================
@@ -8,8 +8,8 @@ struct S0 {
// CHECK-LABEL: define dso_local void @_Z3fS0v
// CHECK: alloca %struct.S0, align 4
// CHECK-NEXT: [[TMP0:%.*]] = alloca %struct.S0, align 4
-// CHECK: call void @llvm.dbg.declare(metadata ptr [[TMP0]], metadata
[[S0_A:![0-9]+]], metadata !DIExpression())
-// CHECK-NEXT: call void @llvm.dbg.declare(metadata ptr [[TMP0]], metadata
[[S0_B:![0-9]+]], metadata !DIExpression(DW_OP_plus_uconst, 2))
+// CHECK: call void @llvm.dbg.declare(metadata ptr [[TMP0]], metadata
[[S0_A:![0-9]+]], metadata !DIExpression(DW_OP_bit_piece, 16, 0))
----------------
john-brawn-arm wrote:
The LLVM langref says "Note that contrary to DW_OP_bit_piece, the offset is
describing the location within the described source variable". What I think
this means is that while DW_OP_bit_piece means "you get the source variable by
extracting these bits of this location", DW_OP_LLVM_fragment means "the value
of this location is these bits of the source variable".
Trying out DW_OP_LLVM_fragment in
```
struct {
unsigned int a : 8;
unsigned int b : 8;
} X = { 1, 2 };
int main() {
auto [a, b] = X;
return a + b;
}
```
the resulting dwarf info looks pretty strange
```
<2><7f>: Abbrev Number: 7 (DW_TAG_variable)
<80> DW_AT_location : 4 byte block: 93 1 91 78 (DW_OP_piece:
1; DW_OP_fbreg: -8)
<85> DW_AT_name : (indirect string, offset: 0x9c): a
<89> DW_AT_decl_file : 1
<8a> DW_AT_decl_line : 7
<8b> DW_AT_type : <0x5f>
<2><8f>: Abbrev Number: 7 (DW_TAG_variable)
<90> DW_AT_location : 6 byte block: 93 1 91 78 93 1 (DW_OP_piece:
1; DW_OP_fbreg: -8; DW_OP_piece: 1)
<97> DW_AT_name : (indirect string, offset: 0xab): b
<9b> DW_AT_decl_file : 1
<9c> DW_AT_decl_line : 7
<9d> DW_AT_type : <0x5f>
```
lldb says (at line 8)
```
(lldb) p a
(unsigned int) 513
(lldb) p b
(unsigned int) 256
```
gdb appears to not like this dwarf info and just says
```
(gdb) p a
$1 = <optimized out>
(gdb) p b
$2 = <optimized out>
```
https://github.com/llvm/llvm-project/pull/85665
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