From 403a8c76293bdef17bf5ee8af6be92ee4eba6bb1 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Sat, 26 Mar 2022 14:29:29 -0400
Subject: [PATCH] Suppress compiler warning in relptr_store().
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
---
src/include/utils/relptr.h | 17 +++++++++++++++--
1 file changed, 15 insertions(+), 2 deletions(-)
diff --git a/src/include/utils/relptr.h b/src/include/utils/relptr.h
index 06e592e0c64..6d49d58ca6c 100644
--- a/src/include/utils/relptr.h
+++ b/src/include/utils/relptr.h
@@ -56,11 +56,24 @@
#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
@@ -68,7 +81,7 @@
*/
#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) \
--
2.39.5