[llvm] [DWARF] Emit line 0 source locations for BB padding nops (PR #99496)

[Nabeel Omer via llvm-commits]

================
@@ -3668,3 +3668,19 @@ bool DwarfDebug::alwaysUseRanges(const DwarfCompileUnit &CU) const {
     return true;
   return false;
 }
+
+void DwarfDebug::beginCodeAlignment(const MachineBasicBlock &MBB) {
+  auto *SP = MBB.getParent()->getFunction().getSubprogram();
+
+  bool NoDebug =
+      !SP || SP->getUnit()->getEmissionKind() == DICompileUnit::NoDebug;
+
+  if (NoDebug)
+    return;
+
+  auto PrevLoc = Asm->OutStreamer->getContext().getCurrentDwarfLoc();
+  Asm->OutStreamer->emitDwarfLocDirective(
----------------
omern1 wrote:

I see what you mean.

```
0000000000000000 <f>:
       ; ...
       3: 7e 2b                         jle     0x30 <f+0x30>
       5: 66 2e 0f 1f 84 00 00 00 00 00 nopw    %cs:(%rax,%rax)
       f: 90                            nop
      10: e8 00 00 00 00                callq   0x15 <f+0x15>
      15: e8 00 00 00 00                callq   0x1a <f+0x1a>
      1a: e8 00 00 00 00                callq   0x1f <f+0x1f>
      1f: e8 00 00 00 00                callq   0x24 <f+0x24>
      24: e8 00 00 00 00                callq   0x29 <f+0x29>
      29: e8 00 00 00 00                callq   0x2e <f+0x2e>
      2e: eb e0                         jmp     0x10 <f+0x10>

      30: e8 00 00 00 00                callq   0x35 <f+0x35> ; BB starting here is a loop but has no nops
      35: eb f9                         jmp     0x30 <f+0x30>
```

We will end up with a line 0 record for address 0x30 immediately followed by a record for the correct line number.

The `MCAlignFragment` type only allows for the possibility of nops being emitted, it does not guarantee that they'll be emitted. The addresses aren't known till after the assembler computes layout which happens much later than here. I can't think of a way to handle this case as things currently stand though there may be other debug-info related mechanisms to deal with this that I'm not aware of, what do you suggest?

https://github.com/llvm/llvm-project/pull/99496