[InstCombine] Canonicalize switch(X^C) to switch(X)

`switch(X^C)` expressions can be folded to `switch(X)`. Minor
opportunity to generalize simplifications in `visitSwitchInst`
via an inverse function helper as well.

Proof: https://alive2.llvm.org/ce/z/TMRy_3.

Author: antoniofrighetto

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -3878,26 +3878,29 @@ static Value *simplifySwitchOnSelectUsingRanges(SwitchInst &SI,
 Instruction *InstCombinerImpl::visitSwitchInst(SwitchInst &SI) {
   Value *Cond = SI.getCondition();
   Value *Op0;
-  ConstantInt *AddRHS;
-  if (match(Cond, m_Add(m_Value(Op0), m_ConstantInt(AddRHS)))) {
-    // Change 'switch (X+4) case 1:' into 'switch (X) case -3'.
-    for (auto Case : SI.cases()) {
-      Constant *NewCase = ConstantExpr::getSub(Case.getCaseValue(), AddRHS);
-      assert(isa<ConstantInt>(NewCase) &&
-             "Result of expression should be constant");
-      Case.setValue(cast<ConstantInt>(NewCase));
-    }
-    return replaceOperand(SI, 0, Op0);
-  }
+  const APInt *CondOpC;
+  using InvertFn = std::function<APInt(const APInt &Case, const APInt &C)>;
 
-  ConstantInt *SubLHS;
-  if (match(Cond, m_Sub(m_ConstantInt(SubLHS), m_Value(Op0)))) {
-    // Change 'switch (1-X) case 1:' into 'switch (X) case 0'.
-    for (auto Case : SI.cases()) {
-      Constant *NewCase = ConstantExpr::getSub(SubLHS, Case.getCaseValue());
-      assert(isa<ConstantInt>(NewCase) &&
-             "Result of expression should be constant");
-      Case.setValue(cast<ConstantInt>(NewCase));
+  auto MaybeInvertible = [&](Value *Cond) -> std::optional<InvertFn> {
+    if (match(Cond, m_Add(m_Value(Op0), m_APInt(CondOpC))))
+      // Change 'switch (X+C) case A:' into 'switch (X) case C-A'.
+      return [](const APInt &C, const APInt &Case) { return C - Case; };
+
+    if (match(Cond, m_Sub(m_APInt(CondOpC), m_Value(Op0))))
+      // Change 'switch (C-X) case A:' into 'switch (X) case A-C'.
+      return [](const APInt &C, const APInt &Case) { return Case - C; };
+
+    if (match(Cond, m_Xor(m_Value(Op0), m_APInt(CondOpC))))
+      // Change 'switch (X^C) case A:' into 'switch (X) case A^C'.
+      return [](const APInt &C, const APInt &Case) { return C ^ Case; };
+
+    return std::nullopt;
+  };
+
+  if (auto InvertFn = MaybeInvertible(Cond)) {
+    for (auto &Case : SI.cases()) {
+      const APInt &New = (*InvertFn)(Case.getCaseValue()->getValue(), *CondOpC);
+      Case.setValue(ConstantInt::get(SI.getContext(), New));
     }
     return replaceOperand(SI, 0, Op0);
   }

llvm/test/Transforms/InstCombine/narrow-switch.ll

@@ -235,11 +235,10 @@ define i32 @trunc32to16(i32 %a0) #0 {
 ; ALL-NEXT:    [[RETVAL:%.*]] = alloca i32, align 4
 ; ALL-NEXT:    [[XOR:%.*]] = lshr i32 [[A0]], 16
 ; ALL-NEXT:    [[TMP0:%.*]] = trunc nuw i32 [[XOR]] to i16
-; ALL-NEXT:    [[TRUNC:%.*]] = xor i16 [[TMP0]], 15784
-; ALL-NEXT:    switch i16 [[TRUNC]], label %[[SW_EPILOG:.*]] [
-; ALL-NEXT:      i16 63, label %[[SW_BB:.*]]
-; ALL-NEXT:      i16 1, label %[[SW_BB1:.*]]
-; ALL-NEXT:      i16 100, label %[[SW_BB2:.*]]
+; ALL-NEXT:    switch i16 [[TMP0]], label %[[SW_EPILOG:.*]] [
+; ALL-NEXT:      i16 15767, label %[[SW_BB:.*]]
+; ALL-NEXT:      i16 15785, label %[[SW_BB1:.*]]
+; ALL-NEXT:      i16 15820, label %[[SW_BB2:.*]]
 ; ALL-NEXT:    ]
 ; ALL:       [[SW_BB]]:
 ; ALL-NEXT:    store i32 90, ptr [[RETVAL]], align 4

llvm/test/Transforms/InstCombine/switch-xor.ll

@@ -5,11 +5,10 @@ define i1 @test_switch_with_xor(i32 %x) {
 ; CHECK-LABEL: define i1 @test_switch_with_xor(
 ; CHECK-SAME: i32 [[X:%.*]]) {
 ; CHECK-NEXT:  [[ENTRY:.*:]]
-; CHECK-NEXT:    [[XOR:%.*]] = xor i32 [[X]], 2
-; CHECK-NEXT:    switch i32 [[XOR]], label %[[SW_DEFAULT:.*]] [
-; CHECK-NEXT:      i32 1, label %[[SW_BB:.*]]
-; CHECK-NEXT:      i32 2, label %[[SW_BB]]
-; CHECK-NEXT:      i32 3, label %[[SW_BB]]
+; CHECK-NEXT:    switch i32 [[X]], label %[[SW_DEFAULT:.*]] [
+; CHECK-NEXT:      i32 3, label %[[SW_BB:.*]]
+; CHECK-NEXT:      i32 0, label %[[SW_BB]]
+; CHECK-NEXT:      i32 1, label %[[SW_BB]]
 ; CHECK-NEXT:    ]
 ; CHECK:       [[SW_BB]]:
 ; CHECK-NEXT:    ret i1 true