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

[Paul T Robinson 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(
----------------
pogo59 wrote:

Usually we call recordSourceLine(), but in this case we're assuming we're at the top of a basic block and (maybe) emitting nops; so, zeroing out flags and discriminator as well as line seems like a reasonable simplification.

What happens if the block is already aligned? No nops will be emitted, and presumably you'd end up with a .loc for a line 0 immediately followed by a .loc for the correct line. The line table will encode that correctly, but I'm suspicious that some consumers will decide the instruction actually has line 0 (because that's what it will find first for that address). Please test that.

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