[CODEGEN][OPENCL] Fix compile error about ternary expression. (#2821)

Code like this can't be built with NV OpenCL, and it needs an explicit type
  converison for ternary expression if return type is uchar.

       uchar i = 0, j = 0;
       uchar t = max((uchar)j, ((i > 0) ? (uchar)1 : (uchar)0));

src/codegen/codegen_opencl.cc

@@ -208,6 +208,25 @@ void CodeGenOpenCL::VisitExpr_(const Broadcast* op, std::ostream& os) {   // NOL
   os << "))";
 }
 
+void CodeGenOpenCL::VisitExpr_(const Call *op, std::ostream& os) {  // NOLINT(*)
+  /* Return type of ternary expression is not always same as its sub-expressions,
+   * add a cast */
+  if (op->is_intrinsic(intrinsic::tvm_if_then_else)) {
+    os << "(";
+    PrintType(op->args[2].type(), os);
+    os << ")";
+  }
+  CodeGenC::VisitExpr_(op, os);
+}
+
+void CodeGenOpenCL::VisitExpr_(const Select* op, std::ostream& os) {  // NOLINT(*)
+  /* Return type of ternary expression is not always same as its sub-expressions,
+   * add a cast */
+  os << "(";
+  PrintType(op->true_value.type(), os);
+  os << ")";
+  CodeGenC::VisitExpr_(op, os);
+}
 
 runtime::Module BuildOpenCL(Array<LoweredFunc> funcs) {
   using tvm::runtime::Registry;

src/codegen/codegen_opencl.h

@@ -38,6 +38,8 @@ class CodeGenOpenCL final : public CodeGenC {
 
   // overload visitor
   void VisitExpr_(const Broadcast* op, std::ostream& os) final; // NOLINT(*)
+  void VisitExpr_(const Call* op, std::ostream& os) final; // NOLINT(*)
+  void VisitExpr_(const Select* op, std::ostream& os) final; // NOLINT(*)
 
  private:
   // whether enable fp16 and fp64 extension

tests/python/unittest/test_codegen_opencl.py

@@ -0,0 +1,55 @@
+import tvm
+
+target = 'opencl'
+
+def test_opencl_ternary_expression():
+    def check_if_then_else(ctx, n, dtype):
+        A = tvm.placeholder((n,), name='A', dtype=dtype)
+        true_value = tvm.const(1, dtype=dtype)
+        false_value = tvm.const(3, dtype=dtype)
+        max_lhs = tvm.const(2, dtype=dtype)
+        max_rhs = tvm.if_then_else(A[0] > 0, true_value, false_value)
+        C = tvm.compute((n,), lambda i: tvm.max(max_lhs, max_rhs), name='C')
+        s = tvm.create_schedule(C.op)
+        s[C].bind(s[C].op.axis[0], tvm.thread_axis("threadIdx.x"))
+        fun = tvm.build(s, [A, C], target)
+
+        a = tvm.nd.empty((n,), A.dtype, ctx)
+        c = tvm.nd.empty((n,), A.dtype, ctx)
+        # Only need to test compiling here
+        fun(a, c)
+
+    def check_select(ctx, n, dtype):
+        A = tvm.placeholder((n,), name='A', dtype=dtype)
+        true_value = tvm.const(1, dtype=dtype)
+        false_value = tvm.const(3, dtype=dtype)
+        max_lhs = tvm.const(2, dtype=dtype)
+        max_rhs = tvm.expr.Select(A[0] > 0, true_value, false_value)
+        C = tvm.compute((n,), lambda i: tvm.max(max_lhs, max_rhs), name='C')
+        s = tvm.create_schedule(C.op)
+        s[C].bind(s[C].op.axis[0], tvm.thread_axis("threadIdx.x"))
+        fun = tvm.build(s, [A, C], target)
+
+        a = tvm.nd.empty((n,), A.dtype, ctx)
+        c = tvm.nd.empty((n,), A.dtype, ctx)
+        # Only need to test compiling here
+        fun(a, c)
+
+    if not tvm.module.enabled(target):
+        print("skip because opencl is not enabled..")
+        return
+
+    ctx = tvm.context(target, 0)
+
+    check_if_then_else(ctx, 1, 'int8')
+    check_if_then_else(ctx, 1, 'uint8')
+    check_if_then_else(ctx, 1, 'int16')
+    check_if_then_else(ctx, 1, 'uint16')
+    check_select(ctx, 1, 'int8')
+    check_select(ctx, 1, 'uint8')
+    check_select(ctx, 1, 'int16')
+    check_select(ctx, 1, 'uint16')
+
+
+if __name__ == "__main__":
+    test_opencl_ternary_expression()