Re: [PATCH] Fix bug in DW_OP_GNU_variable_value evaluation

[Kevin Buettner <kevinb@redhat.com>]

On Mon, 27 Jul 2020 14:46:44 -0600
Tom Tromey <tromey@adacore.com> wrote:

> A modified version of the gnat compiler (TBH I don't know if the
> modifications are relevant to this bug or not, but I figured I'd
> mention it) can generate a DWARF location expression like:
> 
>  <1><1201>: Abbrev Number: 3 (DW_TAG_dwarf_procedure)
>     <1202>   DW_AT_location    : 32 byte block: 12 31 29 28 4 0 30 2f 12 0 14 30 2d 28 4 0 14 2f 1 0 30 34 1e 23 3 9 fc 1a 16 13 16 13 	(DW_OP_dup; DW_OP_lit1; DW_OP_eq; DW_OP_bra: 4; DW_OP_lit0; DW_OP_skip: 18; DW_OP_over; DW_OP_lit0; DW_OP_lt; DW_OP_bra: 4; DW_OP_over; DW_OP_skip: 1; DW_OP_lit0; DW_OP_lit4; DW_OP_mul; DW_OP_plus_uconst: 3; DW_OP_const1s: -4; DW_OP_and; DW_OP_swap; DW_OP_drop; DW_OP_swap; DW_OP_drop)
> 
>  <2><1279>: Abbrev Number: 9 (DW_TAG_structure_type)
>     <127a>   DW_AT_name        : (indirect string, offset: 0x1a5a): p__logical_channel_t
>     <127e>   DW_AT_byte_size   : 18 byte block: fd 43 12 0 0 97 94 1 99 34 0 0 0 23 7 9 fc 1a 	(DW_OP_GNU_variable_value: <0x1243>; DW_OP_push_object_address; DW_OP_deref_size: 1; DW_OP_call4: <0x1201>; DW_OP_plus_uconst: 7; DW_OP_const1s: -4; DW_OP_and)
> 
> When evaluated, this gives:
> 
>     Incompatible types on DWARF stack
> 
> In Jakub's original message about DW_OP_GNU_variable_value:
> 
>     https://gcc.gnu.org/legacy-ml/gcc-patches/2017-02/msg01499.html
> 
> .. it says:
> 
>     The intended behavior is that the debug info consumer computes the
>     value of that referenced variable at the current PC, and if it can
>     compute it and pushes the value as a generic type integer into the
>     DWARF stack
> 
> Instead, gdb is using the variable's type -- but this fails with some
> operations, like DW_OP_and, which expect the types to match.
> 
> I believe what was intended was for the value to be cast to the DWARF
> "untyped" type, which is what this patch implements.  This patch also
> updates varval.exp to exhibit the bug.
> 
> gdb/ChangeLog
> 2020-07-27  Tom Tromey  <tromey@adacore.com>
> 
> 	* dwarf2/expr.c (dwarf_expr_context::execute_stack_op)
> 	<DW_OP_GNU_variable_value>: Cast to address type.
> 
> gdb/testsuite/ChangeLog
> 2020-07-27  Tom Tromey  <tromey@adacore.com>
> 
> 	* gdb.dwarf2/varval.exp (setup_exec): Add 'or' instruction to
> 	'varval' location.

LGTM.

Kevin