[DFAJumpThreading] Don't thread switch without multiple successors (#71060)

Fixes #56882.
Fixes #60254.

When switch has only one successor, it make no sense to thread it. And
computing the cost of it brings div-by-zero exception. We prevent it in
this patch.

Author: XChy

llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp

@@ -730,6 +730,10 @@ struct TransformDFA {
     CodeMetrics Metrics;
     SwitchInst *Switch = SwitchPaths->getSwitchInst();
 
+    // Don't thread switch without multiple successors.
+    if (Switch->getNumSuccessors() <= 1)
+      return false;
+
     // Note that DuplicateBlockMap is not being used as intended here. It is
     // just being used to ensure (BB, State) pairs are only counted once.
     DuplicateBlockMap DuplicateMap;
@@ -807,6 +811,8 @@ struct TransformDFA {
       // using binary search, hence the LogBase2().
       unsigned CondBranches =
           APInt(32, Switch->getNumSuccessors()).ceilLogBase2();
+      assert(CondBranches > 0 &&
+             "The threaded switch must have multiple branches");
       DuplicationCost = Metrics.NumInsts / CondBranches;
     } else {
       // Compared with jump tables, the DFA optimizer removes an indirect branch

llvm/test/Transforms/DFAJumpThreading/single_succ_switch.ll

@@ -0,0 +1,43 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 3
+; RUN: opt -S -passes=dfa-jump-threading %s | FileCheck %s
+
+define void @pr60254() {
+; CHECK-LABEL: define void @pr60254() {
+; CHECK-NEXT:  entry_1:
+; CHECK-NEXT:    br label [[BB_2:%.*]]
+; CHECK:       bb_2:
+; CHECK-NEXT:    [[PTR_I32_25_0:%.*]] = phi i32 [ 0, [[BB_4:%.*]] ], [ 0, [[BB_2]] ], [ 0, [[ENTRY_1:%.*]] ]
+; CHECK-NEXT:    switch i32 [[PTR_I32_25_0]], label [[BB_2]] [
+; CHECK-NEXT:    ]
+; CHECK:       bb_4:
+; CHECK-NEXT:    br label [[BB_2]]
+;
+entry_1:
+  br label %bb_2
+
+bb_2:                                             ; preds = %bb_4, %bb_2, %entry_1
+  %ptr_i32_25.0 = phi i32 [ 0, %bb_4 ], [ 0, %bb_2 ], [ 0, %entry_1 ]
+  switch i32 %ptr_i32_25.0, label %bb_2 [
+  ]
+
+bb_4:                                             ; No predecessors!
+  br label %bb_2
+}
+
+define void @pr56882() {
+; CHECK-LABEL: define void @pr56882() {
+; CHECK-NEXT:  entry_1:
+; CHECK-NEXT:    br label [[BB_2:%.*]]
+; CHECK:       bb_2:
+; CHECK-NEXT:    [[PTR_I64_16_0:%.*]] = phi i64 [ -1317805584074026212, [[ENTRY_1:%.*]] ], [ -158622699357888703, [[BB_2]] ]
+; CHECK-NEXT:    switch i64 [[PTR_I64_16_0]], label [[BB_2]] [
+; CHECK-NEXT:    ]
+;
+entry_1:
+  br label %bb_2
+
+bb_2:                                             ; preds = %bb_2, %entry_1
+  %ptr_i64_16.0 = phi i64 [ -1317805584074026212, %entry_1 ], [ -158622699357888703, %bb_2 ]
+  switch i64 %ptr_i64_16.0, label %bb_2 [
+  ]
+}