forked from Mirrors/opensbi

Currently, the low-level trap handler (i.e. _trap_handler()) uses branch instructions to conditionally setup exception stack based on which mode trap occured. This patch implements exception stack setup using xor instructions which is faster with same number of instructions due to lack of branch instructions. The new exception stack setup approach can be best described by the following pseudocode: Came_From_M_Mode = ((MSTATUS.MPP < PRV_M) ? 1 : 0) - 1; Exception_Stack = TP ^ (Came_From_M_Mode & (SP ^ TP)) Came_From_M_Mode = 0 ==> Exception_Stack = TP Came_From_M_Mode = -1 ==> Exception_Stack = SP Signed-off-by: Anup Patel <anup.patel@wdc.com> Reviewed-by: Atish Patra <atish.patra@wdc.com>