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icache: Use narrower block RAMs

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We only ever access the cache memory for at most the wishbone bus
width at a time. So having the BRAMs organized as a cache-line-wide
port is a waste of resources.

Instead, use a wishbone-wide memory and store a line as consecutive
rows in the BRAM.

This significantly improves BRAM usage in the FPGA as we can now use
more rows in the BRAM blocks. It also saves a few LUTs and muxes.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
pull/82/head
Benjamin Herrenschmidt 5 years ago
parent d415e5544a
commit d40c1c1a25
3 changed files with 201 additions and 109 deletions
Show Stats Download Patch File Download Diff File

2
core.vhdl
Unescape Escape View File

@ -115,7 +115,7 @@ begin

icache_0: entity work.icache
generic map(
LINE_SIZE_DW => 8,
LINE_SIZE => 64,
NUM_LINES => 16
)
port map(

238
icache.vhdl
Unescape Escape View File

@ -10,8 +10,8 @@ use work.wishbone_types.all;

entity icache is
generic (
-- Line size in 64bit doublewords
LINE_SIZE_DW : natural := 8;
-- Line size in bytes
LINE_SIZE : natural := 64;
-- Number of lines
NUM_LINES : natural := 32
);
@ -51,85 +51,176 @@ architecture rtl of icache is
end if;
end function;

constant LINE_SIZE : natural := LINE_SIZE_DW*8;
constant OFFSET_BITS : natural := log2(LINE_SIZE);
-- BRAM organisation: We never access more than wishbone_data_bits at
-- a time so to save resources we make the array only that wide, and
-- use consecutive indices for to make a cache "line"
--
-- ROW_SIZE is the width in bytes of the BRAM (based on WB, so 64-bits)
constant ROW_SIZE : natural := wishbone_data_bits / 8;
-- ROW_PER_LINE is the number of row (wishbone transactions) in a line
constant ROW_PER_LINE : natural := LINE_SIZE / ROW_SIZE;
-- BRAM_ROWS is the number of rows in BRAM needed to represent the full
-- icache
constant BRAM_ROWS : natural := NUM_LINES * ROW_PER_LINE;
-- INSN_PER_ROW is the number of 32bit instructions per BRAM row
constant INSN_PER_ROW : natural := wishbone_data_bits / 32;
-- Bit fields counts in the address

-- INSN_BITS is the number of bits to select an instruction in a row
constant INSN_BITS : natural := log2(INSN_PER_ROW);
-- ROW_BITS is the number of bits to select a row
constant ROW_BITS : natural := log2(BRAM_ROWS);
-- ROW_LINEBITS is the number of bits to select a row within a line
constant ROW_LINEBITS : natural := log2(ROW_PER_LINE);
-- LINE_OFF_BITS is the number of bits for the offset in a cache line
constant LINE_OFF_BITS : natural := log2(LINE_SIZE);
-- ROW_OFF_BITS is the number of bits for the offset in a row
constant ROW_OFF_BITS : natural := log2(ROW_SIZE);
-- INDEX_BITS is the number if bits to select a cache line
constant INDEX_BITS : natural := log2(NUM_LINES);
constant TAG_BITS : natural := 64 - OFFSET_BITS - INDEX_BITS;

subtype cacheline_type is std_logic_vector((LINE_SIZE*8)-1 downto 0);
type cacheline_array is array(0 to NUM_LINES-1) of cacheline_type;

subtype cacheline_tag_type is std_logic_vector(TAG_BITS-1 downto 0);
type cacheline_tag_array is array(0 to NUM_LINES-1) of cacheline_tag_type;
-- TAG_BITS is the number of bits of the tag part of the address
constant TAG_BITS : natural := 64 - LINE_OFF_BITS - INDEX_BITS;

-- Storage. Hopefully "cachelines" is a BRAM, the rest is LUTs
signal cachelines : cacheline_array;
signal tags : cacheline_tag_array;
-- Example of layout for 32 lines of 64 bytes:
--
-- .. tag |index| line |
-- .. | row | |
-- .. | | | |00| zero (2)
-- .. | | |-| | INSN_BITS (1)
-- .. | |---| | ROW_LINEBITS (3)
-- .. | |--- - --| LINE_OFF_BITS (6)
-- .. | |- --| ROW_OFF_BITS (3)
-- .. |----- ---| | ROW_BITS (8)
-- .. |-----| | INDEX_BITS (5)
-- .. --------| | TAG_BITS (53)

subtype row_t is integer range 0 to BRAM_ROWS-1;
subtype index_t is integer range 0 to NUM_LINES-1;

-- The cache data BRAM organized as described above
subtype cache_row_t is std_logic_vector(wishbone_data_bits-1 downto 0);
type cache_array is array(row_t) of cache_row_t;

-- The cache tags LUTRAM has a row per cache line
subtype cache_tag_t is std_logic_vector(TAG_BITS-1 downto 0);
type cache_tags_array is array(index_t) of cache_tag_t;

-- Storage. Hopefully "cache_rows" is a BRAM, the rest is LUTs
signal cache_rows : cache_array;
signal tags : cache_tags_array;
signal tags_valid : std_ulogic_vector(NUM_LINES-1 downto 0);
attribute ram_style : string;
attribute ram_style of cachelines : signal is "block";
attribute ram_style of cache_rows : signal is "block";
attribute ram_decomp : string;
attribute ram_decomp of cachelines : signal is "power";
attribute ram_decomp of cache_rows : signal is "power";

-- Cache reload state machine
type state_type is (IDLE, WAIT_ACK);
type state_t is (IDLE, WAIT_ACK);

type reg_internal_type is record
type reg_internal_t is record
-- Cache hit state (1 cycle BRAM access)
hit_line : cacheline_type;
hit_row : cache_row_t;
hit_nia : std_ulogic_vector(63 downto 0);
hit_smark : std_ulogic;
hit_valid : std_ulogic;

-- Cache miss state (reload state machine)
state : state_type;
state : state_t;
wb : wishbone_master_out;
store_index : integer range 0 to (NUM_LINES-1);
store_mask : std_ulogic_vector(LINE_SIZE_DW-1 downto 0);
store_index : index_t;
end record;

signal r : reg_internal_type;
signal r : reg_internal_t;

-- Async signals decoding incoming requests
signal req_index : integer range 0 to NUM_LINES-1;
signal req_tag : std_ulogic_vector(TAG_BITS-1 downto 0);
signal req_word : integer range 0 to LINE_SIZE_DW*2-1;
-- Async signals on incoming request
signal req_index : index_t;
signal req_row : row_t;
signal req_tag : cache_tag_t;
signal req_is_hit : std_ulogic;

-- Return the cache line index (tag index) for an address
function get_index(addr: std_ulogic_vector(63 downto 0)) return integer is
function get_index(addr: std_ulogic_vector(63 downto 0)) return index_t is
begin
return to_integer(unsigned(addr(63-TAG_BITS downto LINE_OFF_BITS)));
end;

-- Return the cache row index (data memory) for an address
function get_row(addr: std_ulogic_vector(63 downto 0)) return row_t is
begin
return to_integer(unsigned(addr(63-TAG_BITS downto ROW_OFF_BITS)));
end;

-- Returns whether this is the last row of a line
function is_last_row(addr: std_ulogic_vector(63 downto 0)) return boolean is
constant ones : std_ulogic_vector(ROW_LINEBITS-1 downto 0) := (others => '1');
begin
return to_integer(unsigned(addr((OFFSET_BITS+INDEX_BITS-1) downto OFFSET_BITS)));
return addr(LINE_OFF_BITS-1 downto ROW_OFF_BITS) = ones;
end;

-- Return the word index in a cache line for an address
function get_word(addr: std_ulogic_vector(63 downto 0)) return integer is
-- Return the address of the next row in the current cache line
function next_row_addr(addr: std_ulogic_vector(63 downto 0)) return std_ulogic_vector is
variable row_idx : std_ulogic_vector(ROW_LINEBITS-1 downto 0);
variable result : std_ulogic_vector(63 downto 0);
begin
return to_integer(unsigned(addr(OFFSET_BITS-1 downto 2)));
-- Is there no simpler way in VHDL to generate that 3 bits adder ?
row_idx := addr(LINE_OFF_BITS-1 downto ROW_OFF_BITS);
row_idx := std_ulogic_vector(unsigned(row_idx) + 1);
result := addr;
result(LINE_OFF_BITS-1 downto ROW_OFF_BITS) := row_idx;
return result;
end;

-- Read a word in a cache line for an address
function read_word(word: integer; data: cacheline_type) return std_ulogic_vector is
-- Read the instruction word for the given address in the current cache row
function read_insn_word(addr: std_ulogic_vector(63 downto 0);
data: cache_row_t) return std_ulogic_vector is
variable word: integer range 0 to INSN_PER_ROW-1;
begin
return data((word+1)*32-1 downto word*32);
word := to_integer(unsigned(addr(INSN_BITS+2-1 downto 2)));
return data(31+word*32 downto word*32);
end;

-- Calculate the tag value from the address
function get_tag(addr: std_ulogic_vector(63 downto 0)) return std_ulogic_vector is
-- Get the tag value from the address
function get_tag(addr: std_ulogic_vector(63 downto 0)) return cache_tag_t is
begin
return addr(63 downto OFFSET_BITS+INDEX_BITS);
return addr(63 downto 64-TAG_BITS);
end;

begin
assert ispow2(LINE_SIZE) report "LINE_SIZE not power of 2" severity FAILURE;
assert ispow2(NUM_LINES) report "NUM_LINES not power of 2" severity FAILURE;
assert ispow2(ROW_PER_LINE) report "ROW_PER_LINE not power of 2" severity FAILURE;
assert ispow2(INSN_PER_ROW) report "INSN_PER_ROW not power of 2" severity FAILURE;
assert (ROW_BITS = INDEX_BITS + ROW_LINEBITS)
report "geometry bits don't add up" severity FAILURE;
assert (LINE_OFF_BITS = ROW_OFF_BITS + ROW_LINEBITS)
report "geometry bits don't add up" severity FAILURE;
assert (64 = TAG_BITS + INDEX_BITS + LINE_OFF_BITS)
report "geometry bits don't add up" severity FAILURE;
assert (64 = TAG_BITS + ROW_BITS + ROW_OFF_BITS)
report "geometry bits don't add up" severity FAILURE;

debug: process
begin
report "ROW_SIZE = " & natural'image(ROW_SIZE);
report "ROW_PER_LINE = " & natural'image(ROW_PER_LINE);
report "BRAM_ROWS = " & natural'image(BRAM_ROWS);
report "INSN_PER_ROW = " & natural'image(INSN_PER_ROW);
report "INSN_BITS = " & natural'image(INSN_BITS);
report "ROW_BITS = " & natural'image(ROW_BITS);
report "ROW_LINEBITS = " & natural'image(ROW_LINEBITS);
report "LINE_OFF_BITS = " & natural'image(LINE_OFF_BITS);
report "ROW_OFF_BITS = " & natural'image(ROW_OFF_BITS);
report "INDEX_BITS = " & natural'image(INDEX_BITS);
report "TAG_BITS = " & natural'image(TAG_BITS);
wait;
end process;

icache_comb : process(all)
begin
-- Calculate next index and tag index
-- Extract line, row and tag from request
req_index <= get_index(i_in.nia);
req_row <= get_row(i_in.nia);
req_tag <= get_tag(i_in.nia);
req_word <= get_word(i_in.nia);

-- Test if pending request is a hit
if tags(req_index) = req_tag then
@ -138,19 +229,19 @@ begin
req_is_hit <= '0';
end if;

-- Output instruction from current cache line
-- Output instruction from current cache row
--
-- Note: This is a mild violation of our design principle of having pipeline
-- stages output from a clean latch. In this case we output the result
-- of a mux. The alternative would be output an entire cache line
-- which I prefer not to do just yet.
--
i_out.insn <= read_insn_word(r.hit_nia, r.hit_row);
i_out.valid <= r.hit_valid;
i_out.insn <= read_word(get_word(r.hit_nia), r.hit_line);
i_out.nia <= r.hit_nia;
i_out.stop_mark <= r.hit_smark;

-- This needs to match the latching of a new request in icache_hit
-- This needs to match the latching of a new request in process icache_hit
stall_out <= not req_is_hit;

-- Wishbone requests output (from the cache miss reload machine)
@ -160,8 +251,19 @@ begin
icache_hit : process(clk)
begin
if rising_edge(clk) then
-- Assume we have nothing valid first
r.hit_valid <= '0';
-- Debug
if i_in.req = '1' then
report "cache search for " & to_hstring(i_in.nia) &
" index:" & integer'image(req_index) &
" row:" & integer'image(req_row) &
" want_tag:" & to_hstring(req_tag) & " got_tag:" & to_hstring(req_tag) &
" valid:" & std_ulogic'image(tags_valid(req_index));
if req_is_hit = '1' then
report "is hit !";
else
report "is miss !";
end if;
end if;

-- Are we free to latch a new request ?
--
@ -169,7 +271,7 @@ begin
--
if i_in.req = '1' and req_is_hit = '1' and flush_in = '0' then
-- Read the cache line (BRAM read port) and remember the NIA
r.hit_line <= cachelines(req_index);
r.hit_row <= cache_rows(req_row);
r.hit_nia <= i_in.nia;
r.hit_smark <= i_in.stop_mark;
r.hit_valid <= '1';
@ -178,22 +280,19 @@ begin
" SM:" & std_ulogic'image(i_in.stop_mark) &
" idx:" & integer'image(req_index) &
" tag:" & to_hstring(req_tag);
end if;

-- Flush requested ? discard...
if flush_in then
else
r.hit_valid <= '0';
-- Send stop marks down regardless of validity
r.hit_smark <= i_in.stop_mark;
end if;
end if;
end process;

icache_miss : process(clk)
variable store_dword : std_ulogic_vector(OFFSET_BITS-4 downto 0);
begin
if rising_edge(clk) then
if rst = '1' then
tags_valid <= (others => '0');
r.store_mask <= (others => '0');
r.state <= IDLE;
r.wb.cyc <= '0';
r.wb.stb <= '0';
@ -202,56 +301,49 @@ begin
r.wb.dat <= (others => '0');
r.wb.sel <= "11111111";
r.wb.we <= '0';
end if;

else
-- State machine
case r.state is
when IDLE =>
-- We need to read a cache line
if i_in.req = '1' and req_is_hit = '0' then

report "cache miss nia:" & to_hstring(i_in.nia) &
" SM:" & std_ulogic'image(i_in.stop_mark) &
" idx:" & integer'image(req_index) &
" tag:" & to_hstring(req_tag);

r.state <= WAIT_ACK;
r.store_mask <= (0 => '1', others => '0');
r.store_index <= req_index;

-- Force misses while reloading that line
tags_valid(req_index) <= '0';
tags(req_index) <= req_tag;
r.store_index <= req_index;

-- Prep for first dword read
r.wb.adr <= i_in.nia(63 downto OFFSET_BITS) & (OFFSET_BITS-1 downto 0 => '0');
-- Prep for first wishbone read. We calculate the address off
-- the start of the cache line
r.wb.adr <= i_in.nia(63 downto LINE_OFF_BITS) &
(LINE_OFF_BITS-1 downto 0 => '0');
r.wb.cyc <= '1';
r.wb.stb <= '1';

r.state <= WAIT_ACK;
end if;
when WAIT_ACK =>
if wishbone_in.ack = '1' then
-- Store the current dword in both the cache
for i in 0 to LINE_SIZE_DW-1 loop
if r.store_mask(i) = '1' then
cachelines(r.store_index)(63 + i*64 downto i*64) <= wishbone_in.dat;
end if;
end loop;
cache_rows(get_row(r.wb.adr)) <= wishbone_in.dat;

-- That was the last word ? We are done
if r.store_mask(LINE_SIZE_DW-1) = '1' then
r.state <= IDLE;
if is_last_row(r.wb.adr) then
tags_valid(r.store_index) <= '1';
r.wb.cyc <= '0';
r.wb.stb <= '0';
r.state <= IDLE;
else
store_dword := r.wb.adr(OFFSET_BITS-1 downto 3);
store_dword := std_ulogic_vector(unsigned(store_dword) + 1);
r.wb.adr(OFFSET_BITS-1 downto 3) <= store_dword;
-- Otherwise, calculate the next row address
r.wb.adr <= next_row_addr(r.wb.adr);
end if;
-- Advance to next word
r.store_mask <= r.store_mask(LINE_SIZE_DW-2 downto 0) & '0';
end if;
end case;
end if;
end if;
end process;
end;

2
icache_tb.vhdl
Unescape Escape View File

@ -22,7 +22,7 @@ architecture behave of icache_tb is
begin
icache0: entity work.icache
generic map(
LINE_SIZE_DW => 8,
LINE_SIZE => 64,
NUM_LINES => 4
)
port map(

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