[llvm] Symbolize line zero as if no source info is available (PR #124846)

[David Blaikie via llvm-commits]

dwblaikie wrote:

> Can you link to the code or documentation that explains this DWARF line table size encoding optimization? My reading of `DILocation::getMergedLocation` is that we intend to create DILocations with column zero when we choose to use line zero, but maybe there is a logic bug in there that I can't see yet.

The DWARF spec wording for this in DWARFv5 (https://dwarfstd.org/doc/DWARF5.pdf) is page 151, table 6.3 which says "The compiler may emit the value 0 in cases where an instruction cannot be attributed to any source line." (by "line" read that as "entry" - it doesn't mean "oh, there's no line, but maybe there's a column", the column is a modifier on the line - so if there's no line, the column is meaningless (& if there's zero line, the file doesn't matter, etc))

Admittedly, the DWARF spec isn't /entirely/ clear that the file doesn't matter if line is zero - you could argue that the file is meaningful even if the line is zero and means the instruction came from "somewhere in this file" - but that generally hasn't been the interpretation the LLVM project has taken. (we could file a DWARF issue to get this clarified)

So I wrote a small test case with DILocations, and didn't observe the behavior I was expecting - LLVM lowering did respect the varying column on the instructions.

That's because we rarely use line zero on an instruction, I think - we mostly remove the location from the instruction entirely (and that usually causes the location of a previous instruction to "flow" on to the unlocated instruction, when th eline table is created).

But a place where we then do produce a zero location - is when a flow-on location would be problematic. Especially at the start of a basic block, where a previous block's location would flow across the boundary which would be bad (sample accuracy, human user - it'd show we are in a block we aren't in, might not be reached/reachable/hot, etc) 

Ah, here we go: https://github.com/llvm/llvm-project/blob/8b448842c4766306b74f9dba2ee9ad3af12fea6c/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp#L2101-L2120



https://github.com/llvm/llvm-project/pull/124846