[llvm] [BranchFolding] Kill common hoisted debug instructions (PR #140091)

[Jeremy Morse via llvm-commits]

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@@ -2083,19 +2083,54 @@ bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
   if (TBB == FBB) {
     MBB->splice(Loc, TBB, TBB->begin(), TIB);
   } else {
+    // Merge the debug locations, and hoist and kill the debug instructions from
+    // both branches. FIXME: We could probably try harder to preserve some debug
+    // instructions (but at least this isn't producing wrong locations).
+    MachineInstrBuilder MIRBuilder(*MBB->getParent(), Loc);
+    auto HoistAndKillDbgInstr = [MBB, Loc](MachineBasicBlock::iterator DI) {
+      assert(DI->isDebugInstr() && "Expected a debug instruction");
+      if (DI->isDebugRef()) {
+        const TargetInstrInfo *TII =
+            MBB->getParent()->getSubtarget().getInstrInfo();
+        const MCInstrDesc &DBGV = TII->get(TargetOpcode::DBG_VALUE);
+        DI = BuildMI(*MBB->getParent(), DI->getDebugLoc(), DBGV, false, 0,
+                     DI->getDebugVariable(), DI->getDebugExpression());
+        MBB->insert(Loc, &*DI);
+        return;
+      }
+
+      DI->setDebugValueUndef();
+      DI->moveBefore(&*Loc);
+    };
+
     // TIB and FIB point to the end of the regions to hoist/merge in TBB and
     // FBB.
     MachineBasicBlock::iterator FE = FIB;
     MachineBasicBlock::iterator FI = FBB->begin();
     for (MachineBasicBlock::iterator TI :
          make_early_inc_range(make_range(TBB->begin(), TIB))) {
-      // Move debug instructions and pseudo probes without modifying them.
-      // FIXME: This is the wrong thing to do for debug locations, which
-      // should at least be killed (and hoisted from BOTH blocks).
-      if (TI->isDebugOrPseudoInstr()) {
+
+      // Hoist and kill debug instructions from FBB. Skip over pseudo
+      // instructions. After this loop FI points to the next non-debug
+      // instruction to hoist (checked in assert after the TBB debug
+      // instruction handling code).
+      while (FI->isDebugOrPseudoInstr()) {
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jmorse wrote:

`HoistAndKillDbgInst` asserts that only debug-instrs are fed into it, should this condition should be downgraded?

https://github.com/llvm/llvm-project/pull/140091