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Avoid 'nice' printout for LLVM DWARF extensions
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The 'info address' command tries to use a 'nice' printout mechanism if
a given location description is either made off or contains pieces that
are made of a very simple operations (locexpr_describe_location_piece).

The problem is that the current mechanism uses a linear scan of the
DWARF expression byte stream and doesn't expect the possibility of
nested composite location description.

This implementation keeps the backward compatibility with the
previously expected output, but at the same time gives up on a 'nice'
printout if a given expression contains operations that are not
supported by the locexpr_describe_location_piece function. This is done
by introducing yet another DWARF expression byte stream scanning
function.

A better solution would be to have a pre-parsed representation of the
byte stream where every operation would be represented by its own class
and avoid having more and more large switch/case statement, but that
would require a bigger change.

Change-Id: I471575230773a8a8a7bd25bfc7985248f34f96ed
(cherry picked from commit 856142f)
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@ZaricZoran @lmoriche
ZaricZoran authored and lmoriche committed Jul 1, 2022
1 parent e17b096 commit a8e8f70
Showing 1 changed file with 204 additions and 7 deletions.
211 changes: 204 additions & 7 deletions gdb/dwarf2/loc.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3122,7 +3122,8 @@ locexpr_describe_location_piece (struct symbol *symbol, struct ui_file *stream,
struct gdbarch *gdbarch = objfile->arch ();
size_t leb128_size;

if (data[0] >= DW_OP_reg0 && data[0] <= DW_OP_reg31)
if (data[0] >= DW_OP_reg0 && data[0] <= DW_OP_reg31
&& piece_end_p (data + 1, end))
{
fprintf_filtered (stream, _("a variable in $%s"),
locexpr_regname (gdbarch, data[0] - DW_OP_reg0));
Expand All @@ -3131,8 +3132,15 @@ locexpr_describe_location_piece (struct symbol *symbol, struct ui_file *stream,
else if (data[0] == DW_OP_regx)
{
uint64_t reg;
const gdb_byte *save_data = data;

data = safe_read_uleb128 (data + 1, end, &reg);

/* If the next operation doesn't define the end of a piece, the
expression is too complex to be described here. */
if (!piece_end_p (data, end))
return save_data;

fprintf_filtered (stream, _("a variable in $%s"),
locexpr_regname (gdbarch, reg));
}
Expand Down Expand Up @@ -3172,13 +3180,19 @@ locexpr_describe_location_piece (struct symbol *symbol, struct ui_file *stream,
frame_reg = base_data[0] - DW_OP_breg0;
buf_end = safe_read_sleb128 (base_data + 1, base_data + base_size,
&base_offset);
/* If a location description is allowed to be placed on a DWARF
stack, this becomes a valid case which is too complex to be
described here. */
if (buf_end != base_data + base_size)
error (_("Unexpected opcode after "
"DW_OP_breg%u for symbol \"%s\"."),
frame_reg, symbol->print_name ());
return save_data;
}
else if (base_data[0] >= DW_OP_reg0 && base_data[0] <= DW_OP_reg31)
{
/* If the next operation doesn't define the end of a piece, the
expression is too complex to be described here. */
if (!piece_end_p (base_data + 1, base_data + base_size))
return save_data;

/* The frame base is just the register, with no offset. */
frame_reg = base_data[0] - DW_OP_reg0;
base_offset = 0;
Expand All @@ -3195,13 +3209,18 @@ locexpr_describe_location_piece (struct symbol *symbol, struct ui_file *stream,
locexpr_regname (gdbarch, frame_reg),
plongest (base_offset), plongest (frame_offset));
}
else if (data[0] >= DW_OP_breg0 && data[0] <= DW_OP_breg31
&& piece_end_p (data, end))
else if (data[0] >= DW_OP_breg0 && data[0] <= DW_OP_breg31)
{
int64_t offset;
const gdb_byte *save_data = data;

data = safe_read_sleb128 (data + 1, end, &offset);

/* If the next operation doesn't define the end of a piece,
the expression is too complex to be described here. */
if (!piece_end_p (data, end))
return save_data;

fprintf_filtered (stream,
_("a variable at offset %s from base reg $%s"),
plongest (offset),
Expand Down Expand Up @@ -3265,10 +3284,14 @@ locexpr_describe_location_piece (struct symbol *symbol, struct ui_file *stream,
++data;
}

/* If the next operation (after the DW_OP_litX + DW_OP_stack_value
pair) doesn't define the end of a piece, the expression is too
complex to be described here. */
else if (data[0] >= DW_OP_lit0
&& data[0] <= DW_OP_lit31
&& data + 1 < end
&& data[1] == DW_OP_stack_value)
&& data[1] == DW_OP_stack_value
&& piece_end_p (data + 2, end))
{
fprintf_filtered (stream, _("the constant %d"), data[0] - DW_OP_lit0);
data += 2;
Expand All @@ -3277,6 +3300,171 @@ locexpr_describe_location_piece (struct symbol *symbol, struct ui_file *stream,
return data;
}

/* Check if the expression can be described in a nicer way on a per
piece basis. This function only considers operations currently
handled by the locexpr_describe_location_piece function. */

static bool
locexpr_must_disassemble (const gdb_byte *data, const gdb_byte *end,
unsigned int addr_size)
{
while (data < end)
{
enum dwarf_location_atom op = (enum dwarf_location_atom) *data++;

switch (op)
{
case DW_OP_addr:
data += addr_size;
break;

case DW_OP_const1u:
case DW_OP_const1s:
data += 1;
break;

case DW_OP_const2u:
case DW_OP_const2s:
data += 2;
break;

case DW_OP_const4u:
case DW_OP_const4s:
data += 4;
break;

case DW_OP_const8s:
case DW_OP_const8u:
data += 8;
break;

case DW_OP_constu:
case DW_OP_consts:
data += skip_leb128 (data, end);
break;

case DW_OP_lit0:
case DW_OP_lit1:
case DW_OP_lit2:
case DW_OP_lit3:
case DW_OP_lit4:
case DW_OP_lit5:
case DW_OP_lit6:
case DW_OP_lit7:
case DW_OP_lit8:
case DW_OP_lit9:
case DW_OP_lit10:
case DW_OP_lit11:
case DW_OP_lit12:
case DW_OP_lit13:
case DW_OP_lit14:
case DW_OP_lit15:
case DW_OP_lit16:
case DW_OP_lit17:
case DW_OP_lit18:
case DW_OP_lit19:
case DW_OP_lit20:
case DW_OP_lit21:
case DW_OP_lit22:
case DW_OP_lit23:
case DW_OP_lit24:
case DW_OP_lit25:
case DW_OP_lit26:
case DW_OP_lit27:
case DW_OP_lit28:
case DW_OP_lit29:
case DW_OP_lit30:
case DW_OP_lit31:
case DW_OP_reg0:
case DW_OP_reg1:
case DW_OP_reg2:
case DW_OP_reg3:
case DW_OP_reg4:
case DW_OP_reg5:
case DW_OP_reg6:
case DW_OP_reg7:
case DW_OP_reg8:
case DW_OP_reg9:
case DW_OP_reg10:
case DW_OP_reg11:
case DW_OP_reg12:
case DW_OP_reg13:
case DW_OP_reg14:
case DW_OP_reg15:
case DW_OP_reg16:
case DW_OP_reg17:
case DW_OP_reg18:
case DW_OP_reg19:
case DW_OP_reg20:
case DW_OP_reg21:
case DW_OP_reg22:
case DW_OP_reg23:
case DW_OP_reg24:
case DW_OP_reg25:
case DW_OP_reg26:
case DW_OP_reg27:
case DW_OP_reg28:
case DW_OP_reg29:
case DW_OP_reg30:
case DW_OP_reg31:
case DW_OP_GNU_push_tls_address:
case DW_OP_form_tls_address:
case DW_OP_stack_value:
break;

case DW_OP_regx:
case DW_OP_breg0:
case DW_OP_breg1:
case DW_OP_breg2:
case DW_OP_breg3:
case DW_OP_breg4:
case DW_OP_breg5:
case DW_OP_breg6:
case DW_OP_breg7:
case DW_OP_breg8:
case DW_OP_breg9:
case DW_OP_breg10:
case DW_OP_breg11:
case DW_OP_breg12:
case DW_OP_breg13:
case DW_OP_breg14:
case DW_OP_breg15:
case DW_OP_breg16:
case DW_OP_breg17:
case DW_OP_breg18:
case DW_OP_breg19:
case DW_OP_breg20:
case DW_OP_breg21:
case DW_OP_breg22:
case DW_OP_breg23:
case DW_OP_breg24:
case DW_OP_breg25:
case DW_OP_breg26:
case DW_OP_breg27:
case DW_OP_breg28:
case DW_OP_breg29:
case DW_OP_breg30:
case DW_OP_breg31:
case DW_OP_fbreg:
case DW_OP_piece:
case DW_OP_GNU_const_index:
data += skip_leb128 (data, end);
break;

case DW_OP_bregx:
case DW_OP_bit_piece:
data += skip_leb128 (data, end);
data += skip_leb128 (data, end);
break;

default:
return true;
}
}

return false;
}

/* Disassemble an expression, stopping at the end of a piece or at the
end of the expression. Returns a pointer to the next unread byte
in the input expression. If ALL is nonzero, then this function
Expand Down Expand Up @@ -3744,6 +3932,15 @@ locexpr_describe_location_1 (struct symbol *symbol, CORE_ADDR addr,
int first_piece = 1, bad = 0;
objfile *objfile = per_objfile->objfile;

if (locexpr_must_disassemble (data, end, addr_size))
{
fprintf_filtered (stream, _("a complex DWARF expression:\n"));
data = disassemble_dwarf_expression (stream, objfile->arch (), addr_size,
offset_size, data, data, end, 0, 1,
per_cu, per_objfile);
return;
}

while (data < end)
{
const gdb_byte *here = data;
Expand Down

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