On Tue, Dec 04, 2012 at 12:59:11PM +0530, Senthil Kumar Selvaraj wrote:
> As a follow-up to this discussion
> (http://gcc.gnu.org/ml/gcc/2012-11/msg00307.html), I have a patch that
> makes dwarf2out.c emit correct location information for a FP register spanning
> multiple hard regs.
>
> Before the patch, the DW_AT_location for a local variable on the AVR
> target (whose FP spans registers R29 and R28) looks like this
> <4f> DW_AT_location : 2 byte block: 8c 1 (DW_OP_breg28 (r28): 1)
>
> After the patch, it looks like this
> <4f> DW_AT_location : 11 byte block: 6c 93 1 6d 93 1 11 1 22 94 2
> (DW_OP_reg28 (r28); DW_OP_piece: 1; DW_OP_reg29 (r29); DW_OP_piece: 1;
> DW_OP_consts: 1; DW_OP_plus; DW_OP_deref_size: 2)
>
> What do you guys think?
That is invalid DWARF.
Most DW_OP_* opcodes are usable in DWARF expressions, but not all of them,
and DW_OP_reg0..DW_OP_reg31/DW_OP_regx/DW_OP_piece/DW_OP_bit_piece
/DW_OP_implicit_value/DW_OP_stack_value/DW_OP_GNU_implicit_pointer
are the exceptions that can't be used in DWARF expressions.
Then there are DWARF location descriptions, which can be:
1) memory location descriptions (a sequence of DWARF expression opcodes)
2) register location descriptions (one of DW_OP_reg0..DW_OP_reg31 or DW_OP_regx)
3) implicit location descriptions, either DW_OP_implicit_value alone, or
DWARF expression opcodes followed by DW_OP_stack_value
4) implicit pointer, DW_OP_GNU_implicit_pointer
5) composite location description (a series of 1), 2), 3) or 4) followed
by DW_OP_piece or DW_OP_bit_piece
So, you can't put any DWARF expression opcodes after DW_OP_reg* or
DW_OP_piece. If your registers are 8-bit, but pointers 16-bit, the question
is what the consumers actually push into the DWARF stack on say DW_OP_bregx,
if they push just one 8-bit register (and, is it zero or sign extending?),
then perhaps you could emit for say based_loc_register register 4 with
offset 16 if register 4 is 8-bit something like (assuming little endian):
DW_OP_breg5 <0> DW_OP_lit8 DW_OP_shl DW_OP_breg4 <0> DW_OP_const1u <0xff>
DW_OP_and DW_OP_plus DW_OP_plus_uconst <offset> (for positive offset, for
negative offset you'd want int_loc_descriptor (offset) followed by DW_OP_plus.
That will put there (reg5<<8)+(reg4&0xff)+offset.
DW_OP_deref* shouldn't be used, DWARF expression is simply a memory location
description ( 1) above) by default.
Jakub
