[clang] [CIR] Upstream initial support for switch statements (PR #137106)

[Andy Kaylor via cfe-commits]

================
@@ -753,6 +755,221 @@ def ScopeOp : CIR_Op<"scope", [
   ];
 }
 
+//===----------------------------------------------------------------------===//
+// SwitchOp
+//===----------------------------------------------------------------------===//
+
+def CaseOpKind_DT : I32EnumAttrCase<"Default", 1, "default">;
+def CaseOpKind_EQ : I32EnumAttrCase<"Equal", 2, "equal">;
+def CaseOpKind_AO : I32EnumAttrCase<"Anyof", 3, "anyof">;
+def CaseOpKind_RG : I32EnumAttrCase<"Range", 4, "range">;
+
+def CaseOpKind : I32EnumAttr<
+    "CaseOpKind",
+    "case kind",
+    [CaseOpKind_DT, CaseOpKind_EQ, CaseOpKind_AO, CaseOpKind_RG]> {
+  let cppNamespace = "::cir";
+}
+
+def CaseOp : CIR_Op<"case", [
+       DeclareOpInterfaceMethods<RegionBranchOpInterface>,
+       RecursivelySpeculatable, AutomaticAllocationScope]> {
+  let summary = "Case operation";
+  let description = [{
+    The `cir.case` operation represents a case within a C/C++ switch.
+    The `cir.case` operation must be in a `cir.switch` operation directly
+    or indirectly.
+
+    The `cir.case` have 4 kinds:
+    - `equal, <constant>`: equality of the second case operand against the
+    condition.
+    - `anyof, [constant-list]`: equals to any of the values in a subsequent
+    following list.
+    - `range, [lower-bound, upper-bound]`: the condition is within the closed
+                                           interval.
+    - `default`: any other value.
+
+    Each case region must be explicitly terminated.
+  }];
+
+  let arguments = (ins ArrayAttr:$value, CaseOpKind:$kind);
+  let regions = (region AnyRegion:$caseRegion);
+
+  let assemblyFormat = "`(` $kind `,` $value `)` $caseRegion attr-dict";
+
+  let skipDefaultBuilders = 1;
+  let builders = [
+      OpBuilder<(ins "mlir::ArrayAttr":$value,
+                   "CaseOpKind":$kind,
+                   "mlir::OpBuilder::InsertPoint &":$insertPoint)>
+  ];
+}
+
+def SwitchOp : CIR_Op<"switch",
+      [SameVariadicOperandSize,
+       DeclareOpInterfaceMethods<RegionBranchOpInterface>,
+       RecursivelySpeculatable, AutomaticAllocationScope, NoRegionArguments]> {
+  let summary = "Switch operation";
+  let description = [{
+    The `cir.switch` operation represents C/C++ switch functionality for
+    conditionally executing multiple regions of code. The operand to an switch
+    is an integral condition value.
+
+    The set of `cir.case` operations and their enclosing `cir.switch`
+    represents the semantics of a C/C++ switch statement. Users can use
+    `collectCases(llvm::SmallVector<CaseOp> &cases)` to collect the `cir.case`
+    operation in the `cir.switch` operation easily.
+
+    The `cir.case` operations doesn't have to be in the region of `cir.switch`
+    directly. However, when all the `cir.case` operations lives in the region
+    of `cir.switch` directly and there is no other operations except the ending
+    `cir.yield` operation in the region of `cir.switch` directly, we call the
+    `cir.switch` operation is in a simple form. Users can use
+    `bool isSimpleForm(llvm::SmallVector<CaseOp> &cases)` member function to
+    detect if the `cir.switch` operation is in a simple form. The simple form
+    makes analysis easier to handle the `cir.switch` operation
+    and makes the boundary to give up pretty clear.
+
+    To make the simple form as common as possible, CIR code generation attaches
+    operations corresponding to the statements that lives between top level
+    cases into the closest `cir.case` operation.
+
+    For example,
+
+    ```
+    switch(int cond) {
+      case 4:
+        a++;
+
+      b++;
+      case 5;
+        c++;
+
+      ...
+    }
+    ```
+
+    The statement `b++` is not a sub-statement of the case statement `case 4`.
+    But to make the generated `cir.switch` a simple form, we will attach the
+    statement `b++` into the closest `cir.case` operation. So that the generated
+    code will be like:
+
+    ```
+    cir.switch(int cond) {
+      cir.case(equal, 4) {
+        a++;
+        b++;
+        cir.yield
+      }
+      cir.case(equal, 5) {
+        c++;
+        cir.yield
+      }
+      ...
+    }
+    ```
+
+    For the same reason, we will hoist the case statement as the substatement
+    of another case statement so that they will be in the same level. For
+    example,
+
+    ```
+    switch(int cond) {
+      case 4:
+      default;
+      case 5;
+        a++;
+      ...
+    }
+    ```
+
+    will be generated as
+
+    ```
+    cir.switch(int cond) {
+      cir.case(equal, 4) {
+        cir.yield
+      }
+      cir.case(default) {
+        cir.yield
+      }
+      cir.case(equal, 5) {
+        a++;
+        cir.yield
+      }
+      ...
+    }
+    ```
+
+    The cir.switch might not be considered "simple" if any of the following is
+    true:
+    - There are case statements of the switch statement lives in other scopes
+      other than the top level compound statement scope. Note that a case
+      statement itself doesn't form a scope.
+    - The sub-statement of the switch statement is not a compound statement.
+    - There are codes before the first case statement. For example,
+
+    ```
+    switch(int cond) {
+      l:
+        b++;
+
+      case 4:
+        a++;
+        break;
+
+      case 5:
+        goto l;
+      ...
+    }
+    ```
+
+    the generated CIR for this non-simple switch would be:
+
+    ```
+    cir.switch(int cond) {
+      cir.label "l"
+      b++;
+      cir.case(4) {
+        a++;
+        cir.break
+      }
+      cir.case(5) {
+        goto "l"
+      }
+      cir.yield
+    }
+    ```
+  }];
+
+  let arguments = (ins CIR_IntType:$condition);
+
+  let regions = (region AnyRegion:$body);
+
+  let skipDefaultBuilders = 1;
+  let builders = [
+    OpBuilder<(ins "mlir::Value":$condition,
+               "llvm::function_ref<void(mlir::OpBuilder &, mlir::Location, mlir::OperationState &)>":$switchBuilder)>
----------------
andykaylor wrote:

Can you reformat this to fit within 80 characters? Maybe by defining an alias similar to BuilderCallbackRef (as defined in CIRDialect.h)?

https://github.com/llvm/llvm-project/pull/137106