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

tglsfdc · tglsfdc · commit dc14aa038e20 · 2020-10-07T18:42:02.000-04:00
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
@@ -16075,7 +16075,7 @@ 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)
@@ -16099,10 +16099,10 @@ 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
`@@ -16075,7 +16075,7 @@ makeOrderedSetArgs(List directargs, List orderedargs,`
`16075`	`16075`	`core_yyscan_t yyscanner)`
`16076`	`16076`	`{`
`16077`	`16077`	`FunctionParameter lastd = (FunctionParameter ) llast(directargs);`
`16078`		`- int ndirectargs;`
	`16078`	`+ Value *ndirectargs;`
`16079`	`16079`
`16080`	`16080`	`/* No restriction unless last direct arg is VARIADIC */`
`16081`	`16081`	`if (lastd->mode == FUNC_PARAM_VARIADIC)`
`@@ -16099,10 +16099,10 @@ makeOrderedSetArgs(List directargs, List orderedargs,`
`16099`	`16099`	`}`
`16100`	`16100`
`16101`	`16101`	`/* don't merge into the next line, as list_concat changes directargs */`
`16102`		`- ndirectargs = list_length(directargs);`
	`16102`	`+ ndirectargs = makeInteger(list_length(directargs));`
`16103`	`16103`
`16104`	`16104`	`return list_make2(list_concat(directargs, orderedargs),`
`16105`		`- makeInteger(ndirectargs));`
	`16105`	`+ ndirectargs);`
`16106`	`16106`	`}`
`16107`	`16107`
`16108`	`16108`	`/* insertSelectOptions()`