decode1: Take an extra cycle for predicted branch redirects

This does the addition of NIA plus the branch offset from the
instruction after a clock edge, in order to ease timing, as the path
from the icache RAM through the adder in decode1 to the NIA register
in fetch1 was showing up as a critical path.

This adds one extra cycle of latency when redirecting fetch because of
a predicted-taken branch.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
pull/268/head
Paul Mackerras 4 years ago
parent 6427cab46f
commit cb1e3f6d70

@ -31,6 +31,7 @@ end entity decode1;
architecture behaviour of decode1 is architecture behaviour of decode1 is
signal r, rin : Decode1ToDecode2Type; signal r, rin : Decode1ToDecode2Type;
signal s : Decode1ToDecode2Type; signal s : Decode1ToDecode2Type;
signal f, fin : Decode1ToFetch1Type;


constant illegal_inst : decode_rom_t := constant illegal_inst : decode_rom_t :=
(NONE, NONE, OP_ILLEGAL, NONE, NONE, NONE, NONE, '0', '0', '0', '0', ZERO, '0', NONE, '0', '0', '0', '0', '0', '0', NONE, '0', '0', NONE); (NONE, NONE, OP_ILLEGAL, NONE, NONE, NONE, NONE, '0', '0', '0', '0', ZERO, '0', NONE, '0', '0', '0', '0', '0', '0', NONE, '0', '0', NONE);
@ -47,6 +48,14 @@ architecture behaviour of decode1 is
signal ri, ri_in : reg_internal_t; signal ri, ri_in : reg_internal_t;
signal si : reg_internal_t; signal si : reg_internal_t;


type br_predictor_t is record
br_nia : std_ulogic_vector(61 downto 0);
br_offset : signed(23 downto 0);
predict : std_ulogic;
end record;

signal br, br_in : br_predictor_t;

subtype major_opcode_t is unsigned(5 downto 0); subtype major_opcode_t is unsigned(5 downto 0);
type major_rom_array_t is array(0 to 63) of decode_rom_t; type major_rom_array_t is array(0 to 63) of decode_rom_t;
type minor_valid_array_t is array(0 to 1023) of std_ulogic; type minor_valid_array_t is array(0 to 1023) of std_ulogic;
@ -537,6 +546,13 @@ begin
ri <= ri_in; ri <= ri_in;
end if; end if;
end if; end if;
if rst = '1' then
br.br_nia <= (others => '0');
br.br_offset <= (others => '0');
br.predict <= '0';
else
br <= br_in;
end if;
end if; end if;
end process; end process;
busy_out <= s.valid; busy_out <= s.valid;
@ -544,14 +560,13 @@ begin
decode1_1: process(all) decode1_1: process(all)
variable v : Decode1ToDecode2Type; variable v : Decode1ToDecode2Type;
variable vi : reg_internal_t; variable vi : reg_internal_t;
variable f : Decode1ToFetch1Type;
variable majorop : major_opcode_t; variable majorop : major_opcode_t;
variable minor4op : std_ulogic_vector(10 downto 0); variable minor4op : std_ulogic_vector(10 downto 0);
variable op_19_bits: std_ulogic_vector(2 downto 0); variable op_19_bits: std_ulogic_vector(2 downto 0);
variable sprn : spr_num_t; variable sprn : spr_num_t;
variable br_nia : std_ulogic_vector(61 downto 0);
variable br_target : std_ulogic_vector(61 downto 0); variable br_target : std_ulogic_vector(61 downto 0);
variable br_offset : signed(23 downto 0); variable br_offset : signed(23 downto 0);
variable bv : br_predictor_t;
begin begin
v := Decode1ToDecode2Init; v := Decode1ToDecode2Init;
vi := reg_internal_t_init; vi := reg_internal_t_init;
@ -707,17 +722,19 @@ begin
-- Branch predictor -- Branch predictor
-- Note bclr, bcctr and bctar are predicted not taken as we have no -- Note bclr, bcctr and bctar are predicted not taken as we have no
-- count cache or link stack. -- count cache or link stack.
br_nia := f_in.nia(63 downto 2); bv.br_nia := f_in.nia(63 downto 2);
if f_in.insn(1) = '1' then if f_in.insn(1) = '1' then
br_nia := (others => '0'); bv.br_nia := (others => '0');
end if; end if;
br_target := std_ulogic_vector(signed(br_nia) + br_offset); bv.br_offset := br_offset;
f.redirect := v.br_pred and f_in.valid and not flush_in and not s.valid; bv.predict := v.br_pred and f_in.valid and not flush_in and not busy_out;
f.redirect_nia := br_target & "00"; -- after a clock edge...
br_target := std_ulogic_vector(signed(br.br_nia) + br.br_offset);


-- Update registers -- Update registers
rin <= v; rin <= v;
ri_in <= vi; ri_in <= vi;
br_in <= bv;


-- Update outputs -- Update outputs
d_out <= r; d_out <= r;
@ -729,8 +746,9 @@ begin
if ri.force_single = '1' then if ri.force_single = '1' then
d_out.decode.sgl_pipe <= '1'; d_out.decode.sgl_pipe <= '1';
end if; end if;
f_out <= f; f_out.redirect <= br.predict;
flush_out <= f.redirect; f_out.redirect_nia <= br_target & "00";
flush_out <= bv.predict or br.predict;
end process; end process;


d1_log: if LOG_LENGTH > 0 generate d1_log: if LOG_LENGTH > 0 generate

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