftest.dSYM
+rm -f core conftest.err conftest.$ac_objext conftest_ipa8_conftest.oo \
+      conftest$ac_exeext conftest.$ac_ext
+fi
+{ $as_echo "$as_me:$LINENO: result: $pgac_cv__builtin_unreachable" >&5
+$as_echo "$pgac_cv__builtin_unreachable" >&6; }
+if test x"$pgac_cv__builtin_unreachable" = xyes ; then
+
+cat >>confdefs.h <<\_ACEOF
+#define HAVE__BUILTIN_UNREACHABLE 1
+_ACEOF
+
+fi
+{ $as_echo "$as_me:$LINENO: checking for __VA_ARGS__" >&5
+$as_echo_n "checking for __VA_ARGS__... " >&6; }
+if test "${pgac_cv__va_args+set}" = set; then
+  $as_echo_n "(cached) " >&6
+else
+  cat >conftest.$ac_ext <<_ACEOF
+/* confdefs.h.  */
+_ACEOF
+cat confdefs.h >>conftest.$ac_ext
+cat >>conftest.$ac_ext <<_ACEOF
+/* end confdefs.h.  */
+#include <stdio.h>
+int
+main ()
+{
+#define debug(...) fprintf(stderr, __VA_ARGS__)
+debug("%s", "blarg");
+
+  ;
+  return 0;
+}
+_ACEOF
+rm -f conftest.$ac_objext
+if { (ac_try="$ac_compile"
+case "(($ac_try" in
+  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
+  *) ac_try_echo=$ac_try;;
+esac
+eval ac_try_echo="\"\$as_me:$LINENO: $ac_try_echo\""
+$as_echo "$ac_try_echo") >&5
+  (eval "$ac_compile") 2>conftest.er1
+  ac_status=$?
+  grep -v '^ *+' conftest.er1 >conftest.err
+  rm -f conftest.er1
+  cat conftest.err >&5
+  $as_echo "$as_me:$LINENO: \$? = $ac_status" >&5
+  (exit $ac_status); } && {
+	 test -z "$ac_c_werror_flag" ||
+	 test ! -s conftest.err
+       } && test -s conftest.$ac_objext; then
+  pgac_cv__va_args=yes
+else
+  $as_echo "$as_me: failed program was:" >&5
+sed 's/^/| /' conftest.$ac_ext >&5
+
+	pgac_cv__va_args=no
+fi
+
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+fi
+{ $as_echo "$as_me:$LINENO: result: $pgac_cv__va_args" >&5
+$as_echo "$pgac_cv__va_args" >&6; }
+if test x"$pgac_cv__va_args" = xyes ; then
+
+cat >>confdefs.h <<\_ACEOF
+#define HAVE__VA_ARGS 1
+_ACEOF
+
 fi
 { $as_echo "$as_me:$LINENO: checking whether struct tm is in sys/time.h or time.h" >&5
 $as_echo_n "checking whether struct tm is in sys/time.h or time.h... " >&6; }

configure.in

@@ -1106,6 +1106,9 @@ AC_C_VOLATILE
 PGAC_C_FUNCNAME_SUPPORT
 PGAC_C_STATIC_ASSERT
 PGAC_C_TYPES_COMPATIBLE
+PGAC_C_BUILTIN_CONSTANT_P
+PGAC_C_BUILTIN_UNREACHABLE
+PGAC_C_VA_ARGS
 PGAC_STRUCT_TIMEZONE
 PGAC_UNION_SEMUN
 PGAC_STRUCT_SOCKADDR_UN

contrib/cube/cubescan.l

@@ -11,7 +11,13 @@
 
 /* Avoid exit() on fatal scanner errors (a bit ugly -- see yy_fatal_error) */
 #undef fprintf
-#define fprintf(file, fmt, msg)  ereport(ERROR, (errmsg_internal("%s", msg)))
+#define fprintf(file, fmt, msg)  fprintf_to_ereport(fmt, msg)
+
+static void
+fprintf_to_ereport(const char *fmt, const char *msg)
+{
+	ereport(ERROR, (errmsg_internal("%s", msg)));
+}
 
 /* Handles to the buffer that the lexer uses internally */
 static YY_BUFFER_STATE scanbufhandle;

contrib/seg/segscan.l

@@ -10,7 +10,13 @@
 
 /* Avoid exit() on fatal scanner errors (a bit ugly -- see yy_fatal_error) */
 #undef fprintf
-#define fprintf(file, fmt, msg)  ereport(ERROR, (errmsg_internal("%s", msg)))
+#define fprintf(file, fmt, msg)  fprintf_to_ereport(fmt, msg)
+
+static void
+fprintf_to_ereport(const char *fmt, const char *msg)
+{
+	ereport(ERROR, (errmsg_internal("%s", msg)));
+}
 
 /* Handles to the buffer that the lexer uses internally */
 static YY_BUFFER_STATE scanbufhandle;

src/backend/bootstrap/bootscanner.l

@@ -42,7 +42,13 @@
 
 /* Avoid exit() on fatal scanner errors (a bit ugly -- see yy_fatal_error) */
 #undef fprintf
-#define fprintf(file, fmt, msg)  ereport(ERROR, (errmsg_internal("%s", msg)))
+#define fprintf(file, fmt, msg)  fprintf_to_ereport(fmt, msg)
+
+static void
+fprintf_to_ereport(const char *fmt, const char *msg)
+{
+	ereport(ERROR, (errmsg_internal("%s", msg)));
+}
 
 
 static int	yyline = 1;			/* line number for error reporting */

src/backend/parser/scan.l

@@ -42,7 +42,13 @@
 
 /* Avoid exit() on fatal scanner errors (a bit ugly -- see yy_fatal_error) */
 #undef fprintf
-#define fprintf(file, fmt, msg)  ereport(ERROR, (errmsg_internal("%s", msg)))
+#define fprintf(file, fmt, msg)  fprintf_to_ereport(fmt, msg)
+
+static void
+fprintf_to_ereport(const char *fmt, const char *msg)
+{
+	ereport(ERROR, (errmsg_internal("%s", msg)));
+}
 
 /*
  * GUC variables.  This is a DIRECT violation of the warning given at the

src/backend/replication/repl_scanner.l

@@ -19,7 +19,13 @@
 
 /* Avoid exit() on fatal scanner errors (a bit ugly -- see yy_fatal_error) */
 #undef fprintf
-#define fprintf(file, fmt, msg)  ereport(ERROR, (errmsg_internal("%s", msg)))
+#define fprintf(file, fmt, msg)  fprintf_to_ereport(fmt, msg)
+
+static void
+fprintf_to_ereport(const char *fmt, const char *msg)
+{
+	ereport(ERROR, (errmsg_internal("%s", msg)));
+}
 
 /* Handle to the buffer that the lexer uses internally */
 static YY_BUFFER_STATE scanbufhandle;

src/include/c.h

@@ -748,6 +748,18 @@ typedef NameData *Name;
 #endif /* HAVE__BUILTIN_TYPES_COMPATIBLE_P */
 
 
+/*
+ * Mark a point as unreachable in a portable fashion.  This should preferably
+ * be something that the compiler understands, to aid code generation.
+ * In assert-enabled builds, we prefer abort() for debugging reasons.
+ */
+#if defined(HAVE__BUILTIN_UNREACHABLE) && !defined(USE_ASSERT_CHECKING)
+#define pg_unreachable() __builtin_unreachable()
+#else
+#define pg_unreachable() abort()
+#endif
+
+
 /*
  * Function inlining support -- Allow modules to define functions that may be
  * inlined, if the compiler supports it.

src/include/pg_config.h.in

@@ -635,12 +635,21 @@
 /* Define to 1 if you have the <winldap.h> header file. */
 #undef HAVE_WINLDAP_H
 
+/* Define to 1 if your compiler understands __builtin_constant_p. */
+#undef HAVE__BUILTIN_CONSTANT_P
+
 /* Define to 1 if your compiler understands __builtin_types_compatible_p. */
 #undef HAVE__BUILTIN_TYPES_COMPATIBLE_P
 
+/* Define to 1 if your compiler understands __builtin_unreachable. */
+#undef HAVE__BUILTIN_UNREACHABLE
+
 /* Define to 1 if your compiler understands _Static_assert. */
 #undef HAVE__STATIC_ASSERT
 
+/* Define to 1 if your compiler understands __VA_ARGS__ in macros. */
+#undef HAVE__VA_ARGS
+
 /* Define to the appropriate snprintf format for 64-bit ints. */
 #undef INT64_FORMAT