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[lldb/DWARF] Fix handling of variables with both location and const_value attributes
Class-level static constexpr variables can have both DW_AT_const_value (in the "declaration") and a DW_AT_location (in the "definition") attributes. Our code was trying to handle this, but it was brittle and hard to follow (and broken) because it was processing the attributes in the order in which they were found. Refactor the code to make the intent clearer -- DW_AT_location trumps DW_AT_const_value, and fix the bug which meant that we were not displaying these variables properly (the culprit was the delayed parsing of the const_value attribute due to a need to fetch the variable type. Differential Revision: https://reviews.llvm.org/D86615
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‎lldb/source/Plugins/SymbolFile/DWARF/DWARFFormValue.h‎

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -42,6 +42,7 @@ class DWARFFormValue {
4242
DWARFFormValue(const DWARFUnit *unit) : m_unit(unit) {}
4343
DWARFFormValue(const DWARFUnit *unit, dw_form_t form)
4444
: m_unit(unit), m_form(form) {}
45+
const DWARFUnit *GetUnit() const { return m_unit; }
4546
void SetUnit(const DWARFUnit *unit) { m_unit = unit; }
4647
dw_form_t Form() const { return m_form; }
4748
dw_form_t& FormRef() { return m_form; }

‎lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp‎

Lines changed: 70 additions & 67 deletions
Original file line numberDiff line numberDiff line change
@@ -3111,18 +3111,15 @@ VariableSP SymbolFileDWARF::ParseVariableDIE(const SymbolContext &sc,
31113111
const char *name = nullptr;
31123112
const char *mangled = nullptr;
31133113
Declaration decl;
3114-
uint32_t i;
31153114
DWARFFormValue type_die_form;
31163115
DWARFExpression location;
31173116
bool is_external = false;
31183117
bool is_artificial = false;
3119-
bool location_is_const_value_data = false;
3120-
bool has_explicit_location = false;
3121-
DWARFFormValue const_value;
3118+
DWARFFormValue const_value_form, location_form;
31223119
Variable::RangeList scope_ranges;
31233120
// AccessType accessibility = eAccessNone;
31243121

3125-
for (i = 0; i < num_attributes; ++i) {
3122+
for (size_t i = 0; i < num_attributes; ++i) {
31263123
dw_attr_t attr = attributes.AttributeAtIndex(i);
31273124
DWARFFormValue form_value;
31283125

@@ -3152,65 +3149,11 @@ VariableSP SymbolFileDWARF::ParseVariableDIE(const SymbolContext &sc,
31523149
is_external = form_value.Boolean();
31533150
break;
31543151
case DW_AT_const_value:
3155-
// If we have already found a DW_AT_location attribute, ignore this
3156-
// attribute.
3157-
if (!has_explicit_location) {
3158-
location_is_const_value_data = true;
3159-
// The constant value will be either a block, a data value or a
3160-
// string.
3161-
auto debug_info_data = die.GetData();
3162-
if (DWARFFormValue::IsBlockForm(form_value.Form())) {
3163-
// Retrieve the value as a block expression.
3164-
uint32_t block_offset =
3165-
form_value.BlockData() - debug_info_data.GetDataStart();
3166-
uint32_t block_length = form_value.Unsigned();
3167-
location = DWARFExpression(
3168-
module,
3169-
DataExtractor(debug_info_data, block_offset, block_length),
3170-
die.GetCU());
3171-
} else if (DWARFFormValue::IsDataForm(form_value.Form())) {
3172-
// Constant value size does not have to match the size of the
3173-
// variable. We will fetch the size of the type after we create
3174-
// it.
3175-
const_value = form_value;
3176-
} else if (const char *str = form_value.AsCString()) {
3177-
uint32_t string_length = strlen(str) + 1;
3178-
location = DWARFExpression(
3179-
module,
3180-
DataExtractor(str, string_length,
3181-
die.GetCU()->GetByteOrder(),
3182-
die.GetCU()->GetAddressByteSize()),
3183-
die.GetCU());
3184-
}
3185-
}
3152+
const_value_form = form_value;
3153+
break;
3154+
case DW_AT_location:
3155+
location_form = form_value;
31863156
break;
3187-
case DW_AT_location: {
3188-
location_is_const_value_data = false;
3189-
has_explicit_location = true;
3190-
if (DWARFFormValue::IsBlockForm(form_value.Form())) {
3191-
auto data = die.GetData();
3192-
3193-
uint32_t block_offset =
3194-
form_value.BlockData() - data.GetDataStart();
3195-
uint32_t block_length = form_value.Unsigned();
3196-
location = DWARFExpression(
3197-
module, DataExtractor(data, block_offset, block_length),
3198-
die.GetCU());
3199-
} else {
3200-
DataExtractor data = die.GetCU()->GetLocationData();
3201-
dw_offset_t offset = form_value.Unsigned();
3202-
if (form_value.Form() == DW_FORM_loclistx)
3203-
offset = die.GetCU()->GetLoclistOffset(offset).getValueOr(-1);
3204-
if (data.ValidOffset(offset)) {
3205-
data = DataExtractor(data, offset, data.GetByteSize() - offset);
3206-
location = DWARFExpression(module, data, die.GetCU());
3207-
assert(func_low_pc != LLDB_INVALID_ADDRESS);
3208-
location.SetLocationListAddresses(
3209-
attributes.CompileUnitAtIndex(i)->GetBaseAddress(),
3210-
func_low_pc);
3211-
}
3212-
}
3213-
} break;
32143157
case DW_AT_specification:
32153158
spec_die = form_value.Reference();
32163159
break;
@@ -3236,6 +3179,66 @@ VariableSP SymbolFileDWARF::ParseVariableDIE(const SymbolContext &sc,
32363179
}
32373180
}
32383181

3182+
// Prefer DW_AT_location over DW_AT_const_value. Both can be emitted e.g.
3183+
// for static constexpr member variables -- DW_AT_const_value will be
3184+
// present in the class declaration and DW_AT_location in the DIE defining
3185+
// the member.
3186+
bool location_is_const_value_data = false;
3187+
bool has_explicit_location = false;
3188+
bool use_type_size_for_value = false;
3189+
if (location_form.IsValid()) {
3190+
has_explicit_location = true;
3191+
if (DWARFFormValue::IsBlockForm(location_form.Form())) {
3192+
const DWARFDataExtractor &data = die.GetData();
3193+
3194+
uint32_t block_offset =
3195+
location_form.BlockData() - data.GetDataStart();
3196+
uint32_t block_length = location_form.Unsigned();
3197+
location = DWARFExpression(
3198+
module, DataExtractor(data, block_offset, block_length),
3199+
die.GetCU());
3200+
} else {
3201+
DataExtractor data = die.GetCU()->GetLocationData();
3202+
dw_offset_t offset = location_form.Unsigned();
3203+
if (location_form.Form() == DW_FORM_loclistx)
3204+
offset = die.GetCU()->GetLoclistOffset(offset).getValueOr(-1);
3205+
if (data.ValidOffset(offset)) {
3206+
data = DataExtractor(data, offset, data.GetByteSize() - offset);
3207+
location = DWARFExpression(module, data, die.GetCU());
3208+
assert(func_low_pc != LLDB_INVALID_ADDRESS);
3209+
location.SetLocationListAddresses(
3210+
location_form.GetUnit()->GetBaseAddress(), func_low_pc);
3211+
}
3212+
}
3213+
} else if (const_value_form.IsValid()) {
3214+
location_is_const_value_data = true;
3215+
// The constant value will be either a block, a data value or a
3216+
// string.
3217+
const DWARFDataExtractor &debug_info_data = die.GetData();
3218+
if (DWARFFormValue::IsBlockForm(const_value_form.Form())) {
3219+
// Retrieve the value as a block expression.
3220+
uint32_t block_offset =
3221+
const_value_form.BlockData() - debug_info_data.GetDataStart();
3222+
uint32_t block_length = const_value_form.Unsigned();
3223+
location = DWARFExpression(
3224+
module,
3225+
DataExtractor(debug_info_data, block_offset, block_length),
3226+
die.GetCU());
3227+
} else if (DWARFFormValue::IsDataForm(const_value_form.Form())) {
3228+
// Constant value size does not have to match the size of the
3229+
// variable. We will fetch the size of the type after we create
3230+
// it.
3231+
use_type_size_for_value = true;
3232+
} else if (const char *str = const_value_form.AsCString()) {
3233+
uint32_t string_length = strlen(str) + 1;
3234+
location = DWARFExpression(
3235+
module,
3236+
DataExtractor(str, string_length, die.GetCU()->GetByteOrder(),
3237+
die.GetCU()->GetAddressByteSize()),
3238+
die.GetCU());
3239+
}
3240+
}
3241+
32393242
const DWARFDIE parent_context_die = GetDeclContextDIEContainingDIE(die);
32403243
const dw_tag_t parent_tag = die.GetParent().Tag();
32413244
bool is_static_member =
@@ -3415,12 +3418,12 @@ VariableSP SymbolFileDWARF::ParseVariableDIE(const SymbolContext &sc,
34153418
}
34163419

34173420
if (symbol_context_scope) {
3418-
SymbolFileTypeSP type_sp(
3419-
new SymbolFileType(*this, GetUID(type_die_form.Reference())));
3421+
auto type_sp = std::make_shared<SymbolFileType>(
3422+
*this, GetUID(type_die_form.Reference()));
34203423

3421-
if (const_value.Form() && type_sp && type_sp->GetType())
3424+
if (use_type_size_for_value && type_sp->GetType())
34223425
location.UpdateValue(
3423-
const_value.Unsigned(),
3426+
const_value_form.Unsigned(),
34243427
type_sp->GetType()->GetByteSize(nullptr).getValueOr(0),
34253428
die.GetCU()->GetAddressByteSize());
34263429

‎lldb/test/Shell/SymbolFile/DWARF/DW_AT_location-DW_AT_const_value.s‎

Lines changed: 144 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,144 @@
1+
## Test that we don't get confused by variables with both location and
2+
## const_value attributes. Such values are produced in C++ for class-level
3+
## static constexpr variables.
4+
5+
# REQUIRES: x86
6+
7+
# RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux %s -o %t
8+
# RUN: %lldb %t -o "target variable A::x A::y" -o exit | FileCheck %s
9+
10+
# CHECK-LABEL: target variable
11+
# CHECK: (const int) A::x = 142
12+
# CHECK: (const int) A::y = 242
13+
14+
.section .rodata,"a",@progbits
15+
.p2align 2
16+
_ZN1A1xE:
17+
.long 142
18+
_ZN1A1yE:
19+
.long 242
20+
21+
.section .debug_abbrev,"",@progbits
22+
.byte 1 # Abbreviation Code
23+
.byte 17 # DW_TAG_compile_unit
24+
.byte 1 # DW_CHILDREN_yes
25+
.byte 37 # DW_AT_producer
26+
.byte 8 # DW_FORM_string
27+
.byte 3 # DW_AT_name
28+
.byte 8 # DW_FORM_string
29+
.byte 0 # EOM(1)
30+
.byte 0 # EOM(2)
31+
.byte 3 # Abbreviation Code
32+
.byte 19 # DW_TAG_structure_type
33+
.byte 1 # DW_CHILDREN_yes
34+
.byte 3 # DW_AT_name
35+
.byte 8 # DW_FORM_string
36+
.byte 11 # DW_AT_byte_size
37+
.byte 11 # DW_FORM_data1
38+
.byte 0 # EOM(1)
39+
.byte 0 # EOM(2)
40+
.byte 4 # Abbreviation Code
41+
.byte 13 # DW_TAG_member
42+
.byte 0 # DW_CHILDREN_no
43+
.byte 3 # DW_AT_name
44+
.byte 8 # DW_FORM_string
45+
.byte 73 # DW_AT_type
46+
.byte 19 # DW_FORM_ref4
47+
.byte 60 # DW_AT_declaration
48+
.byte 25 # DW_FORM_flag_present
49+
.byte 28 # DW_AT_const_value
50+
.byte 13 # DW_FORM_sdata
51+
.byte 0 # EOM(1)
52+
.byte 0 # EOM(2)
53+
.byte 5 # Abbreviation Code
54+
.byte 38 # DW_TAG_const_type
55+
.byte 0 # DW_CHILDREN_no
56+
.byte 73 # DW_AT_type
57+
.byte 19 # DW_FORM_ref4
58+
.byte 0 # EOM(1)
59+
.byte 0 # EOM(2)
60+
.byte 6 # Abbreviation Code
61+
.byte 36 # DW_TAG_base_type
62+
.byte 0 # DW_CHILDREN_no
63+
.byte 3 # DW_AT_name
64+
.byte 8 # DW_FORM_string
65+
.byte 62 # DW_AT_encoding
66+
.byte 11 # DW_FORM_data1
67+
.byte 11 # DW_AT_byte_size
68+
.byte 11 # DW_FORM_data1
69+
.byte 0 # EOM(1)
70+
.byte 0 # EOM(2)
71+
.byte 7 # Abbreviation Code
72+
.byte 52 # DW_TAG_variable
73+
.byte 0 # DW_CHILDREN_no
74+
.byte 71 # DW_AT_specification
75+
.byte 19 # DW_FORM_ref4
76+
.byte 2 # DW_AT_location
77+
.byte 24 # DW_FORM_exprloc
78+
.byte 110 # DW_AT_linkage_name
79+
.byte 8 # DW_FORM_string
80+
.byte 0 # EOM(1)
81+
.byte 0 # EOM(2)
82+
## This deliberately inverts the order of the specification and location
83+
## attributes.
84+
.byte 8 # Abbreviation Code
85+
.byte 52 # DW_TAG_variable
86+
.byte 0 # DW_CHILDREN_no
87+
.byte 2 # DW_AT_location
88+
.byte 24 # DW_FORM_exprloc
89+
.byte 71 # DW_AT_specification
90+
.byte 19 # DW_FORM_ref4
91+
.byte 110 # DW_AT_linkage_name
92+
.byte 8 # DW_FORM_string
93+
.byte 0 # EOM(1)
94+
.byte 0 # EOM(2)
95+
.byte 0 # EOM(3)
96+
97+
.section .debug_info,"",@progbits
98+
.Lcu_begin0:
99+
.long .Ldebug_info_end0-.Ldebug_info_start0 # Length of Unit
100+
.Ldebug_info_start0:
101+
.short 4 # DWARF version number
102+
.long .debug_abbrev # Offset Into Abbrev. Section
103+
.byte 8 # Address Size (in bytes)
104+
.byte 1 # Abbrev DW_TAG_compile_unit
105+
.asciz "Hand-written DWARF" # DW_AT_producer
106+
.asciz "a.cc" # DW_AT_name
107+
.byte 7 # Abbrev DW_TAG_variable
108+
.long .LA__x-.Lcu_begin0 # DW_AT_specification
109+
.byte 9 # DW_AT_location
110+
.byte 3
111+
.quad _ZN1A1xE
112+
.asciz "_ZN1A1xE" # DW_AT_linkage_name
113+
.byte 8 # Abbrev DW_TAG_variable
114+
.byte 9 # DW_AT_location
115+
.byte 3
116+
.quad _ZN1A1yE
117+
.long .LA__y-.Lcu_begin0 # DW_AT_specification
118+
.asciz "_ZN1A1yE" # DW_AT_linkage_name
119+
.byte 3 # Abbrev DW_TAG_structure_type
120+
.asciz "A" # DW_AT_name
121+
.byte 1 # DW_AT_byte_size
122+
.LA__x:
123+
.byte 4 # Abbrev DW_TAG_member
124+
.asciz "x" # DW_AT_name
125+
.long .Lconst_int-.Lcu_begin0 # DW_AT_type
126+
# DW_AT_declaration
127+
.sleb128 147 # DW_AT_const_value
128+
.LA__y:
129+
.byte 4 # Abbrev DW_TAG_member
130+
.asciz "y" # DW_AT_name
131+
.long .Lconst_int-.Lcu_begin0 # DW_AT_type
132+
# DW_AT_declaration
133+
.sleb128 247 # DW_AT_const_value
134+
.byte 0 # End Of Children Mark
135+
.Lconst_int:
136+
.byte 5 # Abbrev DW_TAG_const_type
137+
.long .Lint-.Lcu_begin0 # DW_AT_type
138+
.Lint:
139+
.byte 6 # Abbrev DW_TAG_base_type
140+
.asciz "int" # DW_AT_name
141+
.byte 5 # DW_AT_encoding
142+
.byte 4 # DW_AT_byte_size
143+
.byte 0 # End Of Children Mark
144+
.Ldebug_info_end0:

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