[SimplifyCFG]: Switch on umin replaces default

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -7540,6 +7540,58 @@ static bool reduceSwitchRange(SwitchInst *SI, IRBuilder<> &Builder,
   return true;
 }
 
+/// Tries to transform the switch when the condition is umin and a constant.
+/// In that case, the default branch can be replaced by the constant's branch.
+/// For example:
+/// switch(umin(a, 3)) {
+/// case 0:
+/// case 1:
+/// case 2:
+/// case 3:
+///   // ...
+/// default:
+///   unreachable
+/// }
+///
+/// Transforms into:
+///
+/// switch(a) {
+/// case 0:
+/// case 1:
+/// case 2:
+/// default:
+///   // This is case 3
+/// }
+static bool simplifySwitchWhenUMin(SwitchInst *SI, DomTreeUpdater *DTU) {
+  Value *A;
+  ConstantInt *Constant;
+
+  if (!match(SI->getCondition(), m_UMin(m_Value(A), m_ConstantInt(Constant))))
+    return false;
+
+  if (!SI->defaultDestUnreachable())
+    return false;
+
+  auto Case = SI->findCaseValue(Constant);
+
+  // When the case value cannot be found then `findCaseValue` returns the
+  // default cause. This means that there is no `case 3:` and this
+  // simplification fails.
+  if (Case == SI->case_default())
+    return false;
+
+  BasicBlock *Unreachable = SI->getDefaultDest();
+  SI->setDefaultDest(Case->getCaseSuccessor());
+  SI->removeCase(Case);
+  SI->setCondition(A);
+
+  BasicBlock *BB = SI->getParent();
+  if (DTU)
+    DTU->applyUpdates({{DominatorTree::Delete, BB, Unreachable}});
+
+  return true;
+}
+
 /// Tries to transform switch of powers of two to reduce switch range.
 /// For example, switch like:
 /// switch (C) { case 1: case 2: case 64: case 128: }
@@ -7966,6 +8018,9 @@ bool SimplifyCFGOpt::simplifySwitch(SwitchInst *SI, IRBuilder<> &Builder) {
   if (simplifyDuplicateSwitchArms(SI, DTU))
     return requestResimplify();
 
+  if (simplifySwitchWhenUMin(SI, DTU))
+    return requestResimplify();
+
   return false;
 }
 

llvm/test/Transforms/SimplifyCFG/switch-umin.ll

@@ -0,0 +1,105 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
+; RUN: opt -S -passes=simplifycfg < %s | FileCheck %s
+
+declare void @a()
+declare void @b()
+declare void @c()
+
+define void @switch_replace_default(i32 %x) {
+; CHECK-LABEL: define void @switch_replace_default(
+; CHECK-SAME: i32 [[X:%.*]]) {
+; CHECK-NEXT:    [[MIN:%.*]] = call i32 @llvm.umin.i32(i32 [[X]], i32 3)
+; CHECK-NEXT:    switch i32 [[X]], label %[[COMMON_RET:.*]] [
+; CHECK-NEXT:      i32 0, label %[[CASE0:.*]]
+; CHECK-NEXT:      i32 1, label %[[CASE1:.*]]
+; CHECK-NEXT:      i32 2, label %[[CASE2:.*]]
+; CHECK-NEXT:    ]
+; CHECK:       [[COMMON_RET]]:
+; CHECK-NEXT:    ret void
+; CHECK:       [[CASE0]]:
+; CHECK-NEXT:    call void @a()
+; CHECK-NEXT:    br label %[[COMMON_RET]]
+; CHECK:       [[CASE1]]:
+; CHECK-NEXT:    call void @b()
+; CHECK-NEXT:    br label %[[COMMON_RET]]
+; CHECK:       [[CASE2]]:
+; CHECK-NEXT:    call void @c()
+; CHECK-NEXT:    br label %[[COMMON_RET]]
+;
+  %min = call i32 @llvm.umin.i32(i32 %x, i32 3)
+  switch i32 %min, label %unreachable [
+  i32 0, label %case0
+  i32 1, label %case1
+  i32 2, label %case2
+  i32 3, label %case3
+  ]
+
+case0:
+  call void @a()
+  ret void
+
+case1:
+  call void @b()
+  ret void
+
+case2:
+  call void @c()
+  ret void
+
+case3:
+  ret void
+
+unreachable:
+  unreachable
+}
+
+define void @do_not_switch_replace_default(i32 %x, i32 %y) {
+; CHECK-LABEL: define void @do_not_switch_replace_default(
+; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
+; CHECK-NEXT:    [[MIN:%.*]] = call i32 @llvm.umin.i32(i32 [[X]], i32 [[Y]])
+; CHECK-NEXT:    switch i32 [[MIN]], label %[[UNREACHABLE:.*]] [
+; CHECK-NEXT:      i32 0, label %[[CASE0:.*]]
+; CHECK-NEXT:      i32 1, label %[[CASE1:.*]]
+; CHECK-NEXT:      i32 2, label %[[CASE2:.*]]
+; CHECK-NEXT:      i32 3, label %[[COMMON_RET:.*]]
+; CHECK-NEXT:    ]
+; CHECK:       [[COMMON_RET]]:
+; CHECK-NEXT:    ret void
+; CHECK:       [[CASE0]]:
+; CHECK-NEXT:    call void @a()
+; CHECK-NEXT:    br label %[[COMMON_RET]]
+; CHECK:       [[CASE1]]:
+; CHECK-NEXT:    call void @b()
+; CHECK-NEXT:    br label %[[COMMON_RET]]
+; CHECK:       [[CASE2]]:
+; CHECK-NEXT:    call void @c()
+; CHECK-NEXT:    br label %[[COMMON_RET]]
+; CHECK:       [[UNREACHABLE]]:
+; CHECK-NEXT:    unreachable
+;
+  %min = call i32 @llvm.umin.i32(i32 %x, i32 %y)
+  switch i32 %min, label %unreachable [
+  i32 0, label %case0
+  i32 1, label %case1
+  i32 2, label %case2
+  i32 3, label %case3
+  ]
+
+case0:
+  call void @a()
+  ret void
+
+case1:
+  call void @b()
+  ret void
+
+case2:
+  call void @c()
+  ret void
+
+case3:
+  ret void
+
+unreachable:
+  unreachable
+}