[llvm] [AMDGPU] Preserve post dominator tree through SILowerControlFlow (PR #153528)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-amdgpu

Author: Carl Ritson (perlfu)

<details>
<summary>Changes</summary>

Change dominator tree updates to also handle post dominator tree.

---
Full diff: https://github.com/llvm/llvm-project/pull/153528.diff


1 Files Affected:

- (modified) llvm/lib/Target/AMDGPU/SILowerControlFlow.cpp (+36-20) 


``````````diff
diff --git a/llvm/lib/Target/AMDGPU/SILowerControlFlow.cpp b/llvm/lib/Target/AMDGPU/SILowerControlFlow.cpp
index f8878f32f829d..f7a9a584a6b51 100644
--- a/llvm/lib/Target/AMDGPU/SILowerControlFlow.cpp
+++ b/llvm/lib/Target/AMDGPU/SILowerControlFlow.cpp
@@ -57,6 +57,7 @@
 #include "llvm/CodeGen/LiveVariables.h"
 #include "llvm/CodeGen/MachineDominators.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachinePostDominators.h"
 #include "llvm/Target/TargetMachine.h"
 
 using namespace llvm;
@@ -76,6 +77,7 @@ class SILowerControlFlow {
   LiveIntervals *LIS = nullptr;
   LiveVariables *LV = nullptr;
   MachineDominatorTree *MDT = nullptr;
+  MachinePostDominatorTree *PDT = nullptr;
   MachineRegisterInfo *MRI = nullptr;
   SetVector<MachineInstr*> LoweredEndCf;
   DenseSet<Register> LoweredIf;
@@ -138,8 +140,8 @@ class SILowerControlFlow {
 
 public:
   SILowerControlFlow(LiveIntervals *LIS, LiveVariables *LV,
-                     MachineDominatorTree *MDT)
-      : LIS(LIS), LV(LV), MDT(MDT) {}
+                     MachineDominatorTree *MDT, MachinePostDominatorTree *PDT)
+      : LIS(LIS), LV(LV), MDT(MDT), PDT(PDT) {}
   bool run(MachineFunction &MF);
 };
 
@@ -159,6 +161,7 @@ class SILowerControlFlowLegacy : public MachineFunctionPass {
     AU.addUsedIfAvailable<LiveIntervalsWrapperPass>();
     // Should preserve the same set that TwoAddressInstructions does.
     AU.addPreserved<MachineDominatorTreeWrapperPass>();
+    AU.addPreserved<MachinePostDominatorTreeWrapperPass>();
     AU.addPreserved<SlotIndexesWrapperPass>();
     AU.addPreserved<LiveIntervalsWrapperPass>();
     AU.addPreserved<LiveVariablesWrapperPass>();
@@ -506,13 +509,18 @@ MachineBasicBlock *SILowerControlFlow::emitEndCf(MachineInstr &MI) {
   MachineBasicBlock *SplitBB = &MBB;
   if (NeedBlockSplit) {
     SplitBB = MBB.splitAt(MI, /*UpdateLiveIns*/true, LIS);
-    if (MDT && SplitBB != &MBB) {
-      MachineDomTreeNode *MBBNode = (*MDT)[&MBB];
-      SmallVector<MachineDomTreeNode *> Children(MBBNode->begin(),
-                                                 MBBNode->end());
-      MachineDomTreeNode *SplitBBNode = MDT->addNewBlock(SplitBB, &MBB);
-      for (MachineDomTreeNode *Child : Children)
-        MDT->changeImmediateDominator(Child, SplitBBNode);
+    if (SplitBB != &MBB && (MDT || PDT)) {
+      using DomTreeT = DomTreeBase<MachineBasicBlock>;
+      SmallVector<DomTreeT::UpdateType, 16> DTUpdates;
+      for (MachineBasicBlock *Succ : SplitBB->successors()) {
+        DTUpdates.push_back({DomTreeT::Insert, SplitBB, Succ});
+        DTUpdates.push_back({DomTreeT::Delete, &MBB, Succ});
+      }
+      DTUpdates.push_back({DomTreeT::Insert, &MBB, SplitBB});
+      if (MDT)
+        MDT->applyUpdates(DTUpdates);
+      if (PDT)
+        PDT->applyUpdates(DTUpdates);
     }
     Opcode = OrTermrOpc;
     InsPt = MI;
@@ -727,26 +735,27 @@ bool SILowerControlFlow::removeMBBifRedundant(MachineBasicBlock &MBB) {
   MachineBasicBlock *Succ = *MBB.succ_begin();
   MachineBasicBlock *FallThrough = nullptr;
 
+  using DomTreeT = DomTreeBase<MachineBasicBlock>;
+  SmallVector<DomTreeT::UpdateType, 8> DTUpdates;
+
   while (!MBB.predecessors().empty()) {
     MachineBasicBlock *P = *MBB.pred_begin();
     if (P->getFallThrough(false) == &MBB)
       FallThrough = P;
     P->ReplaceUsesOfBlockWith(&MBB, Succ);
+    DTUpdates.push_back({DomTreeT::Insert, P, Succ});
+    DTUpdates.push_back({DomTreeT::Delete, P, &MBB});
   }
   MBB.removeSuccessor(Succ);
   if (LIS) {
     for (auto &I : MBB.instrs())
       LIS->RemoveMachineInstrFromMaps(I);
   }
-  if (MDT) {
-    // If Succ, the single successor of MBB, is dominated by MBB, MDT needs
-    // updating by changing Succ's idom to the one of MBB; otherwise, MBB must
-    // be a leaf node in MDT and could be erased directly.
-    if (MDT->dominates(&MBB, Succ))
-      MDT->changeImmediateDominator(MDT->getNode(Succ),
-                                    MDT->getNode(&MBB)->getIDom());
-    MDT->eraseNode(&MBB);
-  }
+  if (MDT)
+    MDT->applyUpdates(DTUpdates);
+  if (PDT)
+    PDT->applyUpdates(DTUpdates);
+
   MBB.clear();
   MBB.eraseFromParent();
   if (FallThrough && !FallThrough->isLayoutSuccessor(Succ)) {
@@ -875,7 +884,11 @@ bool SILowerControlFlowLegacy::runOnMachineFunction(MachineFunction &MF) {
   LiveVariables *LV = LVWrapper ? &LVWrapper->getLV() : nullptr;
   auto *MDTWrapper = getAnalysisIfAvailable<MachineDominatorTreeWrapperPass>();
   MachineDominatorTree *MDT = MDTWrapper ? &MDTWrapper->getDomTree() : nullptr;
-  return SILowerControlFlow(LIS, LV, MDT).run(MF);
+  auto *PDTWrapper =
+      getAnalysisIfAvailable<MachinePostDominatorTreeWrapperPass>();
+  MachinePostDominatorTree *PDT =
+      PDTWrapper ? &PDTWrapper->getPostDomTree() : nullptr;
+  return SILowerControlFlow(LIS, LV, MDT, PDT).run(MF);
 }
 
 PreservedAnalyses
@@ -885,13 +898,16 @@ SILowerControlFlowPass::run(MachineFunction &MF,
   LiveVariables *LV = MFAM.getCachedResult<LiveVariablesAnalysis>(MF);
   MachineDominatorTree *MDT =
       MFAM.getCachedResult<MachineDominatorTreeAnalysis>(MF);
+  MachinePostDominatorTree *PDT =
+      MFAM.getCachedResult<MachinePostDominatorTreeAnalysis>(MF);
 
-  bool Changed = SILowerControlFlow(LIS, LV, MDT).run(MF);
+  bool Changed = SILowerControlFlow(LIS, LV, MDT, PDT).run(MF);
   if (!Changed)
     return PreservedAnalyses::all();
 
   auto PA = getMachineFunctionPassPreservedAnalyses();
   PA.preserve<MachineDominatorTreeAnalysis>();
+  PA.preserve<MachinePostDominatorTreeAnalysis>();
   PA.preserve<SlotIndexesAnalysis>();
   PA.preserve<LiveIntervalsAnalysis>();
   PA.preserve<LiveVariablesAnalysis>();

``````````

</details>


https://github.com/llvm/llvm-project/pull/153528