Suppress warning about stack_base_ptr with late-model GCC.

tglsfdc · tglsfdc · commit e0d4123bbcfe · 2022-02-17T22:45:34.000-05:00
GCC 12 complains that set_stack_base is storing the address of a local variable in a long-lived pointer. This is an entirely reasonable warning (indeed, it just helped us find a bug); but that behavior is intentional here. We can work around it by using __builtin_frame_address(0) instead of a specific local variable; that produces an address a dozen or so bytes different, in my testing, but we don't care about such a small difference. Maybe someday a compiler lacking that function will start to issue a similar warning, but we'll worry about that when it happens. Patch by me, per a suggestion from Andres Freund. Back-patch to v12, which is as far back as the patch will go without some pain. (Recently-established project policy would permit a back-patch as far as 9.2, but I'm disinclined to expend the work until GCC 12 is much more widespread.) Discussion: https://postgr.es/m/3773792.1645141467@sss.pgh.pa.us
diff --git a/config/c-compiler.m4 b/config/c-compiler.m4
@@ -396,6 +396,28 @@ fi])# PGAC_CHECK_BUILTIN_FUNC
 
 
 
+# PGAC_CHECK_BUILTIN_FUNC_PTR
+# -----------------------
+# Like PGAC_CHECK_BUILTIN_FUNC, except that the function is assumed to
+# return a pointer type, and the argument(s) should be given literally.
+# This handles some cases that PGAC_CHECK_BUILTIN_FUNC doesn't.
+AC_DEFUN([PGAC_CHECK_BUILTIN_FUNC_PTR],
+[AC_CACHE_CHECK(for $1, pgac_cv$1,
+[AC_LINK_IFELSE([AC_LANG_PROGRAM([
+void *
+call$1(void)
+{
+    return $1($2);
+}], [])],
+[pgac_cv$1=yes],
+[pgac_cv$1=no])])
+if test x"${pgac_cv$1}" = xyes ; then
+AC_DEFINE_UNQUOTED(AS_TR_CPP([HAVE$1]), 1,
+                   [Define to 1 if your compiler understands $1.])
+fi])# PGAC_CHECK_BUILTIN_FUNC_PTR
+
+
+
 # PGAC_PROG_VARCC_VARFLAGS_OPT
 # -----------------------
 # Given a compiler, variable name and a string, check if the compiler
diff --git a/configure b/configure
@@ -15763,6 +15763,46 @@ cat >>confdefs.h <<_ACEOF
 #define HAVE__BUILTIN_POPCOUNT 1
 _ACEOF
 
+fi
+# __builtin_frame_address may draw a diagnostic for non-constant argument,
+# so it needs a different test function.
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for __builtin_frame_address" >&5
+$as_echo_n "checking for __builtin_frame_address... " >&6; }
+if ${pgac_cv__builtin_frame_address+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+void *
+call__builtin_frame_address(void)
+{
+    return __builtin_frame_address(0);
+}
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_link "$LINENO"; then :
+  pgac_cv__builtin_frame_address=yes
+else
+  pgac_cv__builtin_frame_address=no
+fi
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext conftest.$ac_ext
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv__builtin_frame_address" >&5
+$as_echo "$pgac_cv__builtin_frame_address" >&6; }
+if test x"${pgac_cv__builtin_frame_address}" = xyes ; then
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE__BUILTIN_FRAME_ADDRESS 1
+_ACEOF
+
 fi
 
 ac_fn_c_check_func "$LINENO" "fseeko" "ac_cv_func_fseeko"
diff --git a/configure.in b/configure.in
@@ -1724,6 +1724,9 @@ PGAC_CHECK_BUILTIN_FUNC([__builtin_bswap64], [long int x])
 PGAC_CHECK_BUILTIN_FUNC([__builtin_clz], [unsigned int x])
 PGAC_CHECK_BUILTIN_FUNC([__builtin_ctz], [unsigned int x])
 PGAC_CHECK_BUILTIN_FUNC([__builtin_popcount], [unsigned int x])
+# __builtin_frame_address may draw a diagnostic for non-constant argument,
+# so it needs a different test function.
+PGAC_CHECK_BUILTIN_FUNC_PTR([__builtin_frame_address], [0])
 
 AC_REPLACE_FUNCS(fseeko)
 case $host_os in
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
@@ -1207,7 +1207,7 @@ PostmasterMain(int argc, char *argv[])
 	/*
 	 * Set reference point for stack-depth checking.
 	 */
-	set_stack_base();
+	(void) set_stack_base();
 
 	/*
 	 * Initialize pipe (or process handle on Windows) that allows children to
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
@@ -3210,7 +3210,9 @@ ia64_get_bsp(void)
 pg_stack_base_t
 set_stack_base(void)
 {
+#ifndef HAVE__BUILTIN_FRAME_ADDRESS
 	char		stack_base;
+#endif
 	pg_stack_base_t old;
 
 #if defined(__ia64__) || defined(__ia64)
@@ -3220,8 +3222,16 @@ set_stack_base(void)
 	old = stack_base_ptr;
 #endif
 
-	/* Set up reference point for stack depth checking */
+	/*
+	 * Set up reference point for stack depth checking.  On recent gcc we use
+	 * __builtin_frame_address() to avoid a warning about storing a local
+	 * variable's address in a long-lived variable.
+	 */
+#ifdef HAVE__BUILTIN_FRAME_ADDRESS
+	stack_base_ptr = __builtin_frame_address(0);
+#else
 	stack_base_ptr = &stack_base;
+#endif
 #if defined(__ia64__) || defined(__ia64)
 	register_stack_base_ptr = ia64_get_bsp();
 #endif
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
@@ -274,6 +274,14 @@ InitPostmasterChild(void)
 {
 	IsUnderPostmaster = true;	/* we are a postmaster subprocess now */
 
+	/*
+	 * Set reference point for stack-depth checking.  This might seem
+	 * redundant in !EXEC_BACKEND builds; but it's not because the postmaster
+	 * launches its children from signal handlers, so we might be running on
+	 * an alternative stack.
+	 */
+	(void) set_stack_base();
+
 	InitProcessGlobals();
 
 	/*
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
@@ -757,6 +757,9 @@
 /* Define to 1 if your compiler understands __builtin_ctz. */
 #undef HAVE__BUILTIN_CTZ
 
+/* Define to 1 if your compiler understands __builtin_frame_address. */
+#undef HAVE__BUILTIN_FRAME_ADDRESS
+
 /* Define to 1 if your compiler understands __builtin_$op_overflow. */
 #undef HAVE__BUILTIN_OP_OVERFLOW
 
