[DFAJumpThreading] Add an early exit heuristic for unpredictable valu…

…es (#85015)

Right now the algorithm does not exit on unpredictable values. It
waits until all the paths have been enumerated to see if any of
those paths have that value. Waiting this late leads to a lot of
wasteful computation and higher compile time.

In this patch I have added a heuristic that checks if the value
comes from the same inner loops as the switch, if so, then it is
likely that the value will also be seen on a threadable path and
the code in `getStateDefMap()` return an empty map.

I tested this on the llvm test suite and the only change in the
number of threaded switches was in 7zip (before 23, after 18).
In all of those cases the current algorithm was partially threading
the loop because it was hitting a limit on the number of paths to
be explored. On increasing this limit even the current algorithm
finds paths where the unpredictable value is seen.

Compile time(with pass enabled by default and this patch):

https://llvm-compile-time-tracker.com/compare.php?from=8c5e9cf737138aba22a4a8f64ef2c5efc80dd7f9&to=42c75d888058b35c6d15901b34e36251d8f766b9&stat=instructions:u

llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp

@@ -96,6 +96,11 @@ static cl::opt<bool>
                     cl::desc("View the CFG before DFA Jump Threading"),
                     cl::Hidden, cl::init(false));
 
+static cl::opt<bool> EarlyExitHeuristic(
+    "dfa-early-exit-heuristic",
+    cl::desc("Exit early if an unpredictable value come from the same loop"),
+    cl::Hidden, cl::init(true));
+
 static cl::opt<unsigned> MaxPathLength(
     "dfa-max-path-length",
     cl::desc("Max number of blocks searched to find a threading path"),
@@ -405,7 +410,7 @@ struct MainSwitch {
   ///
   /// Also, collect select instructions to unfold.
   bool isCandidate(const SwitchInst *SI) {
-    std::deque<Value *> Q;
+    std::deque<std::pair<Value *, BasicBlock *>> Q;
     SmallSet<Value *, 16> SeenValues;
     SelectInsts.clear();
 
@@ -415,25 +420,28 @@ struct MainSwitch {
       return false;
 
     // The switch must be in a loop.
-    if (!LI->getLoopFor(SI->getParent()))
+    const Loop *L = LI->getLoopFor(SI->getParent());
+    if (!L)
       return false;
 
-    addToQueue(SICond, Q, SeenValues);
+    addToQueue(SICond, nullptr, Q, SeenValues);
 
     while (!Q.empty()) {
-      Value *Current = Q.front();
+      Value *Current = Q.front().first;
+      BasicBlock *CurrentIncomingBB = Q.front().second;
       Q.pop_front();
 
       if (auto *Phi = dyn_cast<PHINode>(Current)) {
-        for (Value *Incoming : Phi->incoming_values()) {
-          addToQueue(Incoming, Q, SeenValues);
+        for (BasicBlock *IncomingBB : Phi->blocks()) {
+          Value *Incoming = Phi->getIncomingValueForBlock(IncomingBB);
+          addToQueue(Incoming, IncomingBB, Q, SeenValues);
         }
         LLVM_DEBUG(dbgs() << "\tphi: " << *Phi << "\n");
       } else if (SelectInst *SelI = dyn_cast<SelectInst>(Current)) {
         if (!isValidSelectInst(SelI))
           return false;
-        addToQueue(SelI->getTrueValue(), Q, SeenValues);
-        addToQueue(SelI->getFalseValue(), Q, SeenValues);
+        addToQueue(SelI->getTrueValue(), CurrentIncomingBB, Q, SeenValues);
+        addToQueue(SelI->getFalseValue(), CurrentIncomingBB, Q, SeenValues);
         LLVM_DEBUG(dbgs() << "\tselect: " << *SelI << "\n");
         if (auto *SelIUse = dyn_cast<PHINode>(SelI->user_back()))
           SelectInsts.push_back(SelectInstToUnfold(SelI, SelIUse));
@@ -446,18 +454,31 @@ struct MainSwitch {
         // initial switch values that can be ignored (they will hit the
         // unthreaded switch) but this assumption will get checked later after
         // paths have been enumerated (in function getStateDefMap).
+
+        // If the unpredictable value comes from the same inner loop it is
+        // likely that it will also be on the enumerated paths, causing us to
+        // exit after we have enumerated all the paths. This heuristic save
+        // compile time because a search for all the paths can become expensive.
+        if (EarlyExitHeuristic &&
+            L->contains(LI->getLoopFor(CurrentIncomingBB))) {
+          LLVM_DEBUG(dbgs()
+                     << "\tExiting early due to unpredictability heuristic.\n");
+          return false;
+        }
+
         continue;
       }
     }
 
     return true;
   }
 
-  void addToQueue(Value *Val, std::deque<Value *> &Q,
+  void addToQueue(Value *Val, BasicBlock *BB,
+                  std::deque<std::pair<Value *, BasicBlock *>> &Q,
                   SmallSet<Value *, 16> &SeenValues) {
     if (SeenValues.contains(Val))
       return;
-    Q.push_back(Val);
+    Q.push_back({Val, BB});
     SeenValues.insert(Val);
   }
 

llvm/test/Transforms/DFAJumpThreading/dfa-unfold-select.ll

@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt -S -passes=dfa-jump-threading %s | FileCheck %s
+; RUN: opt -S -passes=dfa-jump-threading -dfa-early-exit-heuristic=false %s | FileCheck %s
 
 ; These tests check if selects are unfolded properly for jump threading
 ; opportunities. There are three different patterns to consider:

llvm/test/Transforms/DFAJumpThreading/unpredictable-heuristic.ll

@@ -0,0 +1,124 @@
+; REQUIRES: asserts
+; RUN: opt -S -passes=dfa-jump-threading %s -debug-only=dfa-jump-threading 2>&1 | FileCheck %s
+
+; CHECK-COUNT-3: Exiting early due to unpredictability heuristic.
+
+@.str.1 = private unnamed_addr constant [3 x i8] c"10\00", align 1
+@.str.2 = private unnamed_addr constant [3 x i8] c"30\00", align 1
+@.str.3 = private unnamed_addr constant [3 x i8] c"20\00", align 1
+@.str.4 = private unnamed_addr constant [3 x i8] c"40\00", align 1
+
+define void @test1(i32 noundef %num, i32 noundef %num2) {
+entry:
+  br label %while.body
+
+while.body:                                       ; preds = %entry, %sw.epilog
+  %num.addr.0 = phi i32 [ %num, %entry ], [ %num.addr.1, %sw.epilog ]
+  switch i32 %num.addr.0, label %sw.default [
+    i32 10, label %sw.bb
+    i32 30, label %sw.bb1
+    i32 20, label %sw.bb2
+    i32 40, label %sw.bb3
+  ]
+
+sw.bb:                                            ; preds = %while.body
+  %call.i = tail call i32 @bar(ptr noundef nonnull @.str.1)
+  br label %sw.epilog
+
+sw.bb1:                                           ; preds = %while.body
+  %call.i4 = tail call i32 @bar(ptr noundef nonnull @.str.2)
+  br label %sw.epilog
+
+sw.bb2:                                           ; preds = %while.body
+  %call.i5 = tail call i32 @bar(ptr noundef nonnull @.str.3)
+  br label %sw.epilog
+
+sw.bb3:                                           ; preds = %while.body
+  %call.i6 = tail call i32 @bar(ptr noundef nonnull @.str.4)
+  %call = tail call noundef i32 @foo()
+  %add = add nsw i32 %call, %num2
+  br label %sw.epilog
+
+sw.default:                                       ; preds = %while.body
+  ret void
+
+sw.epilog:                                        ; preds = %sw.bb3, %sw.bb2, %sw.bb1, %sw.bb
+  %num.addr.1 = phi i32 [ %add, %sw.bb3 ], [ 40, %sw.bb2 ], [ 20, %sw.bb1 ], [ 30, %sw.bb ]
+  br label %while.body
+}
+
+
+define void @test2(i32 noundef %num, i32 noundef %num2) {
+entry:
+  br label %while.body
+
+while.body:                                       ; preds = %entry, %sw.epilog
+  %num.addr.0 = phi i32 [ %num, %entry ], [ %num.addr.1, %sw.epilog ]
+  switch i32 %num.addr.0, label %sw.default [
+    i32 10, label %sw.epilog
+    i32 30, label %sw.bb1
+    i32 20, label %sw.bb2
+    i32 40, label %sw.bb3
+  ]
+
+sw.bb1:                                           ; preds = %while.body
+  br label %sw.epilog
+
+sw.bb2:                                           ; preds = %while.body
+  br label %sw.epilog
+
+sw.bb3:                                           ; preds = %while.body
+  br label %sw.epilog
+
+sw.default:                                       ; preds = %while.body
+  ret void
+
+sw.epilog:                                        ; preds = %while.body, %sw.bb3, %sw.bb2, %sw.bb1
+  %.str.4.sink = phi ptr [ @.str.4, %sw.bb3 ], [ @.str.3, %sw.bb2 ], [ @.str.2, %sw.bb1 ], [ @.str.1, %while.body ]
+  %num.addr.1 = phi i32 [ %num2, %sw.bb3 ], [ 40, %sw.bb2 ], [ 20, %sw.bb1 ], [ 30, %while.body ]
+  %call.i6 = tail call i32 @bar(ptr noundef nonnull %.str.4.sink)
+  br label %while.body
+}
+
+
+define void @test3(i32 noundef %num, i32 noundef %num2) {
+entry:
+  %add = add nsw i32 %num2, 40
+  br label %while.body
+
+while.body:                                       ; preds = %entry, %sw.epilog
+  %num.addr.0 = phi i32 [ %num, %entry ], [ %num.addr.1, %sw.epilog ]
+  switch i32 %num.addr.0, label %sw.default [
+    i32 10, label %sw.bb
+    i32 30, label %sw.bb1
+    i32 20, label %sw.bb2
+    i32 40, label %sw.bb3
+  ]
+
+sw.bb:                                            ; preds = %while.body
+  %call.i = tail call i32 @bar(ptr noundef nonnull @.str.1)
+  br label %sw.epilog
+
+sw.bb1:                                           ; preds = %while.body
+  %call.i5 = tail call i32 @bar(ptr noundef nonnull @.str.2)
+  br label %sw.epilog
+
+sw.bb2:                                           ; preds = %while.body
+  %call.i6 = tail call i32 @bar(ptr noundef nonnull @.str.3)
+  br label %sw.epilog
+
+sw.bb3:                                           ; preds = %while.body
+  %call.i7 = tail call i32 @bar(ptr noundef nonnull @.str.4)
+  br label %sw.epilog
+
+sw.default:                                       ; preds = %while.body
+  ret void
+
+sw.epilog:                                        ; preds = %sw.bb3, %sw.bb2, %sw.bb1, %sw.bb
+  %num.addr.1 = phi i32 [ %add, %sw.bb3 ], [ 40, %sw.bb2 ], [ 20, %sw.bb1 ], [ 30, %sw.bb ]
+  br label %while.body
+}
+
+
+declare noundef i32 @foo()
+declare noundef i32 @bar(ptr nocapture noundef readonly)