[lld] [llvm] [Symbolizer] Support for Missing Line Numbers. (PR #82240)

[via llvm-commits]

================
@@ -1312,32 +1312,52 @@ uint32_t DWARFDebugLine::LineTable::findRowInSeq(
   return RowPos - Rows.begin();
 }
 
-uint32_t DWARFDebugLine::LineTable::lookupAddress(
-    object::SectionedAddress Address) const {
+std::pair<uint32_t, bool>
+DWARFDebugLine::LineTable::lookupAddress(object::SectionedAddress Address,
+                                         bool ApproximateLine) const {
 
   // Search for relocatable addresses
-  uint32_t Result = lookupAddressImpl(Address);
+  std::pair<uint32_t, bool> Result =
+      lookupAddressImpl(Address, ApproximateLine);
 
-  if (Result != UnknownRowIndex ||
+  if (Result.first != UnknownRowIndex ||
       Address.SectionIndex == object::SectionedAddress::UndefSection)
     return Result;
 
   // Search for absolute addresses
   Address.SectionIndex = object::SectionedAddress::UndefSection;
-  return lookupAddressImpl(Address);
+  return lookupAddressImpl(Address, ApproximateLine);
 }
 
-uint32_t DWARFDebugLine::LineTable::lookupAddressImpl(
-    object::SectionedAddress Address) const {
+std::pair<uint32_t, bool>
+DWARFDebugLine::LineTable::lookupAddressImpl(object::SectionedAddress Address,
+                                             bool ApproximateLine) const {
   // First, find an instruction sequence containing the given address.
   DWARFDebugLine::Sequence Sequence;
   Sequence.SectionIndex = Address.SectionIndex;
   Sequence.HighPC = Address.Address;
   SequenceIter It = llvm::upper_bound(Sequences, Sequence,
                                       DWARFDebugLine::Sequence::orderByHighPC);
   if (It == Sequences.end() || It->SectionIndex != Address.SectionIndex)
-    return UnknownRowIndex;
-  return findRowInSeq(*It, Address);
+    return {UnknownRowIndex, false};
+
+  uint32_t RowIndex = findRowInSeq(*It, Address);
+  bool IsApproximate = false;
+  if (ApproximateLine) {
+    uint32_t ApproxRowIndex = RowIndex;
+    while (ApproxRowIndex >= It->FirstRowIndex) {
+      if (ApproxRowIndex != UnknownRowIndex && Rows[ApproxRowIndex].Line)
+        break;
+      IsApproximate = true;
+      if (ApproxRowIndex != UnknownRowIndex &&
+          (Rows[ApproxRowIndex].BasicBlock | Rows[ApproxRowIndex].PrologueEnd))
----------------
bd1976bris wrote:

Yes. Agreed. What we are trying to achieve is that the output is similar what a debugger or "objdump -d -S" etc.. would show, whilst being as simple for users to understand as possible. Looking at what llvm-objdump shows it does seem to stop at prologue end, however, our internal debugger does not, and I think both are reasonable. Therefore, unless we have a good justification, my vote is for the simplest implementation - which is only taking sequence boundaries into account.

https://github.com/llvm/llvm-project/pull/82240