Suppress warning about stack_base_ptr with late-model GCC.

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

src/backend/postmaster/postmaster.c

@@ -1033,7 +1033,7 @@ 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

src/backend/tcop/postgres.c

@@ -3415,7 +3415,9 @@ 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)
@@ -3425,8 +3427,16 @@ 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

src/backend/utils/init/miscinit.c

@@ -95,13 +95,12 @@ InitPostmasterChild(void)
 	IsUnderPostmaster = true;	/* we are a postmaster subprocess now */
 
 	/*
-	 * Set reference point for stack-depth checking. We re-do that even in the
-	 * !EXEC_BACKEND case, because there are some edge cases where processes
-	 * are started with an alternative stack (e.g. starting bgworkers when
-	 * running postgres using the rr debugger, as bgworkers are launched from
-	 * signal handlers).
+	 * 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.
 	 */
-	set_stack_base();
+	(void) set_stack_base();
 
 	InitProcessGlobals();
 

src/include/pg_config.h.in

@@ -745,6 +745,9 @@
 /* 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
 

src/tools/msvc/Solution.pm

@@ -441,6 +441,7 @@ sub GenerateFiles
 		HAVE__BUILTIN_CLZ                        => undef,
 		HAVE__BUILTIN_CONSTANT_P                 => undef,
 		HAVE__BUILTIN_CTZ                        => undef,
+		HAVE__BUILTIN_FRAME_ADDRESS              => undef,
 		HAVE__BUILTIN_OP_OVERFLOW                => undef,
 		HAVE__BUILTIN_POPCOUNT                   => undef,
 		HAVE__BUILTIN_TYPES_COMPATIBLE_P         => undef,