[llvm] [DebugInfo] Don't apply is_stmt on MBB branches that preserve lines (PR #108251)

[Jeremy Morse via llvm-commits]

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@@ -2229,6 +2230,112 @@ void DwarfDebug::beginFunctionImpl(const MachineFunction *MF) {
   // Record beginning of function.
   PrologEndLoc = emitInitialLocDirective(
       *MF, Asm->OutStreamer->getContext().getDwarfCompileUnitID());
+
+  // For each MBB we try to determine whether and what source line is *always*
+  // the last stepped-on line before entering MBB. Such a line exists if every
+  // predecessor has an instruction with source line N, which is the last valid
+  // source line to be seen before entering MBB, and if N is the same for all
+  // predecessors. If this is true, and the first instruction with a valid
+  // source line in MBB also has source line N, then that instruction should
+  // *not* use is_stmt. Otherwise, the first instruction with a valid source
+  // line in MBB should always use is_stmt, since we may step to it from a
+  // different line.
+  ForceIsStmtInstrs.clear();
+
+  // First, collect the last stepped line for each MBB.
+  SmallDenseMap<std::pair<const MachineBasicBlock *, const MachineBasicBlock *>,
+                unsigned>
+      MBBExitLines;
+  const auto *TII = MF->getSubtarget().getInstrInfo();
+
+  // We only need to examine MBBs that could have is_stmt set by this logic.
+  // We use const_cast even though we won't actually modify MF, because some
+  // methods we need take a non-const MBB.
+  SetVector<MachineBasicBlock *> PredMBBsToExamine;
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jmorse wrote:

`SmallSetVector` maybe?

https://github.com/llvm/llvm-project/pull/108251