From 0144c9c7e73dc75f9cd5297c7c380d55927beb75 Mon Sep 17 00:00:00 2001 From: Tom Lane 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 55965f28d5a..e71c7d0bdca 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) \