[Moore] Add builtins for simulation time measurements (llvm#8970)

This PR adds builtins for `$time`, `$stime` and `$realtime` in the
`moore` dialect. All these builtins correspond to System Verilog system 
tasks and only slightly differ in their functionality.

To represent the return value of `$time`, this PR adds `TwoValueI64`, 
which represents a 64-Bit integer used to express the simulation time 
value in.

Author: MoritzScherer

include/circt/Dialect/Moore/MooreOps.td

@@ -1761,6 +1761,25 @@ def UrandomBIOp : Builtin<"urandom"> {
   let assemblyFormat = "($seed^)? attr-dict";
 }
 
+//===----------------------------------------------------------------------===//
+// Simulation Time Measurement Builtins
+//===----------------------------------------------------------------------===//
+
+def TimeBIOp : Builtin<"time"> {
+  let summary = "Return the current simulation time";
+  let description = [{
+    Corresponds to the `$time` system function. Returns a int-rounded 64-bit
+    integer corresponding to the elapsed simulation time scaled by the
+    simulation's timescale.
+
+    See IEEE 1800-2023 § 20.3 "Simulation time system functions".
+  }];
+
+  // Optional positional attribute for the seed
+  let results = (outs TimeType:$result);
+  let assemblyFormat = "attr-dict";
+}
+
 //===----------------------------------------------------------------------===//
 // Simulation Control Builtins
 //===----------------------------------------------------------------------===//

lib/Conversion/ImportVerilog/Expressions.cpp

@@ -1594,6 +1594,15 @@ Context::convertSystemCallArity0(const slang::ast::SystemSubroutine &subroutine,
                 [&]() -> Value {
                   return moore::UrandomBIOp::create(builder, loc, nullptr);
                 })
+          .Case(
+              "$time",
+              [&]() -> Value { return moore::TimeBIOp::create(builder, loc); })
+          .Case(
+              "$stime",
+              [&]() -> Value { return moore::TimeBIOp::create(builder, loc); })
+          .Case(
+              "$realtime",
+              [&]() -> Value { return moore::TimeBIOp::create(builder, loc); })
           .Default([&]() -> Value { return {}; });
   return systemCallRes();
 }

test/Conversion/ImportVerilog/builtins.sv

@@ -274,3 +274,15 @@ function RandomBuiltins(int x);
     // CHECK-NEXT: call @dummyA([[RAND1]]) : (!moore.i32) -> ()
    dummyA($urandom(x));
 endfunction
+
+// CHECK-LABEL: func.func private @TimeBuiltins(
+function TimeBuiltins();
+  // CHECK: [[TIME:%.+]] = moore.builtin.time
+  // CHECK-NEXT: [[TIMETOLOGIC:%.+]] = moore.time_to_logic [[TIME]]
+  dummyA($time());
+  // CHECK: [[STIME:%.+]] = moore.builtin.time
+  dummyA($stime());
+  // CHECK: [[REALTIME:%.+]] = moore.builtin.time
+  // TODO: There is no int-to-real conversion yet; change this to dummyB once int-to-real works!
+  dummyA($realtime());
+endfunction