[PATCH] D126835: Round up zero-sized symbols to 1 byte in `.debug_aranges` (without breaking other logic).

[David Blaikie via Phabricator via llvm-commits]

dblaikie added a comment.

In D126835#3582639 <https://reviews.llvm.org/D126835#3582639>, @ayermolo wrote:

> In D126835#3579779 <https://reviews.llvm.org/D126835#3579779>, @dblaikie wrote:
>
>> In D126835#3577753 <https://reviews.llvm.org/D126835#3577753>, @hans wrote:
>>
>>> When trying this on Chromium (in the context of https://crbug.com/1335630) we hit a densemap assert in llvm::DwarfDebug::emitDebugARanges()
>>> See the attached reproducer.
>>>
>>> F23428761: string_piece_unittest-c72fb0.sh <https://reviews.llvm.org/F23428761>
>>>
>>> F23428765: string_piece_unittest-c72fb0.cpp <https://reviews.llvm.org/F23428765>
>>
>> Thanks for the reproducer, @hansw - sounds worth addressing before this moves forward.
>>
>> (I'm still sort of feeling like we should fix the underlying entities to be non-zero length always (even if they're zero length at the language level - they can still be non-zero length as an implementation detail providing unique addresses))
>
> Doesn't it rely on various feeders into llvm doing the right thing, or can this be done in some kind of language agnostic way?

Possible it could require changes to producers if we wanted to make it invalid at the LLVM IR level to have a zero-length global variable (we already can't have a zero-length IR function - it has to have a terminator, even if it's "unreachable").

But I don't think that's necessary/required - this could be implemented in the LLVM backend(s) (ideally as target-neutrally as possible) to emit a trap for any functions that end up not ending in a terminator instruction (including truly zero-length functions) (various slight variations of this support already exist in LLVM - but I wasn't able to stare at them long enough to get the particular variant I really wanted - but maybe that was a "perfect being the enemy of the good" situation and using one of the existing features (either the non-zero-length thing implemented for Windows (I think that just puts a zero rather than trap) or the "trap on unreachable" (that puts traps on even non-final unreachables, which is more pessimism than I was hoping for - but maybe low cost enough) that Sony and Apple use), extending it for global variables as well as for functions)


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