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Optimizing Index Computations in SWG for Improved Timing #1509

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Optimizing Index Computations in SWG for Improved Timing #1509

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4ae38c7
Switch MUX and ADD for better fabric mapping.
preusser Jan 20, 2026
893c4bc
Registered pre-computation of ReadingLast & ReadingDone.
preusser Jan 20, 2026
84d91d2
Reset element counter to -1.
preusser Jan 20, 2026
936f712
Reformulating position compute.
preusser Jan 20, 2026
4dc9923
Fix operator precedence.
preusser Jan 20, 2026
6d072d0
Prevent triggering on neg. one.
preusser Jan 21, 2026
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86 changes: 57 additions & 29 deletions finn-rtllib/swg/swg_template_default.sv
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Original file line number Diff line number Diff line change
Expand Up @@ -112,12 +112,20 @@ module $TOP_MODULE_NAME$_impl #(
// Add a sign bit even to (most) unsigned counters and Window_buffer_read_addr_reg,
// so we can use automatic sign extension and simplify calculations w/ signed increment.
// Alternatively, we could manually sign-extend and shave off a bit here or there.
logic signed [$clog2(LAST_READ_ELEM+1)+1-1:0] Newest_buffered_elem = -1;

// v ReadingLast v ReadingDone
logic signed [$clog2(LAST_READ_ELEM+1)+1-1:0] Newest_buffered_elem = -1; // -1, 0, 1, ..., LAST_READ_ELEM-1, LAST_READ_ELEM
logic ReadingLast = LAST_READ_ELEM == 0;
logic ReadingDone = 0;
logic [$clog2(LAST_READ_ELEM+1)+1-1:0] Current_elem = 0;
logic [$clog2(LAST_READ_ELEM+1)+1-1:0] First_elem_next_window = 0;
logic [$clog2(ELEM_PER_WINDOW) -1:0] Position_in_window = 0;
logic [$clog2(BUF_ELEM_TOTAL)+1 -1:0] Window_buffer_read_addr_reg = 0;
logic [$clog2(BUF_ELEM_TOTAL)-1:0] Window_buffer_write_addr_reg = 0;

// v PositionZero
logic [$clog2(ELEM_PER_WINDOW)-1:0] Position_in_window = 0; // 0, 1, ..., ELEM_PER_WINDOW-1
logic PositionZero = 1;

logic [$clog2(BUF_ELEM_TOTAL)+1-1:0] Window_buffer_read_addr_reg = 0;
logic [$clog2(BUF_ELEM_TOTAL) -1:0] Window_buffer_write_addr_reg = 0;

// Control signals/registers
logic Write_cmd = 0;
Expand All @@ -128,9 +136,8 @@ module $TOP_MODULE_NAME$_impl #(
logic Fetching_done = 0;
uwire fetch_cmd = !($signed(Current_elem) > Newest_buffered_elem) && !write_blocked && !Fetching_done;

uwire reading_done = Newest_buffered_elem == LAST_READ_ELEM;
uwire read_cmd =
!reading_done && ( // if there is still an input element left to read
!ReadingDone && ( // if there is still an input element left to read
Fetching_done || ( // if fetching is done (e.g. for skipped rows at FM end due to stride)
$signed(((Newest_buffered_elem - (BUF_ELEM_TOTAL - 1)))) < $signed(First_elem_next_window) &&
$signed(((Newest_buffered_elem - (BUF_ELEM_TOTAL - 1)))) < $signed(Current_elem)
Expand All @@ -155,28 +162,38 @@ module $TOP_MODULE_NAME$_impl #(
always_ff @(posedge ap_clk) begin
if(!ap_rst_n) begin
Newest_buffered_elem <= -1;
ReadingLast <= LAST_READ_ELEM == 0;
ReadingDone <= 0;

Current_elem <= 0;
First_elem_next_window <= 0;

Position_in_window <= 0;
PositionZero <= 1;

Window_buffer_read_addr_reg <= 0;
Window_buffer_write_addr_reg <= 0;
Fetching_done <= 0;
Write_cmd <= 0;
Writing_done <= 0;
end
else begin
automatic logic nbe_rst = 0;
automatic logic nbe_inc = 0;

if (read_ok) begin
Window_buffer_write_addr_reg <= (Window_buffer_write_addr_reg == BUF_ELEM_TOTAL-1)? 0 : Window_buffer_write_addr_reg + 1;
Newest_buffered_elem <= Newest_buffered_elem+1;
automatic logic wa_rst = ReadingLast ||
((~Window_buffer_write_addr_reg & (BUF_ELEM_TOTAL-1)) == 0); // (Window_buffer_write_addr_reg == BUF_ELEM_TOTAL-1)
Window_buffer_write_addr_reg <= Window_buffer_write_addr_reg - (wa_rst? Window_buffer_write_addr_reg : -1);

if (Newest_buffered_elem == LAST_READ_ELEM-1) begin
Window_buffer_write_addr_reg <= 0;
end
//check if this is the last read cycle (reading_done will be true afterwards)
if ((Newest_buffered_elem == LAST_READ_ELEM-1) && Writing_done) begin
nbe_inc = 1;

//check if this is the last read cycle (ReadingDone will be true afterwards)
if(ReadingLast && Writing_done) begin
//start processing of next FM if writing is done already (possible due to unused input elements at the tail end)
//todo: allow for read overlapping between feature maps (i.e., reading first elements from next FM while still writing last window of current FM)
Newest_buffered_elem <= -1;
nbe_rst = 1;

Current_elem <= 0;
Window_buffer_read_addr_reg <= 0;
First_elem_next_window <= 0;
Expand All @@ -186,6 +203,8 @@ module $TOP_MODULE_NAME$_impl #(
end

if (fetch_cmd) begin
automatic logic wrap_position;

//count up to track which element index is about to be read from the buffer, and where it is located within the buffer
//use increment value calculated by controller

Expand All @@ -197,11 +216,12 @@ module $TOP_MODULE_NAME$_impl #(
Window_buffer_read_addr_reg <= ra + ra_correct;

//keep track where we are within a window
Position_in_window <= (Position_in_window != ELEM_PER_WINDOW - 1)? Position_in_window+1 : 0;
wrap_position = (~Position_in_window & (ELEM_PER_WINDOW - 1)) == 0;
Position_in_window <= Position_in_window + (wrap_position? -ELEM_PER_WINDOW+1 : 1);
PositionZero <= wrap_position;

//update first element of next window to allow buffer overwrite up until that point
if (Position_in_window == 0)
First_elem_next_window <= First_elem_next_window + tail_incr;
First_elem_next_window <= First_elem_next_window + (PositionZero? tail_incr : 0);

//check if this is the last write cycle (Writing_done will be true afterwards)
if (Current_elem == LAST_WRITE_ELEM)
Expand All @@ -216,21 +236,29 @@ module $TOP_MODULE_NAME$_impl #(
Write_cmd <= fetch_cmd;
end

if (write_ok)
if(write_ok) begin
Write_cmd <= fetch_cmd;

if (write_ok && Fetching_done) begin
//check if this is the last write cycle (Writing_done will be true afterwards)
if (reading_done || (read_ok && (Newest_buffered_elem == LAST_READ_ELEM - 1))) begin
//start processing of next FM if reading is done already, or completes in the same cycle
Newest_buffered_elem <= -1;
Current_elem <= 0;
Window_buffer_read_addr_reg <= 0;
First_elem_next_window <= 0;
Fetching_done <= 0;
end else
Writing_done <= 1;
if(Fetching_done) begin
//check if this is the last write cycle (Writing_done will be true afterwards)
if(ReadingDone || (read_ok && ReadingLast)) begin
//start processing of next FM if reading is done already, or completes in the same cycle
nbe_rst = 1;

Current_elem <= 0;
Window_buffer_read_addr_reg <= 0;
First_elem_next_window <= 0;
Fetching_done <= 0;
end
else Writing_done <= 1;
end
end

Newest_buffered_elem <= Newest_buffered_elem + (nbe_rst? ~Newest_buffered_elem : nbe_inc);
ReadingLast <= nbe_rst? LAST_READ_ELEM == 0 : !nbe_inc? ReadingLast :
// Newest_buffered_elem == LAST_READ_ELEM-1 in the next cycle knowing we only get there by increments
((~Newest_buffered_elem & (LAST_READ_ELEM-2)) == 0) && !ReadingLast && !Newest_buffered_elem[$left(Newest_buffered_elem)];
ReadingDone <= nbe_rst? 0 : !nbe_inc? ReadingDone : ReadingLast;
end
end

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