[DWARFYAML][debug_aranges] Replace InitialLength with Format and Length.

[Higuoxing]

Summary

This patch addresses the comment in D80972.

Before this patch, the initial length field of .debug_aranges section should be declared as:

## 32-bit DWARF
debug_aranges:
  - Length:
      TotalLength: 0x20
    Version: 2
    ...

## 64-bit DWARF
debug_aranges:
  - Length:
      TotalLength:   0xffffffff
      TotalLength64: 0x20
    Version: 2
    ...

After this patch:

## 32-bit DWARF
debug_aranges:
  - [[Format:  DWARF32]] ## Optional
    Length:  0x20
    Version: 2
    ...

## 64-bit DWARF
debug_aranges:
  - Format:  DWARF64
    Length:  0x20
    Version: 2

Current implementation of generating DWARF64 .debug_aranges section is buggy. A follow-up patch will improve it and add test cases for DWARF64.

[jhenderson]

llvm/include/llvm/ObjectYAML/DWARFYAML.h

68

I wouldn't bother with a length prefix. I'm not sure it's important for testing, because either the parser will read the DWARF64 marker (i.e. 0xffffffff) and treat the next 8 bytes as the rest of the length, or it won't in which case, it will treat those 4 bytes as the actual length, regardless of their value (with possible exceptions for the rest of the reserved range, but those values can be entered as the length in the YAML document anyway).

llvm/lib/ObjectYAML/DWARFYAML.cpp

78

I'd make this optional, with DWARF32 as the default. That's going to be the common case.

[Higuoxing]

llvm/include/llvm/ObjectYAML/DWARFYAML.h

68

Sorry, I'm a little bit lost here. What do you mean by saying

"but those values can be entered as the length in the YAML document anyway"?

I'm not sure if I understand it correctly. The 12-bit initial length is valid only when the format is DWARF64 (the prefix is 0xffffffff). When the prefix is in 0xfffffff0-0xfffffffe, the DWARF parser will not interpret the last 8-byte as length?

So we don't need LengthPrefix here, right? We can test the prefix in the preserved range by crafting something like

Format: DWARF32
Length: 0xfffffff0

[jhenderson]

llvm/include/llvm/ObjectYAML/DWARFYAML.h

68

Yes, exactly. The LengthPrefix isn't needed at all, I believe. The users options are:

Format: DWARF32
Length: <some value, including special values, that will just be written in the 4 bytes range, interpreted as 32-bits>

(the parser should bail on values in the reserved range other than 0xffffffff, and not try to carry on reading)

or

Format: DWARF64 (implies 0xffffffff for first 4 bytes)
Length: <64-bit length value>

I guess there might be an edge case where people want DWARF32 bit 8-byte offsets or DWARF64 with 4-byte offsets, but let's not worry about that. If people actually need that behaviour, it can be added later.

[Higuoxing]

• Make "Format" optional. (DWARF32 by default).
• Remove LengthPrefix.

Test case for 64-bit will be added in a follow-up patch.

[Higuoxing]

This comment was removed by Higuoxing.

[jhenderson]

LGTM, thanks.

Nit: in the title: replaced ... by -> replaced ... with

(It should be either "X is replaced by Y", or "Y replaces X", along with appropriate tense modifications etc)