Also, fixed-size DIEs are much easier when quickly scanning for something; you can derive the size of the DIE from the abbrev without having to look at the DIE content. When you have variable-size values such as LEB128 then you need to parse the values in order to determine where the next DIE starts. (Greg Clayton [LLDB] in particular has expressed a strong preference for fixed-size values for this reason.) --paulr
On Mon, Feb 3, 2025 at 1:09 AM Cary Coutant <ccout...@gmail.com> wrote: > As an alternative to the indexed proposal in 250130.1, which proposes >> allowing a constant classed value for DW_AT_object_pointer, as an index >> into the formal parameters of the subprogram, I was wondering how folks >> would feel about something a bit different: >> >> What about a reference classed form that is a (signed) LEB128 relative >> DIE offset - relative to the start of the DIE containing the attribute >> value? >> >> This would allow for other shortenings of DIEs that are commonly nearby. >> >> I don't have measurements on how much it could decrease DWARF size, but >> could maybe prototype such a thing (bit expensive, because it makes DWARF >> byte size dependent on itself in some ways - and LLVM's DWARF generation >> precomputes DIE offsets, etc, rather than relying on assembler relaxation) >> > > Yes, it's worth discussing, but I suspect the downsides you've mentioned > would outweigh any benefits obtained by using an offset rather than an > index. If anything, I'd think the index would be more useful if > you read and internalize the DIE tree. An index would also almost always be > a single byte (and its size would be predictable), while an offset would be > more likely to require 2 bytes. > > -cary > >
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