Suppress compiler warning in relptr_store().

tglsfdc · tglsfdc · commit e07d4ddc55fd · 2022-03-26T14:30:09.000-04:00
clang 13 with -Wextra warns that "performing pointer subtraction with a null pointer has undefined behavior" in the places where freepage.c tries to set a relptr variable to constant NULL. This appears to be a compiler bug, but it's unlikely to get fixed instantly. Fortunately, we can work around it by introducing an inline support function, which seems like a good change anyway because it removes the macro's existing double-evaluation hazard. Backpatch to v10 where this code was introduced. Patch by me, based on an idea of Andres Freund's. Discussion: https://postgr.es/m/48826.1648310694@sss.pgh.pa.us
diff --git a/src/include/utils/relptr.h b/src/include/utils/relptr.h
@@ -56,19 +56,32 @@
 #define relptr_is_null(rp) \
 	((rp).relptr_off == 0)
 
+/* We use this inline to avoid double eval of "val" in relptr_store */
+static inline Size
+relptr_store_eval(char *base, char *val)
+{
+	if (val == NULL)
+		return 0;
+	else
+	{
+		Assert(val > base);
+		return val - base;
+	}
+}
+
 #ifdef HAVE__BUILTIN_TYPES_COMPATIBLE_P
 #define relptr_store(base, rp, val) \
 	(AssertVariableIsOfTypeMacro(base, char *), \
 	 AssertVariableIsOfTypeMacro(val, __typeof__((rp).relptr_type)), \
-	 (rp).relptr_off = ((val) == NULL ? 0 : ((char *) (val)) - (base)))
+	 (rp).relptr_off = relptr_store_eval(base, (char *) (val)))
 #else
 /*
  * If we don't have __builtin_types_compatible_p, assume we might not have
  * __typeof__ either.
  */
 #define relptr_store(base, rp, val) \
 	(AssertVariableIsOfTypeMacro(base, char *), \
-	 (rp).relptr_off = ((val) == NULL ? 0 : ((char *) (val)) - (base)))
+	 (rp).relptr_off = relptr_store_eval(base, (char *) (val)))
 #endif
 
 #define relptr_copy(rp1, rp2) \
