* arm-tdep.c (thumb_analyze_prologue): Always fallback on the SP
register when the frame can't be determined.
* arm-tdep.c (arm_analyze_prologue): Ditto.
gdb/testsuite/
* gdb.arch/thumb-prologue.c (switch_stack_to_same): New test function.
(switch_stack_to_other): New test function.
* gdb.arch/thumb-prologue.exp: New test cases.
* arm-tdep.c (arm_frame_is_thumb): New.
(arm_pc_is_thumb): Clarify comment.
(thumb_analyze_prologue): Remove PC special case.
(thumb_scan_prologue): Take a block_addr argument. Use it for
find_pc_partial_function. Remove unused variables.
(arm_scan_prologue): Use arm_frame_is_thumb. Use the block address
for find_pc_partial_function. Remove PC special case.
(arm_prologue_prev_register): Add special handling for PC and CPSR.
(arm_dwarf2_prev_register, arm_dwarf2_frame_init_reg): New.
(arm_get_next_pc): Use arm_frame_is_thumb.
(arm_write_pc): Use CPSR_T instead of 0x20.
(arm_gdbarch_init): Call dwarf2_frame_set_init_reg.
* arm-tdep.h (enum gdb_regnum): Add ARM_CPSR_REGNUM.
(CPSR_T): Define.
* dwarf2-frame.c (dwarf2_frame_prev_register): Handle
DWARF2_FRAME_REG_FN.
* dwarf2-frame.h (enum dwarf2_frame_reg_rule): Add
DWARF2_FRAME_REG_FN.
(struct dwarf2_frame_state_reg): Add FN to loc union.
* gdb.arch/thumb-prologue.exp: Do not expect a saved PC.
