Reapply "[DebugLine] Correct debug line emittion" (#158343) (#158379)

This reverts commit aabf18d7184298566993e3141606cd79ff617d2d.
#157529 included a test that used clang, which doesn't exists in some
CI. #158343 reverts it.

This PR reapplies the original patch with the incorrect test removed.

### Context

#99710 introduced `.loc_label` so we can terminate a line sequence.
However, it did not advance PC properly. This is problematic for
1-instruction functions as it will have zero-length sequence. The test
checked in that PR shows the problem:

```
# CHECK-LINE-TABLE:                  Address            Line   Column File   ISA Discriminator OpIndex Flags
# CHECK-LINE-TABLE-NEXT:             ------------------ ------ ------ ------ --- ------------- ------- -------------
# CHECK-LINE-TABLE-NEXT: 0x00000028: 05 DW_LNS_set_column (1)
# CHECK-LINE-TABLE-NEXT: 0x0000002a: 00 DW_LNE_set_address (0x0000000000000000)
# CHECK-LINE-TABLE-NEXT: 0x00000035: 01 DW_LNS_copy
# CHECK-LINE-TABLE-NEXT:             0x0000000000000000      1      1      1   0             0       0  is_stmt
# CHECK-LINE-TABLE-NEXT: 0x00000036: 00 DW_LNE_end_sequence
# CHECK-LINE-TABLE-NEXT:             0x0000000000000000      1      1      1   0             0       0  is_stmt end_sequence
```

Both rows having PC 0x0 is incorrect, and parsers won't be able to parse
them. See more explanation why this is wrong in #154851.

### Design

This PR attempts to fix this by advancing the PC to the next available
Label, and advance to the end of the section if no Label is available.

### Implementation

- `emitDwarfLineEndEntry` will advance PC to the `CurrLabel`
- If `CurrLabel` is null, its probably a fake LineEntry we introduced in
#110192. In that case look for the next Label
- If still not label can be found, use `null` and
`emitDwarfLineEndEntry` is smart enough to advance PC to the end of the
section
- Rename `LastLabel` to `PrevLabel`, "last" can mean "previous" or
"final", this is ambigous.
- Updated the tests to emit a correct label.

### Note

This fix should render #154986 and #154851 obsolete, they were temporary
fixes and don't resolve the root cause.

1 files changed, 20 insertions, 10 deletions

diff --git a/llvm/lib/MC/MCDwarf.cpp b/llvm/lib/MC/MCDwarf.cpp
index e7c0d37..e8f000a 100644
--- a/llvm/lib/MC/MCDwarf.cpp
+++ b/llvm/lib/MC/MCDwarf.cpp
@@ -181,7 +181,7 @@ void MCDwarfLineTable::emitOne(
 
   unsigned FileNum, LastLine, Column, Flags, Isa, Discriminator;
   bool IsAtStartSeq;
-  MCSymbol *LastLabel;
+  MCSymbol *PrevLabel;
   auto init = [&]() {
     FileNum = 1;
     LastLine = 1;
@@ -189,21 +189,31 @@ void MCDwarfLineTable::emitOne(
     Flags = DWARF2_LINE_DEFAULT_IS_STMT ? DWARF2_FLAG_IS_STMT : 0;
     Isa = 0;
     Discriminator = 0;
-    LastLabel = nullptr;
+    PrevLabel = nullptr;
     IsAtStartSeq = true;
   };
   init();
 
   // Loop through each MCDwarfLineEntry and encode the dwarf line number table.
   bool EndEntryEmitted = false;
-  for (const MCDwarfLineEntry &LineEntry : LineEntries) {
-    MCSymbol *Label = LineEntry.getLabel();
+  for (auto It = LineEntries.begin(); It != LineEntries.end(); ++It) {
+    auto LineEntry = *It;
+    MCSymbol *CurrLabel = LineEntry.getLabel();
     const MCAsmInfo *asmInfo = MCOS->getContext().getAsmInfo();
 
     if (LineEntry.LineStreamLabel) {
       if (!IsAtStartSeq) {
-        MCOS->emitDwarfLineEndEntry(Section, LastLabel,
-                                    /*EndLabel =*/LastLabel);
+        auto *Label = CurrLabel;
+        auto NextIt = It + 1;
+        // LineEntry with a null Label is probably a fake LineEntry we added
+        // when `-emit-func-debug-line-table-offsets` in order to terminate the
+        // sequence. Look for the next Label if possible, otherwise we will set
+        // the PC to the end of the section.
+        if (!Label && NextIt != LineEntries.end()) {
+          Label = NextIt->getLabel();
+        }
+        MCOS->emitDwarfLineEndEntry(Section, PrevLabel,
+                                    /*EndLabel =*/Label);
         init();
       }
       MCOS->emitLabel(LineEntry.LineStreamLabel, LineEntry.StreamLabelDefLoc);
@@ -211,7 +221,7 @@ void MCDwarfLineTable::emitOne(
     }
 
     if (LineEntry.IsEndEntry) {
-      MCOS->emitDwarfAdvanceLineAddr(INT64_MAX, LastLabel, Label,
+      MCOS->emitDwarfAdvanceLineAddr(INT64_MAX, PrevLabel, CurrLabel,
                                      asmInfo->getCodePointerSize());
       init();
       EndEntryEmitted = true;
@@ -258,12 +268,12 @@ void MCDwarfLineTable::emitOne(
     // At this point we want to emit/create the sequence to encode the delta in
     // line numbers and the increment of the address from the previous Label
     // and the current Label.
-    MCOS->emitDwarfAdvanceLineAddr(LineDelta, LastLabel, Label,
+    MCOS->emitDwarfAdvanceLineAddr(LineDelta, PrevLabel, CurrLabel,
                                    asmInfo->getCodePointerSize());
 
     Discriminator = 0;
     LastLine = LineEntry.getLine();
-    LastLabel = Label;
+    PrevLabel = CurrLabel;
     IsAtStartSeq = false;
   }
 
@@ -273,7 +283,7 @@ void MCDwarfLineTable::emitOne(
   // does not track ranges nor terminate the line table. In that case,
   // conservatively use the section end symbol to end the line table.
   if (!EndEntryEmitted && !IsAtStartSeq)
-    MCOS->emitDwarfLineEndEntry(Section, LastLabel);
+    MCOS->emitDwarfLineEndEntry(Section, PrevLabel);
 }
 
 void MCDwarfLineTable::endCurrentSeqAndEmitLineStreamLabel(MCStreamer *MCOS,