Support 2D bit arrays in structures. Optimise array indexing.

2 files changed, 101 insertions, 30 deletions

diff --git a/frontends/ast/simplify.cc b/frontends/ast/simplify.cc
index 6970135d0..c9da8ba48 100644
--- a/frontends/ast/simplify.cc
+++ b/frontends/ast/simplify.cc
@@ -380,31 +380,100 @@ static int size_packed_struct(AstNode *snode, int base_offset)
 
 static AstNode *node_int(int ival)
 {
-	// maybe mkconst_int should have default values for the common integer case
-	return AstNode::mkconst_int(ival, true, 32);
+	return AstNode::mkconst_int(ival, true);
 }
 
-static AstNode *offset_indexed_range(int offset_right, int stride, AstNode *left_expr, AstNode *right_expr)
+static AstNode *node_uint(uint ival)
+{
+	return AstNode::mkconst_int(ival, false);
+}
+
+static unsigned int power_of_two(int n)
+{
+	// iff n is a power of two then return the power, else return 0
+	// caller must ensure n > 1
+	log_assert(n > 1);
+	if (n & (n - 1)) {
+		// not a power of 2
+		return 0;
+	}
+	// brute force the shift
+	for (unsigned int i = 1; i < 32; i++) {
+		n >>= 1;
+		if (n & 1) {
+			return i;
+		}
+	}
+	return 0;
+}
+
+static AstNode *multiply_by_const(AstNode *expr_node, int stride)
+{
+	// the stride is very likely a power of 2, e.g. 8 for bytes
+	// and so could be optimised with a shift
+	AstNode *node;
+	unsigned int shift;
+	if ((shift = power_of_two(stride)) > 0) {
+		node = new AstNode(AST_SHIFT_LEFT, expr_node, node_uint(shift));
+	}
+	else {
+		node = new AstNode(AST_MUL, expr_node, node_int(stride));
+	}
+	return node;
+}
+
+static AstNode *offset_indexed_range(int offset, int stride, AstNode *left_expr, AstNode *right_expr)
 {
 	// adjust the range expressions to add an offset into the struct
 	// and maybe index using an array stride
 	auto left  = left_expr->clone();
 	auto right = right_expr->clone();
-	if (stride == 1) {
-		// just add the offset
-		left  = new AstNode(AST_ADD, node_int(offset_right), left);
-		right = new AstNode(AST_ADD, node_int(offset_right), right);
+	if (stride > 1) {
+		// newleft = (left + 1) * stride - 1
+		left  = new AstNode(AST_SUB, multiply_by_const(new AstNode(AST_ADD, left, node_int(1)), stride), node_int(1));
+		// newright = right * stride
+		right = multiply_by_const(right, stride);
+	}
+	// add the offset
+	if (offset) {
+		left  = new AstNode(AST_ADD, node_int(offset), left);
+		right = new AstNode(AST_ADD, node_int(offset), right);
+	}
+	return new AstNode(AST_RANGE, left, right);
+}
+
+static AstNode *make_struct_index_range(AstNode *node, AstNode *rnode, int stride, int offset)
+{
+	// generate a range node to perform either bit or array indexing
+	if (rnode->children.size() == 1) {
+		// index e.g. s.a[i]
+		return offset_indexed_range(offset, stride, rnode->children[0], rnode->children[0]);
+	}
+	else if (rnode->children.size() == 2) {
+		// slice e.g. s.a[i:j]
+		return offset_indexed_range(offset, stride, rnode->children[0], rnode->children[1]);
 	}
 	else {
-		// newleft = offset_right - 1 + (left + 1) * stride
-		left  = new AstNode(AST_ADD, new AstNode(AST_SUB, node_int(offset_right), node_int(1)),
-				new AstNode(AST_MUL, node_int(stride), new AstNode(AST_ADD, left, node_int(1))));
-		// newright = offset_right + right * stride
-		right = new AstNode(AST_ADD, node_int(offset_right), new AstNode(AST_MUL, right, node_int(stride)));
+		struct_op_error(node);
 	}
+}
+
+static AstNode *slice_range(AstNode *rnode, AstNode *snode)
+{
+	// apply the bit slice indicated by snode to the range rnode
+	log_assert(rnode->type==AST_RANGE);
+	auto left  = rnode->children[0];
+	auto right = rnode->children[1];
+	log_assert(snode->type==AST_RANGE);
+	auto slice_left  = snode->children[0];
+	auto slice_right = snode->children[1];
+	auto width = new AstNode(AST_SUB, slice_left->clone(), slice_right->clone());
+	right = new AstNode(AST_ADD, right->clone(), slice_right->clone());
+	left  = new AstNode(AST_ADD, right->clone(), width);
 	return new AstNode(AST_RANGE, left, right);
 }
 
+
 static AstNode *make_struct_member_range(AstNode *node, AstNode *member_node)
 {
 	// Work out the range in the packed array that corresponds to a struct member
@@ -414,27 +483,26 @@ static AstNode *make_struct_member_range(AstNode *node, AstNode *member_node)
 	int range_right = member_node->range_right;
 	if (node->children.empty()) {
 		// no range operations apply, return the whole width
+		return make_range(range_left, range_right);
 	}
-	else if (node->children.size() == 1 && node->children[0]->type == AST_RANGE) {
-		auto rnode = node->children[0];
-		int stride = get_struct_array_width(member_node);
-		if (rnode->children.size() == 1) {
-			// index e.g. s.a[i]
-			return offset_indexed_range(range_right, stride, rnode->children[0], rnode->children[0]);
-		}
-		else if (rnode->children.size() == 2) {
-			// slice e.g. s.a[i:j]
-			return offset_indexed_range(range_right, stride, rnode->children[0], rnode->children[1]);
-		}
-		else {
-			struct_op_error(node);
-		}
+	int stride = get_struct_array_width(member_node);
+	if (node->children.size() == 1 && node->children[0]->type == AST_RANGE) {
+		// bit or array indexing e.g. s.a[2] or s.a[1:0]
+		return make_struct_index_range(node, node->children[0], stride, range_right);
+	}
+	else if (node->children.size() == 1 && node->children[0]->type == AST_MULTIRANGE) {
+		// multirange, i.e. bit slice after array index, e.g. s.a[i][p:q]
+		log_assert(stride > 1);
+		auto mrnode = node->children[0];
+		auto element_range = make_struct_index_range(node, mrnode->children[0], stride, range_right);
+		// then apply bit slice range
+		auto range = slice_range(element_range, mrnode->children[1]);
+		delete element_range;
+		return range;
 	}
 	else {
-		// TODO multirange, i.e. bit slice after array index s.a[i][p:q]
 		struct_op_error(node);
 	}
-	return make_range(range_left, range_right);
 }
 
 static void add_members_to_scope(AstNode *snode, std::string name)
diff --git a/tests/svtypes/struct_array.sv b/tests/svtypes/struct_array.sv
index 022ad56c6..9c90375ee 100644
--- a/tests/svtypes/struct_array.sv
+++ b/tests/svtypes/struct_array.sv
@@ -3,7 +3,7 @@
 module top;
 	
 	struct packed {
-		bit [5:0] [7:0] a;	// 6 element packed array of bytes
+		bit [7:0] [7:0] a;	// 8 element packed array of bytes
 		bit [15:0] b;		// filler for non-zero offset
 	} s;
 
@@ -13,10 +13,13 @@ module top;
 		s.a[2:1] = 16'h1234;
 		s.a[5] = 8'h42;
 
+		s.a[7] = '1;
+		s.a[7][1:0] = '0;
+
 		s.b = '1;
 		s.b[1:0] = '0;
 	end
 
-	always_comb assert(s==64'h4200_0012_3400_FFFC);
+	always_comb assert(s==80'hFC00_4200_0012_3400_FFFC);
 
 endmodule