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[lockstep] Introduce optimization barrier around lockstep Ibex

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Certain synthesis tools like DC are very smart at optimizing away redundant logic.
Hence, we have to insert an optimization barrier at the IOs of the lockstep Ibex.
This is achieved by manually buffering each bit using prim_buf.
Our Xilinx and DC synthesis flows make sure that these buffers cannot be optimized
away using keep attributes (Vivado) and size_only constraints (DC).

Signed-off-by: Michael Schaffner <msf@google.com>
This commit is contained in:
Michael Schaffner 2021-04-15 18:23:38 -07:00 • committed by Tom Roberts
parent 1b59c67b50
commit 0e7117fbec
2 changed files with 278 additions and 46 deletions
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1
ibex_top.core
View file

@ -10,6 +10,7 @@ filesets:
depend: depend:
- lowrisc:ibex:ibex_pkg - lowrisc:ibex:ibex_pkg
- lowrisc:ibex:ibex_core - lowrisc:ibex:ibex_core
- lowrisc:prim:buf
files: files:
- rtl/ibex_register_file_ff.sv # generic FF-based - rtl/ibex_register_file_ff.sv # generic FF-based
- rtl/ibex_register_file_fpga.sv # FPGA - rtl/ibex_register_file_fpga.sv # FPGA

323
rtl/ibex_top.sv
View file

@ -416,6 +416,225 @@ module ibex_top #(
// Redundant lockstep core implementation // Redundant lockstep core implementation
if (Lockstep) begin : gen_lockstep if (Lockstep) begin : gen_lockstep
// Note: certain synthesis tools like DC are very smart at optimizing away redundant logic.
// Hence, we have to insert an optimization barrier at the IOs of the lockstep Ibex.
// This is achieved by manually buffering each bit using prim_buf.
// Our Xilinx and DC synthesis flows make sure that these buffers cannot be optimized away
// using keep attributes (Vivado) and size_only constraints (DC).
localparam int NumBufferBits = $bits({
hart_id_i,
boot_addr_i,
instr_req_o,
instr_gnt_i,
instr_rvalid_i,
instr_addr_o,
instr_rdata_i,
instr_err_i,
data_req_o,
data_gnt_i,
data_rvalid_i,
data_we_o,
data_be_o,
data_addr_o,
data_wdata_o,
data_rdata_i,
data_err_i,
dummy_instr_id,
rf_raddr_a,
rf_raddr_b,
rf_waddr_wb,
rf_we_wb,
rf_wdata_wb_ecc,
rf_rdata_a_ecc,
rf_rdata_b_ecc,
ic_tag_req,
ic_tag_write,
ic_tag_addr,
ic_tag_wdata,
ic_data_req,
ic_data_write,
ic_data_addr,
ic_data_wdata,
irq_software_i,
irq_timer_i,
irq_external_i,
irq_fast_i,
irq_nm_i,
irq_pending,
debug_req_i,
crash_dump_o,
core_busy_d
});
logic [NumBufferBits-1:0] buf_in, buf_out;
logic [31:0] hart_id_local;
logic [31:0] boot_addr_local;
logic instr_req_local;
logic instr_gnt_local;
logic instr_rvalid_local;
logic [31:0] instr_addr_local;
logic [31:0] instr_rdata_local;
logic instr_err_local;
logic data_req_local;
logic data_gnt_local;
logic data_rvalid_local;
logic data_we_local;
logic [3:0] data_be_local;
logic [31:0] data_addr_local;
logic [31:0] data_wdata_local;
logic [31:0] data_rdata_local;
logic data_err_local;
logic dummy_instr_id_local;
logic [4:0] rf_raddr_a_local;
logic [4:0] rf_raddr_b_local;
logic [4:0] rf_waddr_wb_local;
logic rf_we_wb_local;
logic [RegFileDataWidth-1:0] rf_wdata_wb_ecc_local;
logic [RegFileDataWidth-1:0] rf_rdata_a_ecc_local;
logic [RegFileDataWidth-1:0] rf_rdata_b_ecc_local;
logic [IC_NUM_WAYS-1:0] ic_tag_req_local;
logic ic_tag_write_local;
logic [IC_INDEX_W-1:0] ic_tag_addr_local;
logic [TagSizeECC-1:0] ic_tag_wdata_local;
logic [IC_NUM_WAYS-1:0] ic_data_req_local;
logic ic_data_write_local;
logic [IC_INDEX_W-1:0] ic_data_addr_local;
logic [LineSizeECC-1:0] ic_data_wdata_local;
logic irq_software_local;
logic irq_timer_local;
logic irq_external_local;
logic [14:0] irq_fast_local;
logic irq_nm_local;
logic irq_pending_local;
logic debug_req_local;
crash_dump_t crash_dump_local;
logic core_busy_local;
assign buf_in = {
hart_id_i,
boot_addr_i,
instr_req_o,
instr_gnt_i,
instr_rvalid_i,
instr_addr_o,
instr_rdata_i,
instr_err_i,
data_req_o,
data_gnt_i,
data_rvalid_i,
data_we_o,
data_be_o,
data_addr_o,
data_wdata_o,
data_rdata_i,
data_err_i,
dummy_instr_id,
rf_raddr_a,
rf_raddr_b,
rf_waddr_wb,
rf_we_wb,
rf_wdata_wb_ecc,
rf_rdata_a_ecc,
rf_rdata_b_ecc,
ic_tag_req,
ic_tag_write,
ic_tag_addr,
ic_tag_wdata,
ic_data_req,
ic_data_write,
ic_data_addr,
ic_data_wdata,
irq_software_i,
irq_timer_i,
irq_external_i,
irq_fast_i,
irq_nm_i,
irq_pending,
debug_req_i,
crash_dump_o,
core_busy_d
};
assign {
hart_id_local,
boot_addr_local,
instr_req_local,
instr_gnt_local,
instr_rvalid_local,
instr_addr_local,
instr_rdata_local,
instr_err_local,
data_req_local,
data_gnt_local,
data_rvalid_local,
data_we_local,
data_be_local,
data_addr_local,
data_wdata_local,
data_rdata_local,
data_err_local,
dummy_instr_id_local,
rf_raddr_a_local,
rf_raddr_b_local,
rf_waddr_wb_local,
rf_we_wb_local,
rf_wdata_wb_ecc_local,
rf_rdata_a_ecc_local,
rf_rdata_b_ecc_local,
ic_tag_req_local,
ic_tag_write_local,
ic_tag_addr_local,
ic_tag_wdata_local,
ic_data_req_local,
ic_data_write_local,
ic_data_addr_local,
ic_data_wdata_local,
irq_software_local,
irq_timer_local,
irq_external_local,
irq_fast_local,
irq_nm_local,
irq_pending_local,
debug_req_local,
crash_dump_local,
core_busy_local
} = buf_out;
// Manually buffer all input signals.
for (genvar k = 0; k < NumBufferBits; k++) begin : gen_buffers
prim_buf u_prim_buf (
.in_i(buf_in[k]),
.out_o(buf_out[k])
);
end
logic [TagSizeECC-1:0] ic_tag_rdata_local [IC_NUM_WAYS];
logic [LineSizeECC-1:0] ic_data_rdata_local [IC_NUM_WAYS];
for (genvar k = 0; k < IC_NUM_WAYS; k++) begin : gen_ways
for (genvar j = 0; j < TagSizeECC; j++) begin : gen_tag_bufs
prim_buf u_prim_buf (
.in_i(ic_tag_rdata[k][j]),
.out_o(ic_tag_rdata_local[k][j])
);
end
for (genvar j = 0; j < TagSizeECC; j++) begin : gen_data_bufs
prim_buf u_prim_buf (
.in_i(ic_data_rdata[k][j]),
.out_o(ic_data_rdata_local[k][j])
);
end
end
logic lockstep_alert_minor_local, lockstep_alert_major_local;
ibex_lockstep #( ibex_lockstep #(
.PMPEnable ( PMPEnable ), .PMPEnable ( PMPEnable ),
.PMPGranularity ( PMPGranularity ), .PMPGranularity ( PMPGranularity ),
@ -445,60 +664,72 @@ module ibex_top #(
.clk_i (clk), .clk_i (clk),
.rst_ni (rst_ni), .rst_ni (rst_ni),
.hart_id_i (hart_id_i), .hart_id_i (hart_id_local),
.boot_addr_i (boot_addr_i), .boot_addr_i (boot_addr_local),
.instr_req_i (instr_req_o), .instr_req_i (instr_req_local),
.instr_gnt_i (instr_gnt_i), .instr_gnt_i (instr_gnt_local),
.instr_rvalid_i (instr_rvalid_i), .instr_rvalid_i (instr_rvalid_local),
.instr_addr_i (instr_addr_o), .instr_addr_i (instr_addr_local),
.instr_rdata_i (instr_rdata_i), .instr_rdata_i (instr_rdata_local),
.instr_err_i (instr_err_i), .instr_err_i (instr_err_local),
.data_req_i (data_req_o), .data_req_i (data_req_local),
.data_gnt_i (data_gnt_i), .data_gnt_i (data_gnt_local),
.data_rvalid_i (data_rvalid_i), .data_rvalid_i (data_rvalid_local),
.data_we_i (data_we_o), .data_we_i (data_we_local),
.data_be_i (data_be_o), .data_be_i (data_be_local),
.data_addr_i (data_addr_o), .data_addr_i (data_addr_local),
.data_wdata_i (data_wdata_o), .data_wdata_i (data_wdata_local),
.data_rdata_i (data_rdata_i), .data_rdata_i (data_rdata_local),
.data_err_i (data_err_i), .data_err_i (data_err_local),
.dummy_instr_id_i (dummy_instr_id), .dummy_instr_id_i (dummy_instr_id_local),
.rf_raddr_a_i (rf_raddr_a), .rf_raddr_a_i (rf_raddr_a_local),
.rf_raddr_b_i (rf_raddr_b), .rf_raddr_b_i (rf_raddr_b_local),
.rf_waddr_wb_i (rf_waddr_wb), .rf_waddr_wb_i (rf_waddr_wb_local),
.rf_we_wb_i (rf_we_wb), .rf_we_wb_i (rf_we_wb_local),
.rf_wdata_wb_ecc_i (rf_wdata_wb_ecc), .rf_wdata_wb_ecc_i (rf_wdata_wb_ecc_local),
.rf_rdata_a_ecc_i (rf_rdata_a_ecc), .rf_rdata_a_ecc_i (rf_rdata_a_ecc_local),
.rf_rdata_b_ecc_i (rf_rdata_b_ecc), .rf_rdata_b_ecc_i (rf_rdata_b_ecc_local),
.ic_tag_req_i (ic_tag_req), .ic_tag_req_i (ic_tag_req_local),
.ic_tag_write_i (ic_tag_write), .ic_tag_write_i (ic_tag_write_local),
.ic_tag_addr_i (ic_tag_addr), .ic_tag_addr_i (ic_tag_addr_local),
.ic_tag_wdata_i (ic_tag_wdata), .ic_tag_wdata_i (ic_tag_wdata_local),
.ic_tag_rdata_i (ic_tag_rdata), .ic_tag_rdata_i (ic_tag_rdata_local),
.ic_data_req_i (ic_data_req), .ic_data_req_i (ic_data_req_local),
.ic_data_write_i (ic_data_write), .ic_data_write_i (ic_data_write_local),
.ic_data_addr_i (ic_data_addr), .ic_data_addr_i (ic_data_addr_local),
.ic_data_wdata_i (ic_data_wdata), .ic_data_wdata_i (ic_data_wdata_local),
.ic_data_rdata_i (ic_data_rdata), .ic_data_rdata_i (ic_data_rdata_local),
.irq_software_i (irq_software_i), .irq_software_i (irq_software_local),
.irq_timer_i (irq_timer_i), .irq_timer_i (irq_timer_local),
.irq_external_i (irq_external_i), .irq_external_i (irq_external_local),
.irq_fast_i (irq_fast_i), .irq_fast_i (irq_fast_local),
.irq_nm_i (irq_nm_i), .irq_nm_i (irq_nm_local),
.irq_pending_i (irq_pending), .irq_pending_i (irq_pending_local),
.debug_req_i (debug_req_i), .debug_req_i (debug_req_local),
.crash_dump_i (crash_dump_o), .crash_dump_i (crash_dump_local),
.alert_minor_o (lockstep_alert_minor), .alert_minor_o (lockstep_alert_minor_local),
.alert_major_o (lockstep_alert_major), .alert_major_o (lockstep_alert_major_local),
.core_busy_i (core_busy_d) .core_busy_i (core_busy_local)
); );
// Manually buffer the output signals.
prim_buf u_prim_buf_alert_minor (
.in_i(lockstep_alert_minor_local),
.out_o(lockstep_alert_minor)
);
prim_buf u_prim_buf_alert_major (
.in_i(lockstep_alert_major_local),
.out_o(lockstep_alert_major)
);
end else begin : gen_no_lockstep end else begin : gen_no_lockstep
assign lockstep_alert_major = 1'b0; assign lockstep_alert_major = 1'b0;
assign lockstep_alert_minor = 1'b0; assign lockstep_alert_minor = 1'b0;