DWARF location descriptor and multi-register frame pointer

[Senthil Kumar Selvaraj <senthil_kumar dot selvaraj at atmel dot com>]

Hi all,

 For the AVR target, the FP register spans two registers, yet the DWARF
 location information (DW_AT_location) for a local variable refers to only one 
 of them.

 Looking at the code, I found that based_loc_descr in dwar2out.c, which handles
 base+offset based location descriptions, gets the register number from
 the rtx representing the reg and directly proceeds to create DW_OP_breg0+regno,
 without considering the mode. This is unlike reg_loc_descriptor, which
 checks hard_regno_nregs[regno][mode] and acts accordingly.

 How are frame pointer registers that span more than one hard register
 handled? Would it be appropriate to check the mode and do a
 multiple_reg_loc_descriptor call or something similar to handle this
 case?

 A related question - why does based_loc_descr generate DW_OP_fbreg 
 only if eliminate_regs eliminates the frame/arg pointer register? I
 traced through the execution for my native (x86_64) compiler, and found
 that the elimination is actually from the soft frame pointer to the hard frame
 pointer. What if HARD_FRAME_POINTER_REGNUM is the same as
 FRAME_POINTER_REGNUM? Shouldn't DW_OP_fbreg be generated for those
 targets too?

 Regards
 Senthil