Add "nowrshmsk" attribute, fix shift-and-mask bit slice write for signed offset, fixes #1990

Signed-off-by: Claire Wolf <claire@symbioticeda.com>

README.md

@@ -281,6 +281,9 @@ Verilog Attributes and non-standard features
   temporary variable within an always block. This is mostly used internally
   by Yosys to synthesize Verilog functions and access arrays.
 
+- The ``nowrshmsk`` attribute on a register prohibits the generation of
+  shift-and-mask type circuits for writing to bit slices of that register.
+
 - The ``onehot`` attribute on wires mark them as one-hot state register. This
   is used for example for memory port sharing and set by the fsm_map pass.
 

frontends/ast/ast.cc

@@ -110,6 +110,8 @@ std::string AST::type2str(AstNodeType type)
 	X(AST_SHIFT_RIGHT)
 	X(AST_SHIFT_SLEFT)
 	X(AST_SHIFT_SRIGHT)
+	X(AST_SHIFTX)
+	X(AST_SHIFT)
 	X(AST_LT)
 	X(AST_LE)
 	X(AST_EQ)
@@ -628,6 +630,8 @@ void AstNode::dumpVlog(FILE *f, std::string indent) const
 	if (0) { case AST_SHIFT_RIGHT:  txt = ">>";  }
 	if (0) { case AST_SHIFT_SLEFT:  txt = "<<<"; }
 	if (0) { case AST_SHIFT_SRIGHT: txt = ">>>"; }
+	if (0) { case AST_SHIFTX:       txt = "@shiftx@"; }
+	if (0) { case AST_SHIFT:        txt = "@shift@"; }
 	if (0) { case AST_LT:           txt = "<";   }
 	if (0) { case AST_LE:           txt = "<=";  }
 	if (0) { case AST_EQ:           txt = "==";  }

frontends/ast/ast.h

@@ -91,6 +91,8 @@ namespace AST
 		AST_SHIFT_RIGHT,
 		AST_SHIFT_SLEFT,
 		AST_SHIFT_SRIGHT,
+		AST_SHIFTX,
+		AST_SHIFT,
 		AST_LT,
 		AST_LE,
 		AST_EQ,

frontends/ast/genrtlil.cc

@@ -856,6 +856,8 @@ void AstNode::detectSignWidthWorker(int &width_hint, bool &sign_hint, bool *foun
 	case AST_SHIFT_RIGHT:
 	case AST_SHIFT_SLEFT:
 	case AST_SHIFT_SRIGHT:
+	case AST_SHIFTX:
+	case AST_SHIFT:
 	case AST_POW:
 		children[0]->detectSignWidthWorker(width_hint, sign_hint, found_real);
 		break;
@@ -1356,6 +1358,8 @@ RTLIL::SigSpec AstNode::genRTLIL(int width_hint, bool sign_hint)
 	if (0) { case AST_SHIFT_RIGHT:  type_name = ID($shr); }
 	if (0) { case AST_SHIFT_SLEFT:  type_name = ID($sshl); }
 	if (0) { case AST_SHIFT_SRIGHT: type_name = ID($sshr); }
+	if (0) { case AST_SHIFTX:       type_name = ID($shiftx); }
+	if (0) { case AST_SHIFT:        type_name = ID($shift); }
 		{
 			if (width_hint < 0)
 				detectSignWidth(width_hint, sign_hint);

frontends/ast/simplify.cc

@@ -1786,7 +1786,18 @@ bool AstNode::simplify(bool const_fold, bool at_zero, bool in_lvalue, int stage,
 			result_width = abs(int(left_at_zero_ast->integer - right_at_zero_ast->integer)) + 1;
 		}
 
-		if (0)
+		bool use_case_method = false;
+
+		if (children[0]->id2ast->attributes.count(ID::nowrshmsk)) {
+			AstNode *node = children[0]->id2ast->attributes.at(ID::nowrshmsk);
+			while (node->simplify(true, false, false, stage, -1, false, false)) { }
+			if (node->type != AST_CONSTANT)
+				log_file_error(filename, location.first_line, "Non-constant value for `nowrshmsk' attribute on `%s'!\n", children[0]->id2ast->str.c_str());
+			if (node->asAttrConst().as_bool())
+				use_case_method = true;
+		}
+
+		if (use_case_method)
 		{
 			// big case block
 
@@ -1794,10 +1805,10 @@ bool AstNode::simplify(bool const_fold, bool at_zero, bool in_lvalue, int stage,
 			newNode = new AstNode(AST_CASE, shift_expr);
 			for (int i = 0; i < source_width; i++) {
 				int start_bit = children[0]->id2ast->range_right + i;
+				int end_bit = std::min(start_bit+result_width,source_width) - 1;
 				AstNode *cond = new AstNode(AST_COND, mkconst_int(start_bit, true));
 				AstNode *lvalue = children[0]->clone();
 				lvalue->delete_children();
-				int end_bit = std::min(start_bit+result_width,source_width) - 1;
 				lvalue->children.push_back(new AstNode(AST_RANGE,
 						mkconst_int(end_bit, true), mkconst_int(start_bit, true)));
 				cond->children.push_back(new AstNode(AST_BLOCK, new AstNode(type, lvalue, children[1]->clone())));
@@ -1844,11 +1855,36 @@ bool AstNode::simplify(bool const_fold, bool at_zero, bool in_lvalue, int stage,
 
 			AstNode *shamt = shift_expr;
 
-			newNode->children.push_back(new AstNode(AST_ASSIGN_EQ, ref_mask->clone(),
-					new AstNode(AST_SHIFT_LEFT, mkconst_bits(std::vector<RTLIL::State>(result_width, State::S1), false), shamt->clone())));
-			newNode->children.push_back(new AstNode(AST_ASSIGN_EQ, ref_data->clone(),
-					new AstNode(AST_SHIFT_LEFT, new AstNode(AST_BIT_AND, mkconst_bits(std::vector<RTLIL::State>(result_width, State::S1), false), children[1]->clone()), shamt)));
-			newNode->children.push_back(new AstNode(type, lvalue, new AstNode(AST_BIT_OR, new AstNode(AST_BIT_AND, old_data, new AstNode(AST_BIT_NOT, ref_mask)), ref_data)));
+			int start_bit = children[0]->id2ast->range_right;
+			bool use_shift = shamt->is_signed;
+
+			if (start_bit != 0) {
+				shamt = new AstNode(AST_SUB, shamt, mkconst_int(start_bit, true));
+				use_shift = true;
+			}
+
+			AstNode *t;
+
+			t = mkconst_bits(std::vector<RTLIL::State>(result_width, State::S1), false);
+			if (use_shift)
+				t = new AstNode(AST_SHIFT, t, new AstNode(AST_NEG, shamt->clone()));
+			else
+				t = new AstNode(AST_SHIFT_LEFT, t, shamt->clone());
+			t = new AstNode(AST_ASSIGN_EQ, ref_mask->clone(), t);
+			newNode->children.push_back(t);
+
+			t = new AstNode(AST_BIT_AND, mkconst_bits(std::vector<RTLIL::State>(result_width, State::S1), false), children[1]->clone());
+			if (use_shift)
+				t = new AstNode(AST_SHIFT, t, new AstNode(AST_NEG, shamt));
+			else
+				t = new AstNode(AST_SHIFT_LEFT, t, shamt);
+			t = new AstNode(AST_ASSIGN_EQ, ref_data->clone(), t);
+			newNode->children.push_back(t);
+
+			t = new AstNode(AST_BIT_AND, old_data, new AstNode(AST_BIT_NOT, ref_mask));
+			t = new AstNode(AST_BIT_OR, t, ref_data);
+			t = new AstNode(type, lvalue, t);
+			newNode->children.push_back(t);
 		}
 
 		goto apply_newNode;

kernel/constids.inc

@@ -123,6 +123,7 @@ X(nomem2init)
 X(nomem2reg)
 X(nomeminit)
 X(nosync)
+X(nowrshmsk)
 X(O)
 X(OFFSET)
 X(onehot)