[llvm] r237237 - [DebugInfo] Debug locations for constant SD nodes

[Sergey Dmitrouk]

> This still seems to have some oddities that are increasing the size of -gmlt
> in particular by creating discontiguous instruction ranges for inlined
> subroutines in ways that are possibly incorrect-ish.
>
> Take this example:
>
> volatile int x;
> int y;
> static __attribute__((always_inline)) int f1() {
>   if (x * 3 < 14) return 1;
>   return 2;
> }
> int main() {
>   x = f1();
>   x = x ? 1 : 2;
> }
>
> & the assembly difference: https://www.diffchecker.com/p2ladh2w
>
> What happens is that the 1 stored to x from the inlined f1() ends up being
> emitted indirectly (in this particular instance - setg/movzbl/incl so if the
> condition is false it's 1 + 1 and if it's true it's 0 + 1) so when the 1 is
> needed in main it's emitted directly there, but still attributed to the
> location inside f1.

Thanks for detailed explanation, David.

> Does this still qualify as two uses of the constant '1' & should not get
> locations? Or does that first indirect emission not count as a use at this
> level - yet we somehow still use that use's location for the other use... ?

These are two uses, but this time constant node is queried in a
different way: in SelectionDAGBuilder::getNonRegisterValue() method via
value-to-sdnode map during matching phi-node values.  Later first use
disappears and we left with incorrect location.

Not sure how to handle this yet, a couple of obvious tries causes test
failures, will look for something better.

-- 
Sergey