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It appears that commit cf63c641c, 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
src/backend/parser/README Parser ====== This directory does more than tokenize and parse SQL queries. It also creates Query structures for the various complex queries that are passed to the optimizer and then executor. parser.c things start here scan.l break query into tokens scansup.c handle escapes in input strings gram.y parse the tokens and produce a "raw" parse tree analyze.c top level of parse analysis for optimizable queries parse_agg.c handle aggregates, like SUM(col1), AVG(col2), ... parse_clause.c handle clauses like WHERE, ORDER BY, GROUP BY, ... parse_coerce.c handle coercing expressions to different data types parse_collate.c assign collation information in completed expressions parse_cte.c handle Common Table Expressions (WITH clauses) parse_expr.c handle expressions like col, col + 3, x = 3 or x = 4 parse_func.c handle functions, table.column and column identifiers parse_node.c create nodes for various structures parse_oper.c handle operators in expressions parse_param.c handle Params (for the cases used in the core backend) parse_relation.c support routines for tables and column handling parse_target.c handle the result list of the query parse_type.c support routines for data type handling parse_utilcmd.c parse analysis for utility commands (done at execution time) See also src/common/keywords.c, which contains the table of standard keywords and the keyword lookup function. We separated that out because various frontend code wants to use it too.