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Suppress compiler warning in relptr_store().

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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
This commit is contained in:
Tom Lane
2022-03-26 14:29:29 -04:00
parent d09f765b9d
commit cb8586d051
1 changed files with 15 additions and 2 deletions
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17
src/include/utils/relptr.h
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@@ -56,11 +56,24 @@
#define relptr_is_null(rp) \ #define relptr_is_null(rp) \
((rp).relptr_off == 0) ((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 #ifdef HAVE__BUILTIN_TYPES_COMPATIBLE_P
#define relptr_store(base, rp, val) \ #define relptr_store(base, rp, val) \
(AssertVariableIsOfTypeMacro(base, char *), \ (AssertVariableIsOfTypeMacro(base, char *), \
AssertVariableIsOfTypeMacro(val, __typeof__((rp).relptr_type)), \ 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 #else
/* /*
* If we don't have __builtin_types_compatible_p, assume we might not have * If we don't have __builtin_types_compatible_p, assume we might not have
@@ -68,7 +81,7 @@
*/ */
#define relptr_store(base, rp, val) \ #define relptr_store(base, rp, val) \
(AssertVariableIsOfTypeMacro(base, char *), \ (AssertVariableIsOfTypeMacro(base, char *), \
(rp).relptr_off = ((val) == NULL ? 0 : ((char *) (val)) - (base))) (rp).relptr_off = relptr_store_eval(base, (char *) (val)))
#endif #endif
#define relptr_copy(rp1, rp2) \ #define relptr_copy(rp1, rp2) \