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microwatt/control.vhdl

265 lines
9.1 KiB
VHDL

library ieee;
use ieee.std_logic_1164.all;
library work;
use work.common.all;
entity control is
generic (
EX1_BYPASS : boolean := true;
PIPELINE_DEPTH : natural := 3
);
port (
clk : in std_ulogic;
rst : in std_ulogic;
complete_in : in instr_tag_t;
valid_in : in std_ulogic;
flush_in : in std_ulogic;
deferred : in std_ulogic;
serialize : in std_ulogic;
stop_mark_in : in std_ulogic;
gpr_write_valid_in : in std_ulogic;
gpr_write_in : in gspr_index_t;
gpr_a_read_valid_in : in std_ulogic;
gpr_a_read_in : in gspr_index_t;
gpr_b_read_valid_in : in std_ulogic;
gpr_b_read_in : in gspr_index_t;
gpr_c_read_valid_in : in std_ulogic;
gpr_c_read_in : in gspr_index_t;
execute_next_tag : in instr_tag_t;
execute_next_cr_tag : in instr_tag_t;
execute2_next_tag : in instr_tag_t;
execute2_next_cr_tag : in instr_tag_t;
cr_read_in : in std_ulogic;
cr_write_in : in std_ulogic;
valid_out : out std_ulogic;
stopped_out : out std_ulogic;
gpr_bypass_a : out std_ulogic_vector(1 downto 0);
gpr_bypass_b : out std_ulogic_vector(1 downto 0);
gpr_bypass_c : out std_ulogic_vector(1 downto 0);
cr_bypass : out std_ulogic_vector(1 downto 0);
instr_tag_out : out instr_tag_t
);
end entity control;
architecture rtl of control is
signal gpr_write_valid : std_ulogic;
signal cr_write_valid : std_ulogic;
type tag_register is record
wr_gpr : std_ulogic;
reg : gspr_index_t;
recent : std_ulogic;
wr_cr : std_ulogic;
valid : std_ulogic;
end record;
type tag_regs_array is array(tag_number_t) of tag_register;
signal tag_regs : tag_regs_array;
signal instr_tag : instr_tag_t;
signal gpr_tag_stall : std_ulogic;
signal cr_tag_stall : std_ulogic;
signal serial_stall : std_ulogic;
signal curr_tag : tag_number_t;
signal next_tag : tag_number_t;
signal curr_cr_tag : tag_number_t;
signal prev_tag : tag_number_t;
begin
control0: process(clk)
begin
if rising_edge(clk) then
for i in tag_number_t loop
if rst = '1' or flush_in = '1' then
tag_regs(i).wr_gpr <= '0';
tag_regs(i).wr_cr <= '0';
tag_regs(i).valid <= '0';
else
if complete_in.valid = '1' and i = complete_in.tag then
assert tag_regs(i).valid = '1' report "spurious completion" severity failure;
tag_regs(i).wr_gpr <= '0';
tag_regs(i).wr_cr <= '0';
tag_regs(i).valid <= '0';
report "tag " & integer'image(i) & " not valid";
end if;
Add a second execute stage to the pipeline This adds a second execute stage to the pipeline, in order to match up the length of the pipeline through loadstore and dcache with the length through execute1. This will ultimately enable us to get rid of the 1-cycle bubble that we currently have when issuing ALU instructions after one or more LSU instructions. Most ALU instructions execute in the first stage, except for count-zeroes and popcount instructions (which take two cycles and do some of their work in the second stage) and mfspr/mtspr to "slow" SPRs (TB, DEC, PVR, LOGA/LOGD, CFAR). Multiply and divide/mod instructions take several cycles but the instruction stays in the first stage (ex1) and ex1.busy is asserted until the operation is complete. There is currently a bypass from the first stage but not the second stage. Performance is down somewhat because of that and because this doesn't yet eliminate the bubble between LSU and ALU instructions. The forwarding of XER common bits has been changed somewhat because now there is another pipeline stage between ex1 and the committed state in cr_file. The simplest thing for now is to record the last value written and use that, unless there has been a flush, in which case the committed state (obtained via e_in.xerc) is used. Note that this fixes what was previously a benign bug in control.vhdl, where it was possible for control to forget an instructions dependency on a value from a previous instruction (a GPR or the CR) if this instruction writes the value and the instruction gets to the point where it could issue but is blocked by the busy signal from execute1. In that situation, control may incorrectly not indicate that a bypass should be used. That didn't matter previously because, for ALU and FPU instructions, there was only one previous instruction in flight and once the current instruction could issue, the previous instruction was completing and the correct value would be obtained from register_file or cr_file. For loadstore instructions there could be two being executed, but because there are no bypass paths, failing to indicate use of a bypass path is fine. Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
2 years ago
if instr_tag.valid = '1' and gpr_write_valid = '1' and
tag_regs(i).reg = gpr_write_in then
tag_regs(i).recent <= '0';
if tag_regs(i).recent = '1' and tag_regs(i).wr_gpr = '1' then
report "tag " & integer'image(i) & " not recent";
end if;
end if;
if instr_tag.valid = '1' and i = instr_tag.tag then
tag_regs(i).wr_gpr <= gpr_write_valid;
tag_regs(i).reg <= gpr_write_in;
tag_regs(i).recent <= gpr_write_valid;
tag_regs(i).wr_cr <= cr_write_valid;
tag_regs(i).valid <= '1';
if gpr_write_valid = '1' then
report "tag " & integer'image(i) & " valid for gpr " & to_hstring(gpr_write_in);
end if;
end if;
end if;
end loop;
if rst = '1' then
curr_tag <= 0;
curr_cr_tag <= 0;
prev_tag <= 0;
else
curr_tag <= next_tag;
Add a second execute stage to the pipeline This adds a second execute stage to the pipeline, in order to match up the length of the pipeline through loadstore and dcache with the length through execute1. This will ultimately enable us to get rid of the 1-cycle bubble that we currently have when issuing ALU instructions after one or more LSU instructions. Most ALU instructions execute in the first stage, except for count-zeroes and popcount instructions (which take two cycles and do some of their work in the second stage) and mfspr/mtspr to "slow" SPRs (TB, DEC, PVR, LOGA/LOGD, CFAR). Multiply and divide/mod instructions take several cycles but the instruction stays in the first stage (ex1) and ex1.busy is asserted until the operation is complete. There is currently a bypass from the first stage but not the second stage. Performance is down somewhat because of that and because this doesn't yet eliminate the bubble between LSU and ALU instructions. The forwarding of XER common bits has been changed somewhat because now there is another pipeline stage between ex1 and the committed state in cr_file. The simplest thing for now is to record the last value written and use that, unless there has been a flush, in which case the committed state (obtained via e_in.xerc) is used. Note that this fixes what was previously a benign bug in control.vhdl, where it was possible for control to forget an instructions dependency on a value from a previous instruction (a GPR or the CR) if this instruction writes the value and the instruction gets to the point where it could issue but is blocked by the busy signal from execute1. In that situation, control may incorrectly not indicate that a bypass should be used. That didn't matter previously because, for ALU and FPU instructions, there was only one previous instruction in flight and once the current instruction could issue, the previous instruction was completing and the correct value would be obtained from register_file or cr_file. For loadstore instructions there could be two being executed, but because there are no bypass paths, failing to indicate use of a bypass path is fine. Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
2 years ago
if instr_tag.valid = '1' and cr_write_valid = '1' then
curr_cr_tag <= instr_tag.tag;
end if;
if valid_out = '1' then
prev_tag <= instr_tag.tag;
end if;
end if;
end if;
end process;
control_hazards : process(all)
variable gpr_stall : std_ulogic;
variable tag_a : instr_tag_t;
variable tag_b : instr_tag_t;
variable tag_c : instr_tag_t;
variable tag_s : instr_tag_t;
variable tag_t : instr_tag_t;
variable incr_tag : tag_number_t;
variable byp_a : std_ulogic_vector(1 downto 0);
variable byp_b : std_ulogic_vector(1 downto 0);
variable byp_c : std_ulogic_vector(1 downto 0);
variable tag_cr : instr_tag_t;
variable byp_cr : std_ulogic_vector(1 downto 0);
variable tag_prev : instr_tag_t;
begin
tag_a := instr_tag_init;
for i in tag_number_t loop
if tag_regs(i).wr_gpr = '1' and tag_regs(i).recent = '1' and tag_regs(i).reg = gpr_a_read_in then
tag_a.valid := gpr_a_read_valid_in;
tag_a.tag := i;
end if;
end loop;
tag_b := instr_tag_init;
for i in tag_number_t loop
if tag_regs(i).wr_gpr = '1' and tag_regs(i).recent = '1' and tag_regs(i).reg = gpr_b_read_in then
tag_b.valid := gpr_b_read_valid_in;
tag_b.tag := i;
end if;
end loop;
tag_c := instr_tag_init;
for i in tag_number_t loop
if tag_regs(i).wr_gpr = '1' and tag_regs(i).recent = '1' and tag_regs(i).reg = gpr_c_read_in then
tag_c.valid := gpr_c_read_valid_in;
tag_c.tag := i;
end if;
end loop;
byp_a := "00";
if EX1_BYPASS and tag_match(execute_next_tag, tag_a) then
byp_a := "01";
elsif EX1_BYPASS and tag_match(execute2_next_tag, tag_a) then
byp_a := "10";
elsif tag_match(complete_in, tag_a) then
byp_a := "11";
end if;
byp_b := "00";
if EX1_BYPASS and tag_match(execute_next_tag, tag_b) then
byp_b := "01";
elsif EX1_BYPASS and tag_match(execute2_next_tag, tag_b) then
byp_b := "10";
elsif tag_match(complete_in, tag_b) then
byp_b := "11";
end if;
byp_c := "00";
if EX1_BYPASS and tag_match(execute_next_tag, tag_c) then
byp_c := "01";
elsif EX1_BYPASS and tag_match(execute2_next_tag, tag_c) then
byp_c := "10";
elsif tag_match(complete_in, tag_c) then
byp_c := "11";
end if;
gpr_bypass_a <= byp_a;
gpr_bypass_b <= byp_b;
gpr_bypass_c <= byp_c;
gpr_tag_stall <= (tag_a.valid and not (or (byp_a))) or
(tag_b.valid and not (or (byp_b))) or
(tag_c.valid and not (or (byp_c)));
incr_tag := curr_tag;
instr_tag.tag <= curr_tag;
instr_tag.valid <= valid_out and not deferred;
if instr_tag.valid = '1' then
incr_tag := (curr_tag + 1) mod TAG_COUNT;
end if;
next_tag <= incr_tag;
instr_tag_out <= instr_tag;
-- CR hazards
tag_cr.tag := curr_cr_tag;
tag_cr.valid := cr_read_in and tag_regs(curr_cr_tag).wr_cr;
if tag_match(tag_cr, complete_in) then
tag_cr.valid := '0';
end if;
byp_cr := "00";
if EX1_BYPASS and tag_match(execute_next_cr_tag, tag_cr) then
byp_cr := "10";
elsif EX1_BYPASS and tag_match(execute2_next_cr_tag, tag_cr) then
byp_cr := "11";
end if;
cr_bypass <= byp_cr;
cr_tag_stall <= tag_cr.valid and not byp_cr(1);
tag_prev.tag := prev_tag;
tag_prev.valid := tag_regs(prev_tag).valid;
if tag_match(tag_prev, complete_in) then
tag_prev.valid := '0';
end if;
serial_stall <= tag_prev.valid;
end process;
control1 : process(all)
variable valid_tmp : std_ulogic;
begin
-- asynchronous
valid_tmp := valid_in and not flush_in;
if rst = '1' then
gpr_write_valid <= '0';
cr_write_valid <= '0';
valid_tmp := '0';
end if;
-- Handle debugger stop
stopped_out <= stop_mark_in and not serial_stall;
-- Don't let it go out if there are GPR or CR hazards
-- or we are waiting for the previous instruction to complete
if (gpr_tag_stall or cr_tag_stall or (serialize and serial_stall)) = '1' then
valid_tmp := '0';
end if;
gpr_write_valid <= gpr_write_valid_in and valid_tmp;
cr_write_valid <= cr_write_in and valid_tmp;
-- update outputs
valid_out <= valid_tmp;
end process;
end;