[mlir][LLVMIR] Support switch without cases

[aengelke]

Summary

LLVM supports switch ops without case values, for which the optional
caseValues has no value (i.e., std::nullopt, no the empty array).
Handle this case properly when translating MLIR-LLVM to LLVM-IR.

[aengelke]

Rebase to latest main.

[Dinistro]

Thanks for the fix, LGTM!

mlir/lib/Target/LLVMIR/Dialect/LLVMIR/LLVMToLLVMIRTranslation.cpp

if (auto condbrOp = dyn_cast<LLVM::CondBrOp>(opInst)) {
     return success();
   }
   if (auto switchOp = dyn_cast<LLVM::SwitchOp>(opInst)) {
     llvm::SwitchInst *switchInst = builder.CreateSwitch(
         moduleTranslation.lookupValue(switchOp.getValue()),
         moduleTranslation.lookupBlock(switchOp.getDefaultDestination()),
         switchOp.getCaseDestinations().size());
 
+    // Handle switch with zero cases.
+    if (!switchOp.getCaseValues())
+      return success();
+
     auto *ty = llvm::cast<llvm::IntegerType>(
         moduleTranslation.convertType(switchOp.getValue().getType()));
     for (auto i :
          llvm::zip(llvm::cast<DenseIntElementsAttr>(*switchOp.getCaseValues()),
                    switchOp.getCaseDestinations()))
       switchInst->addCase(
           llvm::ConstantInt::get(ty, std::get<0>(i).getLimitedValue()),
           moduleTranslation.lookupBlock(std::get<1>(i)));

mlir/test/Target/LLVMIR/llvmir.mlir

// CHECK: call nnan float @llvm.vector.reduce.fminimum.v2f32(<2 x float> {{.*}})
   %25 = llvm.mlir.constant(true) : i1
 // CHECK: select contract i1
   %26 = llvm.select %25, %arg0, %20 {fastmathFlags = #llvm.fastmath<contract>} : i1, f32
   llvm.return
 }
 
 // -----
 
+// CHECK-LABEL: @switch_empty
+llvm.func @switch_empty(%arg0 : i32) -> i32 {
+  // CHECK:      switch i32 %[[SWITCH_arg0:[0-9]+]], label %[[SWITCHDEFAULT_bb1:[0-9]+]] [
+  // CHECK-NEXT: ]
+  llvm.switch %arg0 : i32, ^bb1 [
+  ]
+
+  // CHECK:      [[SWITCHDEFAULT_bb1]]:
+  // CHECK-NEXT:   ret i32 %[[SWITCH_arg0]]
+^bb1:
+  llvm.return %arg0 : i32
+}
+
+// -----
+
 // CHECK-LABEL: @switch_args
 llvm.func @switch_args(%arg0: i32) -> i32 {
   %0 = llvm.mlir.constant(5 : i32) : i32
   %1 = llvm.mlir.constant(7 : i32) : i32
   %2 = llvm.mlir.constant(11 : i32) : i32
   // CHECK:      switch i32 %[[SWITCH_arg0:[0-9]+]], label %[[SWITCHDEFAULT_bb1:[0-9]+]] [
   // CHECK-NEXT:   i32 -1, label %[[SWITCHCASE_bb2:[0-9]+]]
   // CHECK-NEXT:   i32 1, label %[[SWITCHCASE_bb3:[0-9]+]]