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Added nordshift attribute
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nordshift on a register yields generation of muxes instead of shift circuits
for dynamic rvalue indexing, akin to nowrshmsk for lvalue indexing.

To facilitate this, the AST transformations for rvalue indexing are moved
from genrtlil.cc to simplify.cc, bringing them in line with transformations
for lvalue indexing.
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@daglem
daglem committed Aug 6, 2023
1 parent f37ce5c commit f1ca67a
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Showing 5 changed files with 171 additions and 66 deletions.
3 changes: 3 additions & 0 deletions README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -296,6 +296,9 @@ Verilog Attributes and non-standard features
- The ``nowrshmsk`` attribute on a register prohibits the generation of
shift-and-mask type circuits for writing to bit slices of that register.

- The ``nordshift`` attribute on a packed array prohibits the generation of
shift type circuits for reads from that array.

- 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.

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94 changes: 28 additions & 66 deletions frontends/ast/genrtlil.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -1365,8 +1365,6 @@ RTLIL::SigSpec AstNode::genRTLIL(int width_hint, bool sign_hint)
}

// simply return the corresponding RTLIL::SigSpec for an AST_IDENTIFIER node
// for identifiers with dynamic bit ranges (e.g. "foo[bar]" or "foo[bar+3:bar]") a
// shifter cell is created and the output signal of this cell is returned
case AST_IDENTIFIER:
{
RTLIL::Wire *wire = NULL;
Expand Down Expand Up @@ -1445,8 +1443,8 @@ RTLIL::SigSpec AstNode::genRTLIL(int width_hint, bool sign_hint)

use_const_chunk:
if (children.size() != 0) {
if (children[0]->type != AST_RANGE)
input_error("Single range expected.\n");
if (children[0]->type != AST_RANGE || !children[0]->range_valid)
input_error("Single static range expected.\n");
int source_width = id2ast->range_left - id2ast->range_right + 1;
int source_offset = id2ast->range_right;
int chunk_left = source_width - 1;
Expand All @@ -1459,70 +1457,34 @@ RTLIL::SigSpec AstNode::genRTLIL(int width_hint, bool sign_hint)
chunk_right = chunk.offset;
}

if (!children[0]->range_valid) {
AstNode *left_at_zero_ast = children[0]->children[0]->clone_at_zero();
AstNode *right_at_zero_ast = children[0]->children.size() >= 2 ? children[0]->children[1]->clone_at_zero() : left_at_zero_ast->clone();
while (left_at_zero_ast->simplify(true, false, 1, -1, false, false)) { }
while (right_at_zero_ast->simplify(true, false, 1, -1, false, false)) { }
if (left_at_zero_ast->type != AST_CONSTANT || right_at_zero_ast->type != AST_CONSTANT)
input_error("Unsupported expression on dynamic range select on signal `%s'!\n", str.c_str());
int width = abs(int(left_at_zero_ast->integer - right_at_zero_ast->integer)) + 1;
AstNode *fake_ast = new AstNode(AST_NONE, clone(), children[0]->children.size() >= 2 ?
children[0]->children[1]->clone() : children[0]->children[0]->clone());
fake_ast->children[0]->delete_children();
if (member_node)
fake_ast->children[0]->attributes[ID::wiretype] = member_node->clone();

int fake_ast_width = 0;
bool fake_ast_sign = true;
fake_ast->children[1]->detectSignWidth(fake_ast_width, fake_ast_sign);
RTLIL::SigSpec shift_val = fake_ast->children[1]->genRTLIL(fake_ast_width, fake_ast_sign);

if (source_offset != 0) {
shift_val = current_module->Sub(NEW_ID, shift_val, source_offset, fake_ast_sign);
fake_ast->children[1]->is_signed = true;
}
if (id2ast->range_swapped) {
shift_val = current_module->Sub(NEW_ID, RTLIL::SigSpec(source_width - width), shift_val, fake_ast_sign);
fake_ast->children[1]->is_signed = true;
}
if (GetSize(shift_val) >= 32)
fake_ast->children[1]->is_signed = true;
RTLIL::SigSpec sig = binop2rtlil(fake_ast, ID($shiftx), width, fake_ast->children[0]->genRTLIL(), shift_val);
delete left_at_zero_ast;
delete right_at_zero_ast;
delete fake_ast;
return sig;
chunk.width = children[0]->range_left - children[0]->range_right + 1;
chunk.offset += children[0]->range_right - source_offset;
if (id2ast->range_swapped)
chunk.offset = source_width - (chunk.offset + chunk.width);
if (chunk.offset > chunk_left || chunk.offset + chunk.width < chunk_right) {
if (chunk.width == 1)
log_file_warning(filename, location.first_line, "Range select out of bounds on signal `%s': Setting result bit to undef.\n",
str.c_str());
else
log_file_warning(filename, location.first_line, "Range select [%d:%d] out of bounds on signal `%s': Setting all %d result bits to undef.\n",
children[0]->range_left, children[0]->range_right, str.c_str(), chunk.width);
chunk = RTLIL::SigChunk(RTLIL::State::Sx, chunk.width);
} else {
chunk.width = children[0]->range_left - children[0]->range_right + 1;
chunk.offset += children[0]->range_right - source_offset;
if (id2ast->range_swapped)
chunk.offset = source_width - (chunk.offset + chunk.width);
if (chunk.offset > chunk_left || chunk.offset + chunk.width < chunk_right) {
if (chunk.width == 1)
log_file_warning(filename, location.first_line, "Range select out of bounds on signal `%s': Setting result bit to undef.\n",
str.c_str());
else
log_file_warning(filename, location.first_line, "Range select [%d:%d] out of bounds on signal `%s': Setting all %d result bits to undef.\n",
children[0]->range_left, children[0]->range_right, str.c_str(), chunk.width);
chunk = RTLIL::SigChunk(RTLIL::State::Sx, chunk.width);
} else {
if (chunk.offset + chunk.width - 1 > chunk_left) {
add_undef_bits_msb = (chunk.offset + chunk.width - 1) - chunk_left;
chunk.width -= add_undef_bits_msb;
}
if (chunk.offset < chunk_right) {
add_undef_bits_lsb = chunk_right - chunk.offset;
chunk.width -= add_undef_bits_lsb;
chunk.offset += add_undef_bits_lsb;
}
if (add_undef_bits_lsb)
log_file_warning(filename, location.first_line, "Range [%d:%d] select out of bounds on signal `%s': Setting %d LSB bits to undef.\n",
children[0]->range_left, children[0]->range_right, str.c_str(), add_undef_bits_lsb);
if (add_undef_bits_msb)
log_file_warning(filename, location.first_line, "Range [%d:%d] select out of bounds on signal `%s': Setting %d MSB bits to undef.\n",
children[0]->range_left, children[0]->range_right, str.c_str(), add_undef_bits_msb);
if (chunk.offset + chunk.width - 1 > chunk_left) {
add_undef_bits_msb = (chunk.offset + chunk.width - 1) - chunk_left;
chunk.width -= add_undef_bits_msb;
}
if (chunk.offset < chunk_right) {
add_undef_bits_lsb = chunk_right - chunk.offset;
chunk.width -= add_undef_bits_lsb;
chunk.offset += add_undef_bits_lsb;
}
if (add_undef_bits_lsb)
log_file_warning(filename, location.first_line, "Range [%d:%d] select out of bounds on signal `%s': Setting %d LSB bits to undef.\n",
children[0]->range_left, children[0]->range_right, str.c_str(), add_undef_bits_lsb);
if (add_undef_bits_msb)
log_file_warning(filename, location.first_line, "Range [%d:%d] select out of bounds on signal `%s': Setting %d MSB bits to undef.\n",
children[0]->range_left, children[0]->range_right, str.c_str(), add_undef_bits_msb);
}
}

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138 changes: 138 additions & 0 deletions frontends/ast/simplify.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -2336,6 +2336,144 @@ bool AstNode::simplify(bool const_fold, bool in_lvalue, int stage, int width_hin
goto apply_newNode;
}

// Rewrite dynamic indexing of rvalue packed dimensions.
if (stage > 1 && type == AST_IDENTIFIER && id2ast != NULL && id2ast->type != AST_MEMORY && !in_lvalue &&
GetSize(children) == 1 && children[0]->type == AST_RANGE && !children[0]->range_valid)
{
AST::AstNode *member_node = get_struct_member(this);
int source_width = member_node ?
member_node->range_left - member_node->range_right + 1 :
id2ast->range_left - id2ast->range_right + 1;
int source_offset = id2ast->range_right;
int result_width = 1;

AstNode *shift_expr = NULL;
AstNode *range = children[0];

if (range->children.size() == 1) {
shift_expr = range->children[0]->clone();
} else {
shift_expr = range->children[1]->clone();
if (!try_determine_range_width(range, result_width))
input_error("Unsupported expression on dynamic range select on signal `%s'!\n", str.c_str());
}

bool use_case_method = false;

if (id2ast->attributes.count(ID::nordshift)) {
AstNode *node = id2ast->attributes.at(ID::nordshift);
while (node->simplify(true, false, stage, -1, false, false)) { }
if (node->type != AST_CONSTANT)
input_error("Non-constant value for `nordshift' attribute on `%s'!\n", id2ast->str.c_str());
if (node->asAttrConst().as_bool())
use_case_method = true;
}

AstNode *wire = new AstNode(AST_WIRE, new AstNode(AST_RANGE, node_int(result_width - 1), node_int(0)));
wire->str = stringf("$bitsel$%s$%s:%d$%d", str.c_str(), RTLIL::encode_filename(filename).c_str(), location.first_line, autoidx++);
if (current_block)
wire->attributes[ID::nosync] = AstNode::mkconst_int(1, false);
wire->is_logic = true;
while (wire->simplify(true, false, 1, -1, false, false)) { }
current_ast_mod->children.push_back(wire);

AstNode *lvalue = new AstNode(AST_IDENTIFIER);
lvalue->str = wire->str;
lvalue->was_checked = true;

AstNode *assign;

if (use_case_method) {
// AST_CASE with an AST_COND for each possible bit slice.

int stride = 1;
int div_stride = 1;

// Optimization: Remove multiplication of index by stride for two-dimensional packed arrays and
// variable slices on the form dst = src[i*w +: w].
if (!source_offset && shift_expr->type == AST_MUL &&
((shift_expr->children[0]->type == AST_CONSTANT && (int)shift_expr->children[0]->integer == result_width) ||
(shift_expr->children[1]->type == AST_CONSTANT && (int)shift_expr->children[1]->integer == result_width)))
{
int var_i = shift_expr->children[0]->type == AST_CONSTANT;
AstNode *tmp = shift_expr->children[var_i]->clone();
stride = result_width;
div_stride = stride;
delete shift_expr;
shift_expr = tmp;
}
else if (member_node)
{
// Clamp chunk to range of member within struct/union.
log_assert(!source_offset && !id2ast->range_swapped);

// When the (* nordshift *) attribute is set, a CASE block is generated below
// to select the indexed bit slice. When a multirange array is indexed, the
// start of each possible slice is separated by the bit stride of the last
// index dimension, and we can optimize the CASE block accordingly.
// The dimension of the original array expression is saved in the 'integer' field.
int dims = integer;
stride = source_width;
for (int dim = 0; dim < dims; dim++) {
stride /= get_struct_range_width(member_node, dim);
}
}

assign = new AstNode(AST_CASE, shift_expr);
for (int i = 0; i < source_width; i += stride) {
int start_bit = source_offset + i;
int end_bit = std::min(start_bit+result_width,source_width) - 1;
AstNode *cond = new AstNode(AST_COND, node_int(start_bit/div_stride));
AstNode *rvalue = clone();
rvalue->delete_children();
if (member_node)
rvalue->attributes[ID::wiretype] = member_node->clone();
rvalue->children.push_back(new AstNode(AST_RANGE,
node_int(end_bit), node_int(start_bit)));
cond->children.push_back(new AstNode(AST_BLOCK, new AstNode(AST_ASSIGN_EQ, lvalue->clone(), rvalue)));
assign->children.push_back(cond);
}
} else {
// Shift to access the indexed bit slice.
AstNode *rvalue = clone();
rvalue->delete_children();
if (member_node)
rvalue->attributes[ID::wiretype] = member_node->clone();

if (source_offset != 0) {
// offset the shift amount by the lower bound of the dimension
shift_expr = new AstNode(AST_SUB, shift_expr, node_int(source_offset));
}
if (id2ast->range_swapped) {
// reflect the shift amount if the dimension is swapped
shift_expr = new AstNode(AST_SUB, node_int(source_width - result_width), shift_expr);
}

// Shift rvalue to the right so that the bit slice starts at bit 0.
assign = new AstNode(AST_ASSIGN_EQ,
lvalue->clone(),
new AstNode(AST_SHIFTX, rvalue, shift_expr));
}

if (current_block) {
size_t assign_idx = 0;
while (assign_idx < current_block->children.size() && current_block->children[assign_idx] != current_block_child)
assign_idx++;
log_assert(assign_idx < current_block->children.size());
current_block->children.insert(current_block->children.begin()+assign_idx, assign);
wire->is_reg = true;
} else {
if (!use_case_method) {
assign = new AstNode(AST_BLOCK, assign);
}
AstNode *proc = new AstNode(AST_ALWAYS, assign);
current_ast_mod->children.push_back(proc);
}

newNode = lvalue;
goto apply_newNode;
}

if (type == AST_WHILE)
input_error("While loops are only allowed in constant functions!\n");

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1 change: 1 addition & 0 deletions kernel/constids.inc
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Original file line number Diff line number Diff line change
Expand Up @@ -135,6 +135,7 @@ X(nolatches)
X(nomem2init)
X(nomem2reg)
X(nomeminit)
X(nordshift)
X(nosync)
X(nowrshmsk)
X(no_rw_check)
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1 change: 1 addition & 0 deletions tests/techmap/shiftx2mux.ys
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Original file line number Diff line number Diff line change
Expand Up @@ -104,6 +104,7 @@ wire [7:0] AA = {1'bx, A};
assign Y = AA[B*2 +: 2];
endmodule
EOT
proc
opt
wreduce
equiv_opt -assert techmap

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