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library ieee;
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use ieee.std_logic_1164.all;
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use ieee.numeric_std.all;
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library work;
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use work.common.all;
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entity fetch1 is
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generic(
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RESET_ADDRESS : std_logic_vector(63 downto 0) := (others => '0');
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fetch1: Implement a simple branch target cache
This implements a cache in fetch1, where each entry stores the address
of a simple branch instruction (b or bc) and the target of the branch.
When fetching sequentially, if the address being fetched matches the
cache entry, then fetching will be redirected to the branch target.
The cache has 1024 entries and is direct-mapped, i.e. indexed by bits
11..2 of the NIA.
The bus from execute1 now carries information about taken and
not-taken simple branches, which fetch1 uses to update the cache.
The cache entry is updated for both taken and not-taken branches, with
the valid bit being set if the branch was taken and cleared if the
branch was not taken.
If fetching is redirected to the branch target then that goes down the
pipe as a predicted-taken branch, and decode1 does not do any static
branch prediction. If fetching is not redirected, then the next
instruction goes down the pipe as normal and decode1 does its static
branch prediction.
In order to make timing, the lookup of the cache is pipelined, so on
each cycle the cache entry for the current NIA + 8 is read. This
means that after a redirect (from decode1 or execute1), only the third
and subsequent sequentially-fetched instructions will be able to be
predicted.
This improves the coremark value on the Arty A7-100 from about 180 to
about 190 (more than 5%).
The BTC is optional. Builds for the Artix 7 35-T part have it off by
default because the extra ~1420 LUTs it takes mean that the design
doesn't fit on the Arty A7-35 board.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
4 years ago
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ALT_RESET_ADDRESS : std_logic_vector(63 downto 0) := (others => '0');
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HAS_BTC : boolean := true
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);
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port(
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clk : in std_ulogic;
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rst : in std_ulogic;
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-- Control inputs:
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stall_in : in std_ulogic;
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flush_in : in std_ulogic;
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fetch1: Implement a simple branch target cache
This implements a cache in fetch1, where each entry stores the address
of a simple branch instruction (b or bc) and the target of the branch.
When fetching sequentially, if the address being fetched matches the
cache entry, then fetching will be redirected to the branch target.
The cache has 1024 entries and is direct-mapped, i.e. indexed by bits
11..2 of the NIA.
The bus from execute1 now carries information about taken and
not-taken simple branches, which fetch1 uses to update the cache.
The cache entry is updated for both taken and not-taken branches, with
the valid bit being set if the branch was taken and cleared if the
branch was not taken.
If fetching is redirected to the branch target then that goes down the
pipe as a predicted-taken branch, and decode1 does not do any static
branch prediction. If fetching is not redirected, then the next
instruction goes down the pipe as normal and decode1 does its static
branch prediction.
In order to make timing, the lookup of the cache is pipelined, so on
each cycle the cache entry for the current NIA + 8 is read. This
means that after a redirect (from decode1 or execute1), only the third
and subsequent sequentially-fetched instructions will be able to be
predicted.
This improves the coremark value on the Arty A7-100 from about 180 to
about 190 (more than 5%).
The BTC is optional. Builds for the Artix 7 35-T part have it off by
default because the extra ~1420 LUTs it takes mean that the design
doesn't fit on the Arty A7-35 board.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
4 years ago
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inval_btc : in std_ulogic;
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stop_in : in std_ulogic;
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alt_reset_in : in std_ulogic;
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-- redirect from writeback unit
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w_in : in WritebackToFetch1Type;
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-- redirect from decode1
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d_in : in Decode1ToFetch1Type;
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-- Request to icache
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i_out : out Fetch1ToIcacheType;
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-- outputs to logger
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log_out : out std_ulogic_vector(42 downto 0)
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);
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end entity fetch1;
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architecture behaviour of fetch1 is
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type reg_internal_t is record
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mode_32bit: std_ulogic;
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fetch1: Implement a simple branch target cache
This implements a cache in fetch1, where each entry stores the address
of a simple branch instruction (b or bc) and the target of the branch.
When fetching sequentially, if the address being fetched matches the
cache entry, then fetching will be redirected to the branch target.
The cache has 1024 entries and is direct-mapped, i.e. indexed by bits
11..2 of the NIA.
The bus from execute1 now carries information about taken and
not-taken simple branches, which fetch1 uses to update the cache.
The cache entry is updated for both taken and not-taken branches, with
the valid bit being set if the branch was taken and cleared if the
branch was not taken.
If fetching is redirected to the branch target then that goes down the
pipe as a predicted-taken branch, and decode1 does not do any static
branch prediction. If fetching is not redirected, then the next
instruction goes down the pipe as normal and decode1 does its static
branch prediction.
In order to make timing, the lookup of the cache is pipelined, so on
each cycle the cache entry for the current NIA + 8 is read. This
means that after a redirect (from decode1 or execute1), only the third
and subsequent sequentially-fetched instructions will be able to be
predicted.
This improves the coremark value on the Arty A7-100 from about 180 to
about 190 (more than 5%).
The BTC is optional. Builds for the Artix 7 35-T part have it off by
default because the extra ~1420 LUTs it takes mean that the design
doesn't fit on the Arty A7-35 board.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
4 years ago
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rd_is_niap4: std_ulogic;
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predicted: std_ulogic;
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predicted_nia: std_ulogic_vector(63 downto 0);
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end record;
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signal r, r_next : Fetch1ToIcacheType;
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signal r_int, r_next_int : reg_internal_t;
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fetch1: Implement a simple branch target cache
This implements a cache in fetch1, where each entry stores the address
of a simple branch instruction (b or bc) and the target of the branch.
When fetching sequentially, if the address being fetched matches the
cache entry, then fetching will be redirected to the branch target.
The cache has 1024 entries and is direct-mapped, i.e. indexed by bits
11..2 of the NIA.
The bus from execute1 now carries information about taken and
not-taken simple branches, which fetch1 uses to update the cache.
The cache entry is updated for both taken and not-taken branches, with
the valid bit being set if the branch was taken and cleared if the
branch was not taken.
If fetching is redirected to the branch target then that goes down the
pipe as a predicted-taken branch, and decode1 does not do any static
branch prediction. If fetching is not redirected, then the next
instruction goes down the pipe as normal and decode1 does its static
branch prediction.
In order to make timing, the lookup of the cache is pipelined, so on
each cycle the cache entry for the current NIA + 8 is read. This
means that after a redirect (from decode1 or execute1), only the third
and subsequent sequentially-fetched instructions will be able to be
predicted.
This improves the coremark value on the Arty A7-100 from about 180 to
about 190 (more than 5%).
The BTC is optional. Builds for the Artix 7 35-T part have it off by
default because the extra ~1420 LUTs it takes mean that the design
doesn't fit on the Arty A7-35 board.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
4 years ago
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signal advance_nia : std_ulogic;
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signal log_nia : std_ulogic_vector(42 downto 0);
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fetch1: Implement a simple branch target cache
This implements a cache in fetch1, where each entry stores the address
of a simple branch instruction (b or bc) and the target of the branch.
When fetching sequentially, if the address being fetched matches the
cache entry, then fetching will be redirected to the branch target.
The cache has 1024 entries and is direct-mapped, i.e. indexed by bits
11..2 of the NIA.
The bus from execute1 now carries information about taken and
not-taken simple branches, which fetch1 uses to update the cache.
The cache entry is updated for both taken and not-taken branches, with
the valid bit being set if the branch was taken and cleared if the
branch was not taken.
If fetching is redirected to the branch target then that goes down the
pipe as a predicted-taken branch, and decode1 does not do any static
branch prediction. If fetching is not redirected, then the next
instruction goes down the pipe as normal and decode1 does its static
branch prediction.
In order to make timing, the lookup of the cache is pipelined, so on
each cycle the cache entry for the current NIA + 8 is read. This
means that after a redirect (from decode1 or execute1), only the third
and subsequent sequentially-fetched instructions will be able to be
predicted.
This improves the coremark value on the Arty A7-100 from about 180 to
about 190 (more than 5%).
The BTC is optional. Builds for the Artix 7 35-T part have it off by
default because the extra ~1420 LUTs it takes mean that the design
doesn't fit on the Arty A7-35 board.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
4 years ago
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constant BTC_ADDR_BITS : integer := 10;
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constant BTC_TAG_BITS : integer := 62 - BTC_ADDR_BITS;
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constant BTC_TARGET_BITS : integer := 62;
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constant BTC_SIZE : integer := 2 ** BTC_ADDR_BITS;
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constant BTC_WIDTH : integer := BTC_TAG_BITS + BTC_TARGET_BITS;
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type btc_mem_type is array (0 to BTC_SIZE - 1) of std_ulogic_vector(BTC_WIDTH - 1 downto 0);
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signal btc_rd_data : std_ulogic_vector(BTC_WIDTH - 1 downto 0) := (others => '0');
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signal btc_rd_valid : std_ulogic := '0';
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begin
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regs : process(clk)
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begin
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if rising_edge(clk) then
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log_nia <= r.nia(63) & r.nia(43 downto 2);
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if r /= r_next then
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report "fetch1 rst:" & std_ulogic'image(rst) &
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" IR:" & std_ulogic'image(r_next.virt_mode) &
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" P:" & std_ulogic'image(r_next.priv_mode) &
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" E:" & std_ulogic'image(r_next.big_endian) &
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" 32:" & std_ulogic'image(r_next_int.mode_32bit) &
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" R:" & std_ulogic'image(w_in.redirect) & std_ulogic'image(d_in.redirect) &
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" S:" & std_ulogic'image(stall_in) &
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" T:" & std_ulogic'image(stop_in) &
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fetch1: Implement a simple branch target cache
This implements a cache in fetch1, where each entry stores the address
of a simple branch instruction (b or bc) and the target of the branch.
When fetching sequentially, if the address being fetched matches the
cache entry, then fetching will be redirected to the branch target.
The cache has 1024 entries and is direct-mapped, i.e. indexed by bits
11..2 of the NIA.
The bus from execute1 now carries information about taken and
not-taken simple branches, which fetch1 uses to update the cache.
The cache entry is updated for both taken and not-taken branches, with
the valid bit being set if the branch was taken and cleared if the
branch was not taken.
If fetching is redirected to the branch target then that goes down the
pipe as a predicted-taken branch, and decode1 does not do any static
branch prediction. If fetching is not redirected, then the next
instruction goes down the pipe as normal and decode1 does its static
branch prediction.
In order to make timing, the lookup of the cache is pipelined, so on
each cycle the cache entry for the current NIA + 8 is read. This
means that after a redirect (from decode1 or execute1), only the third
and subsequent sequentially-fetched instructions will be able to be
predicted.
This improves the coremark value on the Arty A7-100 from about 180 to
about 190 (more than 5%).
The BTC is optional. Builds for the Artix 7 35-T part have it off by
default because the extra ~1420 LUTs it takes mean that the design
doesn't fit on the Arty A7-35 board.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
4 years ago
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" nia:" & to_hstring(r_next.nia);
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end if;
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if rst = '1' or w_in.redirect = '1' or d_in.redirect = '1' or stall_in = '0' then
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fetch1: Implement a simple branch target cache
This implements a cache in fetch1, where each entry stores the address
of a simple branch instruction (b or bc) and the target of the branch.
When fetching sequentially, if the address being fetched matches the
cache entry, then fetching will be redirected to the branch target.
The cache has 1024 entries and is direct-mapped, i.e. indexed by bits
11..2 of the NIA.
The bus from execute1 now carries information about taken and
not-taken simple branches, which fetch1 uses to update the cache.
The cache entry is updated for both taken and not-taken branches, with
the valid bit being set if the branch was taken and cleared if the
branch was not taken.
If fetching is redirected to the branch target then that goes down the
pipe as a predicted-taken branch, and decode1 does not do any static
branch prediction. If fetching is not redirected, then the next
instruction goes down the pipe as normal and decode1 does its static
branch prediction.
In order to make timing, the lookup of the cache is pipelined, so on
each cycle the cache entry for the current NIA + 8 is read. This
means that after a redirect (from decode1 or execute1), only the third
and subsequent sequentially-fetched instructions will be able to be
predicted.
This improves the coremark value on the Arty A7-100 from about 180 to
about 190 (more than 5%).
The BTC is optional. Builds for the Artix 7 35-T part have it off by
default because the extra ~1420 LUTs it takes mean that the design
doesn't fit on the Arty A7-35 board.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
4 years ago
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r.virt_mode <= r_next.virt_mode;
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r.priv_mode <= r_next.priv_mode;
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r.big_endian <= r_next.big_endian;
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r_int.mode_32bit <= r_next_int.mode_32bit;
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end if;
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if advance_nia = '1' then
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r.predicted <= r_next.predicted;
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r.nia <= r_next.nia;
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r_int.predicted <= r_next_int.predicted;
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r_int.predicted_nia <= r_next_int.predicted_nia;
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r_int.rd_is_niap4 <= r_next.sequential;
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end if;
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r.sequential <= r_next.sequential and advance_nia;
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-- always send the up-to-date stop mark and req
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r.stop_mark <= stop_in;
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r.req <= not rst;
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end if;
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end process;
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log_out <= log_nia;
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fetch1: Implement a simple branch target cache
This implements a cache in fetch1, where each entry stores the address
of a simple branch instruction (b or bc) and the target of the branch.
When fetching sequentially, if the address being fetched matches the
cache entry, then fetching will be redirected to the branch target.
The cache has 1024 entries and is direct-mapped, i.e. indexed by bits
11..2 of the NIA.
The bus from execute1 now carries information about taken and
not-taken simple branches, which fetch1 uses to update the cache.
The cache entry is updated for both taken and not-taken branches, with
the valid bit being set if the branch was taken and cleared if the
branch was not taken.
If fetching is redirected to the branch target then that goes down the
pipe as a predicted-taken branch, and decode1 does not do any static
branch prediction. If fetching is not redirected, then the next
instruction goes down the pipe as normal and decode1 does its static
branch prediction.
In order to make timing, the lookup of the cache is pipelined, so on
each cycle the cache entry for the current NIA + 8 is read. This
means that after a redirect (from decode1 or execute1), only the third
and subsequent sequentially-fetched instructions will be able to be
predicted.
This improves the coremark value on the Arty A7-100 from about 180 to
about 190 (more than 5%).
The BTC is optional. Builds for the Artix 7 35-T part have it off by
default because the extra ~1420 LUTs it takes mean that the design
doesn't fit on the Arty A7-35 board.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
4 years ago
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btc : if HAS_BTC generate
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signal btc_memory : btc_mem_type;
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attribute ram_style : string;
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attribute ram_style of btc_memory : signal is "block";
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signal btc_valids : std_ulogic_vector(BTC_SIZE - 1 downto 0);
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attribute ram_style of btc_valids : signal is "distributed";
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signal btc_wr : std_ulogic;
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signal btc_wr_data : std_ulogic_vector(BTC_WIDTH - 1 downto 0);
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signal btc_wr_addr : std_ulogic_vector(BTC_ADDR_BITS - 1 downto 0);
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signal btc_wr_v : std_ulogic;
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begin
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btc_wr_data <= w_in.br_nia(63 downto BTC_ADDR_BITS + 2) &
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w_in.redirect_nia(63 downto 2);
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btc_wr_addr <= w_in.br_nia(BTC_ADDR_BITS + 1 downto 2);
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btc_wr <= w_in.br_last;
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btc_wr_v <= w_in.br_taken;
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fetch1: Implement a simple branch target cache
This implements a cache in fetch1, where each entry stores the address
of a simple branch instruction (b or bc) and the target of the branch.
When fetching sequentially, if the address being fetched matches the
cache entry, then fetching will be redirected to the branch target.
The cache has 1024 entries and is direct-mapped, i.e. indexed by bits
11..2 of the NIA.
The bus from execute1 now carries information about taken and
not-taken simple branches, which fetch1 uses to update the cache.
The cache entry is updated for both taken and not-taken branches, with
the valid bit being set if the branch was taken and cleared if the
branch was not taken.
If fetching is redirected to the branch target then that goes down the
pipe as a predicted-taken branch, and decode1 does not do any static
branch prediction. If fetching is not redirected, then the next
instruction goes down the pipe as normal and decode1 does its static
branch prediction.
In order to make timing, the lookup of the cache is pipelined, so on
each cycle the cache entry for the current NIA + 8 is read. This
means that after a redirect (from decode1 or execute1), only the third
and subsequent sequentially-fetched instructions will be able to be
predicted.
This improves the coremark value on the Arty A7-100 from about 180 to
about 190 (more than 5%).
The BTC is optional. Builds for the Artix 7 35-T part have it off by
default because the extra ~1420 LUTs it takes mean that the design
doesn't fit on the Arty A7-35 board.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
4 years ago
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btc_ram : process(clk)
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variable raddr : unsigned(BTC_ADDR_BITS - 1 downto 0);
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begin
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if rising_edge(clk) then
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raddr := unsigned(r.nia(BTC_ADDR_BITS + 1 downto 2)) +
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to_unsigned(2, BTC_ADDR_BITS);
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if advance_nia = '1' then
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btc_rd_data <= btc_memory(to_integer(raddr));
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btc_rd_valid <= btc_valids(to_integer(raddr));
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end if;
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if btc_wr = '1' then
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btc_memory(to_integer(unsigned(btc_wr_addr))) <= btc_wr_data;
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end if;
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if inval_btc = '1' or rst = '1' then
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btc_valids <= (others => '0');
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elsif btc_wr = '1' then
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btc_valids(to_integer(unsigned(btc_wr_addr))) <= btc_wr_v;
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end if;
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end if;
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end process;
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end generate;
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comb : process(all)
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variable v : Fetch1ToIcacheType;
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variable v_int : reg_internal_t;
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begin
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v := r;
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v_int := r_int;
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v.sequential := '0';
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fetch1: Implement a simple branch target cache
This implements a cache in fetch1, where each entry stores the address
of a simple branch instruction (b or bc) and the target of the branch.
When fetching sequentially, if the address being fetched matches the
cache entry, then fetching will be redirected to the branch target.
The cache has 1024 entries and is direct-mapped, i.e. indexed by bits
11..2 of the NIA.
The bus from execute1 now carries information about taken and
not-taken simple branches, which fetch1 uses to update the cache.
The cache entry is updated for both taken and not-taken branches, with
the valid bit being set if the branch was taken and cleared if the
branch was not taken.
If fetching is redirected to the branch target then that goes down the
pipe as a predicted-taken branch, and decode1 does not do any static
branch prediction. If fetching is not redirected, then the next
instruction goes down the pipe as normal and decode1 does its static
branch prediction.
In order to make timing, the lookup of the cache is pipelined, so on
each cycle the cache entry for the current NIA + 8 is read. This
means that after a redirect (from decode1 or execute1), only the third
and subsequent sequentially-fetched instructions will be able to be
predicted.
This improves the coremark value on the Arty A7-100 from about 180 to
about 190 (more than 5%).
The BTC is optional. Builds for the Artix 7 35-T part have it off by
default because the extra ~1420 LUTs it takes mean that the design
doesn't fit on the Arty A7-35 board.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
4 years ago
|
|
|
v.predicted := '0';
|
|
|
|
|
v_int.predicted := '0';
|
|
|
|
|
|
|
|
|
|
if rst = '1' then
|
|
|
|
|
if alt_reset_in = '1' then
|
|
|
|
|
v.nia := ALT_RESET_ADDRESS;
|
|
|
|
|
else
|
|
|
|
|
v.nia := RESET_ADDRESS;
|
|
|
|
|
end if;
|
Add TLB to icache
This adds a direct-mapped TLB to the icache, with 64 entries by default.
Execute1 now sends a "virt_mode" signal from MSR[IR] to fetch1 along
with redirects to indicate whether instruction addresses should be
translated through the TLB, and fetch1 sends that on to icache.
Similarly a "priv_mode" signal is sent to indicate the privilege
mode for instruction fetches. This means that changes to MSR[IR]
or MSR[PR] don't take effect until the next redirect, meaning an
isync, rfid, branch, etc.
The icache uses a hash of the effective address (i.e. next instruction
address) to index the TLB. The hash is an XOR of three fields of the
address; with a 64-entry TLB, the fields are bits 12--17, 18--23 and
24--29 of the address. TLB invalidations simply invalidate the
indexed TLB entry without checking the contents.
If the icache detects a TLB miss with virt_mode=1, it will send a
fetch_failed indication through fetch2 to decode1, which will turn it
into a special OP_FETCH_FAILED opcode with unit=LDST. That will get
sent down to loadstore1 which will currently just raise a Instruction
Storage Interrupt (0x400) exception.
One bit in the PTE obtained from the TLB is used to check whether an
instruction access is allowed -- the privilege bit (bit 3). If bit 3
is 1 and priv_mode=0, then a fetch_failed indication is sent down to
fetch2 and to decode1, which generates an OP_FETCH_FAILED. Any PTEs
with PTE bit 0 (EAA[3]) clear or bit 8 (R) clear should not be put
into the iTLB since such PTEs would not allow execution by any
context.
Tlbie operations get sent from mmu to icache over a new connection.
Unfortunately the privileged instruction tests are broken for now.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
5 years ago
|
|
|
v.virt_mode := '0';
|
|
|
|
|
v.priv_mode := '1';
|
|
|
|
|
v.big_endian := '0';
|
|
|
|
|
v_int.mode_32bit := '0';
|
fetch1: Implement a simple branch target cache
This implements a cache in fetch1, where each entry stores the address
of a simple branch instruction (b or bc) and the target of the branch.
When fetching sequentially, if the address being fetched matches the
cache entry, then fetching will be redirected to the branch target.
The cache has 1024 entries and is direct-mapped, i.e. indexed by bits
11..2 of the NIA.
The bus from execute1 now carries information about taken and
not-taken simple branches, which fetch1 uses to update the cache.
The cache entry is updated for both taken and not-taken branches, with
the valid bit being set if the branch was taken and cleared if the
branch was not taken.
If fetching is redirected to the branch target then that goes down the
pipe as a predicted-taken branch, and decode1 does not do any static
branch prediction. If fetching is not redirected, then the next
instruction goes down the pipe as normal and decode1 does its static
branch prediction.
In order to make timing, the lookup of the cache is pipelined, so on
each cycle the cache entry for the current NIA + 8 is read. This
means that after a redirect (from decode1 or execute1), only the third
and subsequent sequentially-fetched instructions will be able to be
predicted.
This improves the coremark value on the Arty A7-100 from about 180 to
about 190 (more than 5%).
The BTC is optional. Builds for the Artix 7 35-T part have it off by
default because the extra ~1420 LUTs it takes mean that the design
doesn't fit on the Arty A7-35 board.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
4 years ago
|
|
|
v_int.predicted_nia := (others => '0');
|
|
|
|
|
elsif w_in.redirect = '1' then
|
|
|
|
|
v.nia := w_in.redirect_nia(63 downto 2) & "00";
|
|
|
|
|
if w_in.mode_32bit = '1' then
|
|
|
|
|
v.nia(63 downto 32) := (others => '0');
|
|
|
|
|
end if;
|
|
|
|
|
v.virt_mode := w_in.virt_mode;
|
|
|
|
|
v.priv_mode := w_in.priv_mode;
|
|
|
|
|
v.big_endian := w_in.big_endian;
|
|
|
|
|
v_int.mode_32bit := w_in.mode_32bit;
|
|
|
|
|
elsif d_in.redirect = '1' then
|
|
|
|
|
v.nia := d_in.redirect_nia(63 downto 2) & "00";
|
|
|
|
|
if r_int.mode_32bit = '1' then
|
|
|
|
|
v.nia(63 downto 32) := (others => '0');
|
|
|
|
|
end if;
|
fetch1: Implement a simple branch target cache
This implements a cache in fetch1, where each entry stores the address
of a simple branch instruction (b or bc) and the target of the branch.
When fetching sequentially, if the address being fetched matches the
cache entry, then fetching will be redirected to the branch target.
The cache has 1024 entries and is direct-mapped, i.e. indexed by bits
11..2 of the NIA.
The bus from execute1 now carries information about taken and
not-taken simple branches, which fetch1 uses to update the cache.
The cache entry is updated for both taken and not-taken branches, with
the valid bit being set if the branch was taken and cleared if the
branch was not taken.
If fetching is redirected to the branch target then that goes down the
pipe as a predicted-taken branch, and decode1 does not do any static
branch prediction. If fetching is not redirected, then the next
instruction goes down the pipe as normal and decode1 does its static
branch prediction.
In order to make timing, the lookup of the cache is pipelined, so on
each cycle the cache entry for the current NIA + 8 is read. This
means that after a redirect (from decode1 or execute1), only the third
and subsequent sequentially-fetched instructions will be able to be
predicted.
This improves the coremark value on the Arty A7-100 from about 180 to
about 190 (more than 5%).
The BTC is optional. Builds for the Artix 7 35-T part have it off by
default because the extra ~1420 LUTs it takes mean that the design
doesn't fit on the Arty A7-35 board.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
4 years ago
|
|
|
elsif r_int.predicted = '1' then
|
|
|
|
|
v.nia := r_int.predicted_nia;
|
|
|
|
|
v.predicted := '1';
|
|
|
|
|
else
|
|
|
|
|
v.sequential := '1';
|
|
|
|
|
v.nia := std_ulogic_vector(unsigned(r.nia) + 4);
|
|
|
|
|
if r_int.mode_32bit = '1' then
|
|
|
|
|
v.nia(63 downto 32) := x"00000000";
|
|
|
|
|
end if;
|
|
|
|
|
if btc_rd_valid = '1' and r_int.rd_is_niap4 = '1' and
|
|
|
|
|
btc_rd_data(BTC_WIDTH - 1 downto BTC_TARGET_BITS)
|
|
|
|
|
= v.nia(BTC_TAG_BITS + BTC_ADDR_BITS + 1 downto BTC_ADDR_BITS + 2) then
|
|
|
|
|
v_int.predicted := '1';
|
|
|
|
|
end if;
|
|
|
|
|
end if;
|
|
|
|
|
v_int.predicted_nia := btc_rd_data(BTC_TARGET_BITS - 1 downto 0) & "00";
|
|
|
|
|
|
fetch1: Implement a simple branch target cache
This implements a cache in fetch1, where each entry stores the address
of a simple branch instruction (b or bc) and the target of the branch.
When fetching sequentially, if the address being fetched matches the
cache entry, then fetching will be redirected to the branch target.
The cache has 1024 entries and is direct-mapped, i.e. indexed by bits
11..2 of the NIA.
The bus from execute1 now carries information about taken and
not-taken simple branches, which fetch1 uses to update the cache.
The cache entry is updated for both taken and not-taken branches, with
the valid bit being set if the branch was taken and cleared if the
branch was not taken.
If fetching is redirected to the branch target then that goes down the
pipe as a predicted-taken branch, and decode1 does not do any static
branch prediction. If fetching is not redirected, then the next
instruction goes down the pipe as normal and decode1 does its static
branch prediction.
In order to make timing, the lookup of the cache is pipelined, so on
each cycle the cache entry for the current NIA + 8 is read. This
means that after a redirect (from decode1 or execute1), only the third
and subsequent sequentially-fetched instructions will be able to be
predicted.
This improves the coremark value on the Arty A7-100 from about 180 to
about 190 (more than 5%).
The BTC is optional. Builds for the Artix 7 35-T part have it off by
default because the extra ~1420 LUTs it takes mean that the design
doesn't fit on the Arty A7-35 board.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
4 years ago
|
|
|
-- If the last NIA value went down with a stop mark, it didn't get
|
|
|
|
|
-- executed, and hence we shouldn't increment NIA.
|
|
|
|
|
advance_nia <= rst or w_in.redirect or d_in.redirect or (not r.stop_mark and not stall_in);
|
|
|
|
|
|
|
|
|
|
r_next <= v;
|
|
|
|
|
r_next_int <= v_int;
|
|
|
|
|
|
|
|
|
|
-- Update outputs to the icache
|
|
|
|
|
i_out <= r;
|
|
|
|
|
|
|
|
|
|
end process;
|
|
|
|
|
|
|
|
|
|
end architecture behaviour;
|
