[GlobalISel][IRTranslator] Port switch binary tree search optimization. (#77279)

This re-uses some code extracted earlier from SelectionDAG into
SwitchLoweringUtils

Much of the code is a straight port from SDAG's splitWorkItem(), with
minor changes needed for GISel.

Author: aemerson

llvm/include/llvm/CodeGen/GlobalISel/IRTranslator.h

@@ -366,6 +366,10 @@ class IRTranslator : public MachineFunctionPass {
                        BranchProbability BranchProbToNext, Register Reg,
                        SwitchCG::BitTestCase &B, MachineBasicBlock *SwitchBB);
 
+  void splitWorkItem(SwitchCG::SwitchWorkList &WorkList,
+                     const SwitchCG::SwitchWorkListItem &W, Value *Cond,
+                     MachineBasicBlock *SwitchMBB, MachineIRBuilder &MIB);
+
   bool lowerJumpTableWorkItem(
       SwitchCG::SwitchWorkListItem W, MachineBasicBlock *SwitchMBB,
       MachineBasicBlock *CurMBB, MachineBasicBlock *DefaultMBB,

llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp

@@ -751,16 +751,91 @@ bool IRTranslator::translateSwitch(const User &U, MachineIRBuilder &MIB) {
   auto DefaultProb = getEdgeProbability(SwitchMBB, DefaultMBB);
   WorkList.push_back({SwitchMBB, First, Last, nullptr, nullptr, DefaultProb});
 
-  // FIXME: At the moment we don't do any splitting optimizations here like
-  // SelectionDAG does, so this worklist only has one entry.
   while (!WorkList.empty()) {
     SwitchWorkListItem W = WorkList.pop_back_val();
+
+    unsigned NumClusters = W.LastCluster - W.FirstCluster + 1;
+    // For optimized builds, lower large range as a balanced binary tree.
+    if (NumClusters > 3 &&
+        MF->getTarget().getOptLevel() != CodeGenOptLevel::None &&
+        !DefaultMBB->getParent()->getFunction().hasMinSize()) {
+      splitWorkItem(WorkList, W, SI.getCondition(), SwitchMBB, MIB);
+      continue;
+    }
+
     if (!lowerSwitchWorkItem(W, SI.getCondition(), SwitchMBB, DefaultMBB, MIB))
       return false;
   }
   return true;
 }
 
+void IRTranslator::splitWorkItem(SwitchCG::SwitchWorkList &WorkList,
+                                 const SwitchCG::SwitchWorkListItem &W,
+                                 Value *Cond, MachineBasicBlock *SwitchMBB,
+                                 MachineIRBuilder &MIB) {
+  using namespace SwitchCG;
+  assert(W.FirstCluster->Low->getValue().slt(W.LastCluster->Low->getValue()) &&
+         "Clusters not sorted?");
+  assert(W.LastCluster - W.FirstCluster + 1 >= 2 && "Too small to split!");
+
+  auto [LastLeft, FirstRight, LeftProb, RightProb] =
+      SL->computeSplitWorkItemInfo(W);
+
+  // Use the first element on the right as pivot since we will make less-than
+  // comparisons against it.
+  CaseClusterIt PivotCluster = FirstRight;
+  assert(PivotCluster > W.FirstCluster);
+  assert(PivotCluster <= W.LastCluster);
+
+  CaseClusterIt FirstLeft = W.FirstCluster;
+  CaseClusterIt LastRight = W.LastCluster;
+
+  const ConstantInt *Pivot = PivotCluster->Low;
+
+  // New blocks will be inserted immediately after the current one.
+  MachineFunction::iterator BBI(W.MBB);
+  ++BBI;
+
+  // We will branch to the LHS if Value < Pivot. If LHS is a single cluster,
+  // we can branch to its destination directly if it's squeezed exactly in
+  // between the known lower bound and Pivot - 1.
+  MachineBasicBlock *LeftMBB;
+  if (FirstLeft == LastLeft && FirstLeft->Kind == CC_Range &&
+      FirstLeft->Low == W.GE &&
+      (FirstLeft->High->getValue() + 1LL) == Pivot->getValue()) {
+    LeftMBB = FirstLeft->MBB;
+  } else {
+    LeftMBB = FuncInfo.MF->CreateMachineBasicBlock(W.MBB->getBasicBlock());
+    FuncInfo.MF->insert(BBI, LeftMBB);
+    WorkList.push_back(
+        {LeftMBB, FirstLeft, LastLeft, W.GE, Pivot, W.DefaultProb / 2});
+  }
+
+  // Similarly, we will branch to the RHS if Value >= Pivot. If RHS is a
+  // single cluster, RHS.Low == Pivot, and we can branch to its destination
+  // directly if RHS.High equals the current upper bound.
+  MachineBasicBlock *RightMBB;
+  if (FirstRight == LastRight && FirstRight->Kind == CC_Range && W.LT &&
+      (FirstRight->High->getValue() + 1ULL) == W.LT->getValue()) {
+    RightMBB = FirstRight->MBB;
+  } else {
+    RightMBB = FuncInfo.MF->CreateMachineBasicBlock(W.MBB->getBasicBlock());
+    FuncInfo.MF->insert(BBI, RightMBB);
+    WorkList.push_back(
+        {RightMBB, FirstRight, LastRight, Pivot, W.LT, W.DefaultProb / 2});
+  }
+
+  // Create the CaseBlock record that will be used to lower the branch.
+  CaseBlock CB(ICmpInst::Predicate::ICMP_SLT, false, Cond, Pivot, nullptr,
+               LeftMBB, RightMBB, W.MBB, MIB.getDebugLoc(), LeftProb,
+               RightProb);
+
+  if (W.MBB == SwitchMBB)
+    emitSwitchCase(CB, SwitchMBB, MIB);
+  else
+    SL->SwitchCases.push_back(CB);
+}
+
 void IRTranslator::emitJumpTable(SwitchCG::JumpTable &JT,
                                  MachineBasicBlock *MBB) {
   // Emit the code for the jump table

llvm/test/CodeGen/AArch64/GlobalISel/irtranslator-switch-split.ll

@@ -17,31 +17,33 @@ define i32 @scanfile(i32 %call148) {
 ; CHECK-NEXT:    .cfi_offset w30, -8
 ; CHECK-NEXT:    .cfi_offset w29, -16
 ; CHECK-NEXT:    mov w8, w0
+; CHECK-NEXT:    cmp w0, #1
 ; CHECK-NEXT:    mov w0, wzr
-; CHECK-NEXT:    cbz w8, LBB0_7
+; CHECK-NEXT:    b.ge LBB0_3
 ; CHECK-NEXT:  ; %bb.1: ; %entry
-; CHECK-NEXT:    cmp w8, #1
-; CHECK-NEXT:    b.eq LBB0_7
-; CHECK-NEXT:  ; %bb.2: ; %entry
+; CHECK-NEXT:    cbnz w8, LBB0_7
+; CHECK-NEXT:  LBB0_2: ; %common.ret1
+; CHECK-NEXT:    ldp x29, x30, [sp], #16 ; 16-byte Folded Reload
+; CHECK-NEXT:    ret
+; CHECK-NEXT:  LBB0_3: ; %entry
+; CHECK-NEXT:    b.eq LBB0_2
+; CHECK-NEXT:  ; %bb.4: ; %entry
 ; CHECK-NEXT:    cmp w8, #2
-; CHECK-NEXT:    b.eq LBB0_4
-; CHECK-NEXT:  ; %bb.3: ; %entry
+; CHECK-NEXT:    b.eq LBB0_6
+; CHECK-NEXT:  ; %bb.5: ; %entry
 ; CHECK-NEXT:    cmp w8, #3
-; CHECK-NEXT:    b.ne LBB0_5
-; CHECK-NEXT:  LBB0_4: ; %sw.bb300
+; CHECK-NEXT:    b.ne LBB0_2
+; CHECK-NEXT:  LBB0_6: ; %sw.bb300
 ; CHECK-NEXT:    bl _logg
 ; CHECK-NEXT:    ldp x29, x30, [sp], #16 ; 16-byte Folded Reload
 ; CHECK-NEXT:    ret
-; CHECK-NEXT:  LBB0_5: ; %entry
+; CHECK-NEXT:  LBB0_7: ; %entry
 ; CHECK-NEXT:    cmn w8, #2
-; CHECK-NEXT:    b.eq LBB0_8
-; CHECK-NEXT:  ; %bb.6: ; %entry
+; CHECK-NEXT:    b.eq LBB0_9
+; CHECK-NEXT:  ; %bb.8: ; %entry
 ; CHECK-NEXT:    cmn w8, #1
-; CHECK-NEXT:    b.eq LBB0_8
-; CHECK-NEXT:  LBB0_7: ; %common.ret1
-; CHECK-NEXT:    ldp x29, x30, [sp], #16 ; 16-byte Folded Reload
-; CHECK-NEXT:    ret
-; CHECK-NEXT:  LBB0_8: ; %sw.bb150
+; CHECK-NEXT:    b.ne LBB0_2
+; CHECK-NEXT:  LBB0_9: ; %sw.bb150
 ; CHECK-NEXT:    bl _logg
 ; CHECK-NEXT:    brk #0x1
 entry: