[llvm] [DFAJumpThreading] Avoid exploring the paths that never come back (PR #85505)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: XChy (XChy)

<details>
<summary>Changes</summary>

This patch does:
- Preserve loop info when unfolding selects.
- Reduce the search space for loop paths.

---
Full diff: https://github.com/llvm/llvm-project/pull/85505.diff


1 Files Affected:

- (modified) llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp (+21-7) 


``````````diff
diff --git a/llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp b/llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp
index e66a1a2ccb318c..d359b701646722 100644
--- a/llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp
@@ -126,7 +126,7 @@ class SelectInstToUnfold {
   explicit operator bool() const { return SI && SIUse; }
 };
 
-void unfold(DomTreeUpdater *DTU, SelectInstToUnfold SIToUnfold,
+void unfold(DomTreeUpdater *DTU, LoopInfo *LI, SelectInstToUnfold SIToUnfold,
             std::vector<SelectInstToUnfold> *NewSIsToUnfold,
             std::vector<BasicBlock *> *NewBBs);
 
@@ -152,7 +152,7 @@ class DFAJumpThreading {
 
       std::vector<SelectInstToUnfold> NewSIsToUnfold;
       std::vector<BasicBlock *> NewBBs;
-      unfold(&DTU, SIToUnfold, &NewSIsToUnfold, &NewBBs);
+      unfold(&DTU, LI, SIToUnfold, &NewSIsToUnfold, &NewBBs);
 
       // Put newly discovered select instructions into the work list.
       for (const SelectInstToUnfold &NewSIToUnfold : NewSIsToUnfold)
@@ -196,7 +196,7 @@ void createBasicBlockAndSinkSelectInst(
 /// created basic blocks into \p NewBBs.
 ///
 /// TODO: merge it with CodeGenPrepare::optimizeSelectInst() if possible.
-void unfold(DomTreeUpdater *DTU, SelectInstToUnfold SIToUnfold,
+void unfold(DomTreeUpdater *DTU, LoopInfo *LI, SelectInstToUnfold SIToUnfold,
             std::vector<SelectInstToUnfold> *NewSIsToUnfold,
             std::vector<BasicBlock *> *NewBBs) {
   SelectInst *SI = SIToUnfold.getInst();
@@ -302,6 +302,12 @@ void unfold(DomTreeUpdater *DTU, SelectInstToUnfold SIToUnfold,
   DTU->applyUpdates({{DominatorTree::Insert, StartBlock, TT},
                      {DominatorTree::Insert, StartBlock, FT}});
 
+  // Preserve loop info
+  if (Loop *L = LI->getLoopFor(SI->getParent())) {
+    for (BasicBlock *NewBB : *NewBBs)
+      L->addBasicBlockToLoop(NewBB, *LI);
+  }
+
   // The select is now dead.
   assert(SI->use_empty() && "Select must be dead now");
   SI->eraseFromParent();
@@ -501,9 +507,10 @@ struct MainSwitch {
 };
 
 struct AllSwitchPaths {
-  AllSwitchPaths(const MainSwitch *MSwitch, OptimizationRemarkEmitter *ORE)
-      : Switch(MSwitch->getInstr()), SwitchBlock(Switch->getParent()),
-        ORE(ORE) {}
+  AllSwitchPaths(const MainSwitch *MSwitch, OptimizationRemarkEmitter *ORE,
+                 LoopInfo *LI)
+      : Switch(MSwitch->getInstr()), SwitchBlock(Switch->getParent()), ORE(ORE),
+        LI(LI) {}
 
   std::vector<ThreadingPath> &getThreadingPaths() { return TPaths; }
   unsigned getNumThreadingPaths() { return TPaths.size(); }
@@ -575,6 +582,12 @@ struct AllSwitchPaths {
 
     Visited.insert(BB);
 
+    // Stop if we have reached the BB out of loop, since its successors have no
+    // impact on the DFA.
+    // TODO: Do we need to stop exploring if BB is the outer loop of the switch?
+    if (!LI->getLoopFor(BB))
+      return Res;
+
     // Some blocks have multiple edges to the same successor, and this set
     // is used to prevent a duplicate path from being generated
     SmallSet<BasicBlock *, 4> Successors;
@@ -716,6 +729,7 @@ struct AllSwitchPaths {
   BasicBlock *SwitchBlock;
   OptimizationRemarkEmitter *ORE;
   std::vector<ThreadingPath> TPaths;
+  LoopInfo *LI;
 };
 
 struct TransformDFA {
@@ -1283,7 +1297,7 @@ bool DFAJumpThreading::run(Function &F) {
     if (!Switch.getSelectInsts().empty())
       MadeChanges = true;
 
-    AllSwitchPaths SwitchPaths(&Switch, ORE);
+    AllSwitchPaths SwitchPaths(&Switch, ORE, LI);
     SwitchPaths.run();
 
     if (SwitchPaths.getNumThreadingPaths() > 0) {

``````````

</details>


https://github.com/llvm/llvm-project/pull/85505