[llvm] [DebugInfo] Preserve line and column number when merging debug info. (PR #129960)

[Snehasish Kumar via llvm-commits]

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@@ -89,9 +90,14 @@ has a location with an accurate scope attached, and b) to prevent misleading
 single-stepping (or breakpoint) behavior. Often, merged instructions are memory
 accesses which can trap: having an accurate scope attached greatly assists in
 crash triage by identifying the (possibly inlined) function where the bad
-memory access occurred. This rule is also meant to assist SamplePGO by banning
-scenarios in which a sample of a block containing a merged instruction is
-misattributed to a block containing one of the instructions-to-be-merged.
+memory access occurred. 
+
+For SamplePGO, it is often beneficial to retain an arbitrary but deterministic
+location instead of discarding line and column information as part of merging.
+In particular, loss of location information for calls inhibit optimizations 
+such as indirect call promotion. This behavior can be optionally enabled until
+support for accurately representing merged instructions in the line table is
+implemented. 
----------------
snehasish wrote:

> Are branch instruction locations also critical?

Yes, though the design of sample attribution mitigates this somewhat by inferring block weight from the [max](https://github.com/llvm/llvm-project/blob/0adc672ed4a2e9351732389e25cf7b436de347b7/llvm/include/llvm/Transforms/Utils/SampleProfileLoaderBaseImpl.h#L520) of all instructions in the basic block. Calls are more important since they carry additional metadata apart from their own execution count as noted earlier.

Updated text, ptal.

https://github.com/llvm/llvm-project/pull/129960