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

tglsfdc · tglsfdc · commit 9f3d3fb8702b · 2020-10-07T18:42:30.000-04:00
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
@@ -15409,7 +15409,7 @@ 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)
@@ -15433,10 +15433,10 @@ 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
`@@ -15409,7 +15409,7 @@ makeOrderedSetArgs(List directargs, List orderedargs,`
`15409`	`15409`	`core_yyscan_t yyscanner)`
`15410`	`15410`	`{`
`15411`	`15411`	`FunctionParameter lastd = (FunctionParameter ) llast(directargs);`
`15412`		`- int ndirectargs;`
	`15412`	`+ Value *ndirectargs;`
`15413`	`15413`
`15414`	`15414`	`/* No restriction unless last direct arg is VARIADIC */`
`15415`	`15415`	`if (lastd->mode == FUNC_PARAM_VARIADIC)`
`@@ -15433,10 +15433,10 @@ makeOrderedSetArgs(List directargs, List orderedargs,`
`15433`	`15433`	`}`
`15434`	`15434`
`15435`	`15435`	`/* don't merge into the next line, as list_concat changes directargs */`
`15436`		`- ndirectargs = list_length(directargs);`
	`15436`	`+ ndirectargs = makeInteger(list_length(directargs));`
`15437`	`15437`
`15438`	`15438`	`return list_make2(list_concat(directargs, orderedargs),`
`15439`		`- makeInteger(ndirectargs));`
	`15439`	`+ ndirectargs);`
`15440`	`15440`	`}`
`15441`	`15441`
`15442`	`15442`	`/* insertSelectOptions()`