[llvm] [Coverage] Rework Decision/Expansion/Branch (PR #78969)

[Jessica Paquette via llvm-commits]

================
@@ -662,23 +724,39 @@ Error CoverageMapping::loadFunctionRecord(
     }
     Function.pushRegion(Region, *ExecutionCount, *AltExecutionCount);
 
+    if (Region.Kind == CounterMappingRegion::ExpansionRegion) {
+      for (auto &Decision : reverse(Decisions)) {
+        if (Decision.updateExpansion(Region))
+          break;
+      }
+      continue;
+    }
+
+    if (Region.Kind != CounterMappingRegion::MCDCBranchRegion)
+      continue;
+
     // If a MCDCDecisionRegion was seen, store the BranchRegions that
     // correspond to it in a vector, according to the number of conditions
     // recorded for the region (tracked by NumConds).
-    if (NumConds > 0 && Region.Kind == CounterMappingRegion::MCDCBranchRegion) {
-      MCDCBranches.push_back(Region);
+    for (int I = Decisions.size() - 1; I >= 0; --I) {
+      auto &Decision = Decisions[I];
 
       // As we move through all of the MCDCBranchRegions that follow the
       // MCDCDecisionRegion, decrement NumConds to make sure we account for
       // them all before we calculate the bitmap of executed test vectors.
-      if (--NumConds == 0) {
+      switch (Decision.updateBranch(Region)) {
+      case DecisionRow::UpdateResult::NotFound:
+        continue;
+      case DecisionRow::UpdateResult::Updated:
+        goto branch_found;
----------------
ornata wrote:

Why `goto`?

```
bool BranchFound = false;
switch(...) {
case DecisionRow::UpdateResult::Updated:
    BranchFound = true;
    break;
...
}
(void)BranchFound;
assert(BranchFound && "Did not find a branch?!");
```

Alternatively, we could return an `Error` when `BranchFound == false`.


https://github.com/llvm/llvm-project/pull/78969