# My Journey with LLVM (GSoC'20 Phase 2)

- **Author**: @higuoxing
- **Date**: 2020-07-30
- **URL**: https://higuoxing.com/archive/my-journey-with-llvm-ii/
- **Tags**: GSoC, LLVM


During the second coding period, we’ve implemented the `.debug_str_offsets`, `.debug_rnglists` and `.debug_loclists` section. We’re able to handcraft these 3 sections via the following syntax.
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### `debug_str_offsets`

```yaml
debug_str_offsets:
  - Format:  DWARF32 ## Optional
    Length:  0x1234  ## Optional
    Version: 5       ## Optional
    Padding: 0x00    ## Optional
    Offsets: [ 0x01, 0x02, 0x03 ]
```

### `debug_rnglists`

```yaml
debug_rnglists:
  - Format:              DWARF32        ## Optional
    Length:              0x1234         ## Optional
    Version:             5              ## Optional
    AddressSize:         0x08           ## Optional
    SegmentSelectorSize: 0x00           ## Optional
    OffsetEntryCount:    2              ## Optional
    Offsets:             [ 0x01, 0x02 ] ## Optional
    Lists:
      - Entries:
          - Operator: DW_RLE_blah
            Values:  [ 0x01, 0x02 ]
```

### `debug_loclists`

> Note: The `.debug_loclists` section is implemented, but it hasn’t been landed yet.

```yaml
debug_loclists:
  - Format:              DWARF32 ## Optional
    Length:              0x1234  ## Optional
    Version:             5       ## Optional
    AddressSize:         8       ## Optional
    SegmentSelectorSize: 0       ## Optional
    OffsetEntryCount:    1       ## Optional
    Offsets:             [ 1 ]   ## Optional
    Lists:
      - Entries:
          - Operator:          DW_LLE_blah
            Values:            [ 0x1234, 0x4321 ]
            DescriptorsLength: 0x1234   ## Optional
            Descriptors:
              - Operator: DW_OP_blah
                Values:   [ 0x1234 ]
```

We’ve also taught `yaml2obj` to infer the compilation unit’s length for us. Now, we’re able to handcraft the `.debug_info` section without caring about the length field.

### `debug_info`

```yaml
debug_info:
  - Format:     DWARF32 ## Optional
    Length:     0x1234  ## Optional
    Version:    4
    AbbrOffset: 0x00
    AddrSize:   0x08    ## Optional
    Entries:
      - AbbrCode: 1
        Values:
          - Value: 0x1234
          - Value: 0x4321
```

You’ve probably noticed that we still have to calculate the `AbbrOffset` field manually and it makes handcrafting the `.debug_info` section a nightmare. In the next coding period, we’re going to address this issue and make `yaml2obj` able to interlink some DWARF sections. If time permits, we’d also like to add DWARF support to `obj2yaml`.

## Areas in Need of Improvements

In the second coding period, I’m not good at splitting a huge change into several pieces of small patches which brings inconvenience to reviewers. I will try to avoid it in the future. Just as what James has pointed out, I should communicate more with others. When I was implementing these DWARF sections, I should learn about people’s requirements and ask others opinions rather than do it myself and implement what I have in my own mind. I haven’t realized it until I work on the `.debug_loclists` section. Pavel Labath gives me some good advice and comments that I haven’t thought of before!

## Accepted Patches

D84496 [[DWARFYAML] Replace 'Format', 'Version', etc with 'FormParams'. NFC.](https://reviews.llvm.org/D84496)\
D84383 [[DWARFYAML] Pull out common helper functions for rnglist and loclist tables. NFC.](https://reviews.llvm.org/D84383)\
D84008 [[DWARFYAML] Refactor emitDebugInfo() to make the length be inferred.](https://reviews.llvm.org/D84008)\
D84239 [[DWARFYAML] Refactor range list table to hold more data structure.](https://reviews.llvm.org/D84239)\
D83624 [[DWARFYAML] Implement the .debug_rnglists section.](https://reviews.llvm.org/D83624)\
D83853 [[DWARFYAML] Implement the .debug_str_offsets section.](https://reviews.llvm.org/D83853)\
D83749 [[DWARFYAML] Add support for emitting value forms of strx, addrx, etc.](https://reviews.llvm.org/D83749)\
D83452 [[DWARFYAML] Use override instead of virtual for better safety.](https://reviews.llvm.org/D83452)\
D83220 [[DWARFYAML][unittest] Refactor parseDWARFYAML().](https://reviews.llvm.org/D83220)

## Ongoing Patches

D84386 [[DWARFYAML] Add support for emitting custom operands for range list entry.](https://reviews.llvm.org/D84386) <br>
D84234 [[DWARFYAML] Implement the .debug_loclists section.](https://reviews.llvm.org/D84234)