http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56124
--- Comment #1 from bin.cheng <amker.cheng at gmail dot com> 2013-01-28
02:43:10 UTC ---
The root cause is in ira:scan_one_insn function.
It decrease cost of memory for pseudo which are target of loading from memory:
if (set != 0 && REG_P (SET_DEST (set)) && MEM_P (SET_SRC (set))
&& (note = find_reg_note (insn, REG_EQUIV, NULL_RTX)) != NULL_RTX
&& ((MEM_P (XEXP (note, 0)))
|| (CONSTANT_P (XEXP (note, 0))
&& targetm.legitimate_constant_p (GET_MODE (SET_DEST (set)),
XEXP (note, 0))
&& REG_N_SETS (REGNO (SET_DEST (set))) == 1))
&& general_operand (SET_SRC (set), GET_MODE (SET_SRC (set))))
{
enum reg_class cl = GENERAL_REGS;
rtx reg = SET_DEST (set);
int num = COST_INDEX (REGNO (reg));
COSTS (costs, num)->mem_cost
-= ira_memory_move_cost[GET_MODE (reg)][cl][1] * frequency;
record_address_regs (GET_MODE (SET_SRC (set)),
MEM_ADDR_SPACE (SET_SRC (set)),
XEXP (SET_SRC (set), 0), 0, MEM, SCRATCH,
frequency * 2);
counted_mem = true;
}
The problem is if the src memory rtx (like in insn 2) has side effect, the orig
load insn won't be eliminated and causes redundant reload.
Patch will be sent for review.
