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Avoid generating wires for function args which are constant

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This commit is contained in:
Zachary Snow 2020-07-24 21:18:24 -06:00
parent dafe04d559
commit 59c4ad8ed3
3 changed files with 73 additions and 0 deletions
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28
frontends/ast/simplify.cc
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@ -483,6 +483,27 @@ static AstNode *make_packed_struct(AstNode *template_node, std::string &name)
return wnode;
}
// check if a node or its children contains an assignment to the given variable
static bool node_contains_assignment_to(const AstNode* node, const AstNode* var)
{
if (node->type == AST_ASSIGN_EQ || node->type == AST_ASSIGN_LE) {
// current node is iteslf an assignment
log_assert(node->children.size() >= 2);
const AstNode* lhs = node->children[0];
if (lhs->type == AST_IDENTIFIER && lhs->str == var->str)
return false;
}
for (const AstNode* child : node->children) {
// if this child shadows the given variable
if (child != var && child->str == var->str && child->type == AST_WIRE)
break; // skip the remainder of this block/scope
// depth-first short circuit
if (!node_contains_assignment_to(child, var))
return false;
}
return true;
}
// convert the AST into a simpler AST that has all parameters substituted by their
// values, unrolled for-loops, expanded generate blocks, etc. when this function
// is done with an AST it can be converted into RTLIL using genRTLIL().
@ -3196,6 +3217,13 @@ skip_dynamic_range_lvalue_expansion:;
if ((child->is_input || child->is_output) && arg_count < children.size())
{
AstNode *arg = children[arg_count++]->clone();
// convert purely constant arguments into localparams
if (child->is_input && child->type == AST_WIRE && arg->type == AST_CONSTANT && node_contains_assignment_to(decl, child)) {
wire->type = AST_LOCALPARAM;
wire->attributes.erase(ID::nosync);
wire->children.insert(wire->children.begin(), arg->clone());
continue;
}
AstNode *wire_id = new AstNode(AST_IDENTIFIER);
wire_id->str = wire->str;
AstNode *assign = child->is_input ?