Add constexpr IDL pass

Author: Derek Hower

backends/cpp_hart_gen/lib/constexpr_pass.rb

@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+
+module Idl
+  class AstNode
+    def constexpr?(symtab)
+      if children.empty?
+        true
+      else
+        children.all? { |child| child.constexpr?(symtab) }
+      end
+    end
+  end
+  class IdAst
+    def constexpr?(symtab)
+      sym = symtab.get(name)
+      return true if sym.nil? # assuming undefined syms are local (be sure to type check first!!)
+      return true if sym.is_a?(Type)
+
+      !sym.type.global?
+    end
+  end
+  class PcAssignmentAst
+    def constexpr?(symtab) = false
+  end
+  class FunctionCallExpressionAst
+    def constexpr?(symtab) = false # conservative, can do better...
+  end
+  class CsrFieldReadExpressionAst
+    def constexpr?(symtab) = false
+  end
+  class CsrReadExpressionAst
+    def constexpr?(symtab) = false
+  end
+  class CsrSoftwareWriteAst
+    def constexpr?(symtab) = false
+  end
+  class CsrFunctionCallAst
+    def constexpr?(symtab) = function_name == "address"
+  end
+  class CsrWriteAst
+    def constexpr?(symtab) = false
+  end
+  class FunctionDefAst
+    # @return [Boolean] If the function is possibly C++ constexpr (does not access CSRs or registers)
+    def constexpr?(symtab)
+      return false if builtin?
+
+      body.constexpr?(symtab)
+    end
+  end
+end