[llvm-dev] Tracking parts of expanded values in optimized debug

[Ed Jones via llvm-dev]

Hi all,

I'm currently working on an out-of-tree backend and am trying to
improve the debug experience when debugging optimized code. Our
backend only has 8-bit and 16-bit legal types, so any larger values
are expanded. The behavior I am currently seeing is that the expanded
halves of an illegal type lose their debug information. Is this the
expected behavior?

For example, if I have an ISD::DBG_VALUE SDNode associated with a
value, and that value is split up (for example a i64 load expanded
into two i32 loads), I would expect to see a new ISD::DBG_VALUE
tracking each fragment of the input original debug variable. However,
I can't see anywhere this is handled. There seems to be some support
for this, through the DW_OP_LLVM_fragment DIExpression, and there is a
function DIBuilder::createFragmentExpression which could be used to
create an expression to track part of a variable, however these don't
seem to be used much.

Thanks,
Ed