[DebugInfo] When merging locations prefer unannotated empty locs (#157707)

When merging DILocations, we prefer to use DebugLoc::getMergedLocation
when possible to better preserve DebugLoc coverage tracking information
through transformations (as conversion to DILocations drops all coverage
tracking data). Currently, DebugLoc::getMergedLocation checks to see if
either DebugLoc is empty and returns it directly if so, to propagate
that DebugLoc's coverage tracking data to the merged location; however,
it only checks whether either location is valid, not whether they are
annotated.

This is significant because an annotated location is not a bug, while an
empty unannotated location may be one; therefore, we check to see if
either location is unannotated, and prefer to return that location if it
exists rather than an annotated one.

This change is NFC outside of DebugLoc coverage tracking builds.

Author: SLTozer

llvm/lib/IR/DebugLoc.cpp

@@ -181,10 +181,19 @@ DebugLoc DebugLoc::getMergedLocations(ArrayRef<DebugLoc> Locs) {
   return Merged;
 }
 DebugLoc DebugLoc::getMergedLocation(DebugLoc LocA, DebugLoc LocB) {
-  if (!LocA)
-    return LocA;
-  if (!LocB)
+  if (!LocA || !LocB) {
+    // If coverage tracking is enabled, prioritize returning empty non-annotated
+    // locations to empty annotated locations.
+#if LLVM_ENABLE_DEBUGLOC_TRACKING_COVERAGE
+    if (!LocA && LocA.getKind() == DebugLocKind::Normal)
+      return LocA;
+    if (!LocB && LocB.getKind() == DebugLocKind::Normal)
+      return LocB;
+#endif // LLVM_ENABLE_DEBUGLOC_TRACKING_COVERAGE
+    if (!LocA)
+      return LocA;
     return LocB;
+  }
   return DILocation::getMergedLocation(LocA, LocB);
 }