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Suppress compiler warning in relptr_store().

clang 13 with -Wextra warns that "performing pointer subtraction witha null pointer has undefined behavior" in the places where freepage.ctries to set a relptr variable to constant NULL. This appears to bea compiler bug, but it's unlikely to get fixed instantly. Fortunately,we can work around it by introducing an inline support function, whichseems like a good change anyway because it removes the macro's existingdouble-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

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src/include/utils
- relptr.h

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`‎src/include/utils/relptr.h`

Lines changed: 15 additions & 2 deletions

Original file line number	Diff line number	Diff line change
`@@ -56,19 +56,32 @@`
`56`	`56`	`#definerelptr_is_null(rp) \`
`57`	`57`	`((rp).relptr_off == 0)`
`58`	`58`
	`59`	`+/* We use this inline to avoid double eval of "val" in relptr_store */`
	`60`	`+staticinlineSize`
	`61`	`+relptr_store_eval(charbase,charval)`
	`62`	`+{`
	`63`	`+if (val==NULL)`
	`64`	`+return0;`
	`65`	`+else`
	`66`	`+{`
	`67`	`+Assert(val>base);`
	`68`	`+returnval-base;`
	`69`	`+}`
	`70`	`+}`
	`71`	`+`
`59`	`72`	`#ifdefHAVE__BUILTIN_TYPES_COMPATIBLE_P`
`60`	`73`	`#definerelptr_store(base,rp,val) \`
`61`	`74`	`(AssertVariableIsOfTypeMacro(base, char *), \`
`62`	`75`	`AssertVariableIsOfTypeMacro(val, __typeof__((rp).relptr_type)), \`
`63`		`- (rp).relptr_off =((val) == NULL ? 0 : ((char *) (val)) - (base)))`
	`76`	`+ (rp).relptr_off =relptr_store_eval(base, (char *) (val)))`
`64`	`77`	`#else`
`65`	`78`	`/*`
`66`	`79`	`* If we don't have __builtin_types_compatible_p, assume we might not have`
`67`	`80`	`* __typeof__ either.`
`68`	`81`	`*/`
`69`	`82`	`#definerelptr_store(base,rp,val) \`
`70`	`83`	`(AssertVariableIsOfTypeMacro(base, char *), \`
`71`		`- (rp).relptr_off =((val) == NULL ? 0 : ((char *) (val)) - (base)))`
	`84`	`+ (rp).relptr_off =relptr_store_eval(base, (char *) (val)))`
`72`	`85`	`#endif`
`73`	`86`
`74`	`87`	`#definerelptr_copy(rp1,rp2) \`

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`‎src/include/utils/relptr.h`

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