Prior patches in this series were small bug fixes. This lays out the ground rules for following patches.
* config/rs6000/rs6000.c (rs6000_rtx_costs): Expand comment. diff --git a/gcc/config/rs6000/rs6000.c b/gcc/config/rs6000/rs6000.c index 523d029800a..5b3c0ee0e8c 100644 --- a/gcc/config/rs6000/rs6000.c +++ b/gcc/config/rs6000/rs6000.c @@ -21133,7 +21133,45 @@ rs6000_cannot_copy_insn_p (rtx_insn *insn) /* Compute a (partial) cost for rtx X. Return true if the complete cost has been computed, and false if subexpressions should be - scanned. In either case, *TOTAL contains the cost result. */ + scanned. In either case, *TOTAL contains the cost result. + + 1) Calls from places like optabs.c:avoid_expensive_constant will + come here with OUTER_CODE set to an operation such as AND with X + being a CONST_INT or other CONSTANT_P type. This will be compared + against set_src_cost, where we'll come here with OUTER_CODE as SET + and X the same constant. + + 2) Calls from places like combine:distribute_and_simplify_rtx are + asking whether a possibly quite complex SET_SRC can be implemented + more cheaply than some other logically equivalent SET_SRC. + + 3) Calls from places like default_noce_conversion_profitable_p will + come here via seq_cost and pass the pattern of a SET insn in X. + Presuming the insn is valid and set_dest a reg, rs6000_rtx_costs + will next see the SET_SRC. The overall cost should be comparable + to rs6000_insn_cost since the code is comparing one insn sequence + (some of which may be costed by insn_cost) against another insn + sequence. + + 4) Calls from places like cprop.c:try_replace_reg will come here + with OUTER_CODE as INSN, and X either a valid pattern of a SET or + one where some registers have been replaced with constants. The + replacements may make the SET invalid, for example if + (set (reg1) (and (reg2) (const_int 0xfff))) + replaces reg2 as + (set (reg1) (and (symbol_ref) (const_int 0xfff))) + then the replacement can't be implemented in one instruction and + really the cost should be higher by one instruction. However, + the cost for invalid insns doesn't matter much except that a + higher cost may lead to their rejection earlier. + + 5) fwprop.c:should_replace_address puts yet another wrinkle on this + function, where we prefer an address calculation that is more + complex yet has the same address_cost. In this case "more + complex" is determined by having a higher set_src_cost. So for + example, if we want a plain (reg) address to be replaced with + (plus (reg) (const)) when possible then PLUS needs to cost more + than zero here. */ static bool rs6000_rtx_costs (rtx x, machine_mode mode, int outer_code,