I recently found myself in trouble because the crash I had disappeared with -g, so I could not debug the program. This happened because the optimizer did not remember to consider DBG_VALUEs instruction so it changed its behavior, and the bug went hiding.
I then started discussing this on https://reviews.llvm.org/D45878, and since this is something that should be handled by all different machine passes, the immediate question is now what utility functions should be made available for common use.
1. A pass such as MachineSink.cpp must first of all remember to consider DBG_VALUEs when iterating over instructions, so that it does not e.g. "break if next instruction is not X" and therefore change its results with -g.
2. At a second priority,
a\) it should handle DEBUG\_VALUEs when moving / erasing MachineInstrs\. b\) it should " // Merge or erase debug location to ensure consistent stepping in profilers and debuggers\." \(MachineSink\.cpp\)\.
Well, this is in fact the first point here: I think this should be documented somewhere by a comment so that it is clear exactly how these things should be handled. My personal understanding right now is that 2 is not vital but should be done on a best-effort basis. 1 however is really bad if it happens (which it currently does).
We are discussing where to place these utility functions, and if splice() should optionally be made to do this also. It seems that collectDebugValues() in MachineSink.cpp is a good starting point, but we probably want to do an even better search than just looking at the first def operand.
It would be nice to get some general feedback from the community at this point so we know which direction to take...