Re: [PATCH v11 01/10] gdb, dwarf: add support for DW_AT_trampoline in DWARF reader

[Tom Tromey <tom@tromey.com>]

>>>>> Abdul Basit Ijaz <abdul.b.ijaz@intel.com> writes:

> DW_AT_trampoline can be used to describe compiler generated functions
> that serve some intermediary purpose on making a call to another
> function.  A compiler can emit this tag in order to help a debugger hide
> the trampolines from a user.

> This patch adds support to GDB for reading the DW_AT_trampoline
> attribute.  It stores the attribute and its value in the type_specific
> part of a GDB type.  This patch is implemented in preparation of the
> following patches, which will add a mechanism to hide DW_AT_trampoline
> subroutines from the user.

One thing I don't understand is why this data is attached to the type,
and not to the function's symbol (or perhaps the block structure).

> +      TYPE_TRAMPOLINE_TARGET (ftype)
> +	= (trampoline_target *) TYPE_ZALLOC (ftype,
> +					     sizeof (trampoline_target));

This zero-initializes so...

> +	  if (dwarf2_get_pc_bounds (target_die, &lowpc, NULL, target_cu,
> +				    nullptr, nullptr) <= PC_BOUNDS_INVALID)
> +	    complaint (_("DW_AT_trampoline target DIE has invalid "
> +			 "low pc, for referencing DIE %s[in module %s]"),
> +			 sect_offset_str (die->sect_off),
> +			 objfile_name (objfile));
> +	  else
> +	    TYPE_TRAMPOLINE_TARGET (ftype)->set_target_addr (lowpc);

... if the complaint triggers, this will end up with a kind of
TRAMPOLINE_TARGET_ADDR and an unrelocated address of 0.

Perhaps in such a case it's better to avoid marking the function as a
trampoline.  That way, if there is a problem in the DWARF, gdb will fall
back to the function simply being ordinary.

> +      else if (attr->form_is_unsigned ())
> +	TYPE_TRAMPOLINE_TARGET (ftype)->set_target_flag (attr->as_boolean ());
> +      else
> +	TYPE_TRAMPOLINE_TARGET (ftype)->set_target_addr (attr->as_address ());

This else clause also seems perhaps weird.

I had some other nits for this patch but I figured I'd rather find out
about the type issue before writing them up.

Tom