Fix optimization hazard in gram.y's makeOrderedSetArgs(), redux.

tglsfdc · tglsfdc · commit 8d2a01ae12cd · 2020-10-07T18:41:39.000-04:00
It appears that commit cf63c64, which intended to prevent misoptimization of the result-building step in makeOrderedSetArgs, didn't go far enough: buildfarm member hornet's version of xlc is now optimizing back to the old, broken behavior in which list_length(directargs) is fetched only after list_concat() has changed that value. I'm not entirely convinced whether that's an undeniable compiler bug or whether it can be justified by a sufficiently aggressive interpretation of C sequence points. So let's just change the code to make it harder to misinterpret. Back-patch to all supported versions, just in case. Discussion: https://postgr.es/m/1830491.1601944935@sss.pgh.pa.us
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
@@ -16291,7 +16291,7 @@ makeOrderedSetArgs(List *directargs, List *orderedargs,
 				   core_yyscan_t yyscanner)
 {
 	FunctionParameter *lastd = (FunctionParameter *) llast(directargs);
-	int			ndirectargs;
+	Value	   *ndirectargs;
 
 	/* No restriction unless last direct arg is VARIADIC */
 	if (lastd->mode == FUNC_PARAM_VARIADIC)
@@ -16315,10 +16315,10 @@ makeOrderedSetArgs(List *directargs, List *orderedargs,
 	}
 
 	/* don't merge into the next line, as list_concat changes directargs */
-	ndirectargs = list_length(directargs);
+	ndirectargs = makeInteger(list_length(directargs));
 
 	return list_make2(list_concat(directargs, orderedargs),
-					  makeInteger(ndirectargs));
+					  ndirectargs);
 }
 
 /* insertSelectOptions()

Original file line number	Diff line number	Diff line change
`@@ -16291,7 +16291,7 @@ makeOrderedSetArgs(List directargs, List orderedargs,`
`16291`	`16291`	`core_yyscan_t yyscanner)`
`16292`	`16292`	`{`
`16293`	`16293`	`FunctionParameter lastd = (FunctionParameter ) llast(directargs);`
`16294`		`- int ndirectargs;`
	`16294`	`+ Value *ndirectargs;`
`16295`	`16295`
`16296`	`16296`	`/* No restriction unless last direct arg is VARIADIC */`
`16297`	`16297`	`if (lastd->mode == FUNC_PARAM_VARIADIC)`
`@@ -16315,10 +16315,10 @@ makeOrderedSetArgs(List directargs, List orderedargs,`
`16315`	`16315`	`}`
`16316`	`16316`
`16317`	`16317`	`/* don't merge into the next line, as list_concat changes directargs */`
`16318`		`- ndirectargs = list_length(directargs);`
	`16318`	`+ ndirectargs = makeInteger(list_length(directargs));`
`16319`	`16319`
`16320`	`16320`	`return list_make2(list_concat(directargs, orderedargs),`
`16321`		`- makeInteger(ndirectargs));`
	`16321`	`+ ndirectargs);`
`16322`	`16322`	`}`
`16323`	`16323`
`16324`	`16324`	`/* insertSelectOptions()`