| Issue |
144798
|
| Summary |
clang encodes an invalid DWARF register number for sub-registers.
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
amsen20
|
There is a [mapping](https://dwarfstd.org/doc/DWARF5.pdf) from each target register to a DWARF register number, which is used to refer to different registers within the debug information. Sub-registers are [not defined](https://refspecs.linuxfoundation.org/elf/x86_64-abi-0.95.pdf) in this mapping and they should be either considered to have the same DWARF register number as their super registers or as invalid input. However, in LLVM, they are considered different registers with different and incorrect DWARF register numbers (e.g. unsigned -1).
Example `a.s`:
```asm
.text
.type _start,@function
.globl _start
.hidden _start
_start:
.cfi_sections .eh_frame, .debug_frame
.cfi_startproc
.cfi_same_value %rax
.cfi_same_value %eax
retq
.cfi_endproc
.Ltmp0:
.size _start, .Ltmp0-_start
.text
```
If you run `clang` on it, it compiles it successfully and the `objdump` output is:
```
$ clang -c -g b.s -o a.o && llvm-objdump --dwarf=frames a.o
a.o: file format elf64-x86-64
.debug_frame contents:
00000000 00000014 ffffffff CIE
Format: DWARF32
Version: 4
Augmentation: ""
Address size: 8
Segment desc size: 0
Code alignment factor: 1
Data alignment factor: -8
Return address column: 16
DW_CFA_def_cfa: reg7 +8
DW_CFA_offset: reg16 -8
DW_CFA_nop:
DW_CFA_nop:
DW_CFA_nop:
DW_CFA_nop:
CFA=reg7+8: reg16=[CFA-8]
00000018 0000001c 00000000 FDE cie=00000000 pc=00000000...00000001
Format: DWARF32
DW_CFA_same_value: reg0
DW_CFA_same_value: reg4294967294
0x0: CFA=reg7+8: reg0=same, reg16=[CFA-8], reg4294967294=same
.eh_frame contents:
00000000 00000014 00000000 CIE
Format: DWARF32
Version: 1
Augmentation: "zR"
Code alignment factor: 1
Data alignment factor: -8
Return address column: 16
Augmentation data: 1B
DW_CFA_def_cfa: reg7 +8
DW_CFA_offset: reg16 -8
DW_CFA_nop:
DW_CFA_nop:
CFA=reg7+8: reg16=[CFA-8]
00000018 0000001c 0000001c FDE cie=00000000 pc=00000000...00000001
Format: DWARF32
DW_CFA_same_value: reg0
DW_CFA_same_value: reg4294967294
DW_CFA_nop:
DW_CFA_nop:
DW_CFA_nop:
DW_CFA_nop:
DW_CFA_nop:
DW_CFA_nop:
DW_CFA_nop:
0x0: CFA=reg7+8: reg0=same, reg16=[CFA-8], reg4294967294=same
```
As you can see, `eax` is mapped to `reg4294967294` which is incorrect and does not exist.
On the other hand, if you input the same code to gnu as, the following error happens:
```
$ as -g a.s -o a.o
a.s: Assembler messages:
a.s:10: Error: bad register _expression_
```
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