[llvm-dev] Range lists, zero-length functions, linker gc

[Alexey Lapshin via llvm-dev]

>> Paul> As has been mentioned elsewhere, Sony generally fixes up references from
>> Paul> debug info to stripped functions (of any length) using -1, because
>> Paul> that’s a less-likely-to-be-real address than 0x0 or 0x1.  (0x0 is a
>> Paul> typical base address for shared libraries, I’d think using it has the
>> Paul> potential to mislead various consumers.)  For .debug_ranges we use -2,
>> Paul> because both a 0/0 pair and a -1/-1 pair have a reserved meaning in that
>> Paul> section.
>>
>> 0 in an ET_DYN object refers to the base address, where the ELF header
>> (or headers of other binary formats) resides. The ELF header is unlikely
>> to be a meaningful code sequence. In LLD and GNU ld -z separate-code's
>> layout, it is mapped to a (non-executable) PF_R PT_LOAD segment.
>>
>> I suspect special cased 0 values may have been baked into some DWARF
>> consumers.
>>
>> Do other binary formats/platforms allow the base address to be a
>> meaningful place that we can't lose DWARF for? (If 0 is a meaningful
>> place but its debuggability does not matter that much, we can still use
>> 0 as a special value, losing the debuggability of address 0).

>Yeah, that's the thing - while it might not be an issue for ELF, DWARF
>would want a standard that's fairly resilient to quirky/interesting
>use cases (admittedly - such platforms could equally want to make
>their executable code way up in the address space near max or max - 1,
etc?).

>I know Alexey was particularly interested in the problem of
>low-address functions overlapping with the [0, length) ranges created
>by dead code with gold/lld - but I'm not sure if he has a use case for
>literal zero, or just "low but non-zero". Hopefully he can weigh in
>here.

yes. Having 0 as valid executable address is a common case for embedded programs.
Our customers have problems with overlapping address ranges like [0x00, length) and [0xf0, length).
They use linker script to have .text starting from 0. I would consult for the real case and whether they have use case for literal zero. i.e. [0x00, length) and [0x00, length).

Another thing is that at a time when D59553 was tested many tools worked incorrectly for  [0x00, length) and [0xf0, length) case.
To properly handle that situation - [0x00, length) and [0xf0, length) and use 0 as a marker - there would be necessary to teach all of them to handle 0.
At the same time, using -2 makes all of them working without modifications.

Thank you, Alexey.