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Commit 68232c03 authored by Alban Gruin's avatar Alban Gruin
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fixup! frontend: disable branch prediction, mkII

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src/frontend/frontend.sv
+ 61
− 211
View file @ 68232c03
......@@ -89,20 +89,10 @@ module frontend import ariane_pkg::*; #(
// re-aligned instruction and address (coming from cache - combinationally)
logic [INSTR_PER_FETCH-1:0][31:0] instr;
logic [INSTR_PER_FETCH-1:0][riscv::VLEN-1:0] addr;
logic [INSTR_PER_FETCH-1:0] instruction_valid, instruction_really_valid;
logic [INSTR_PER_FETCH-1:0] instr_really_valid;
// BHT, BTB and RAS prediction
bht_prediction_t [INSTR_PER_FETCH-1:0] bht_prediction;
btb_prediction_t [INSTR_PER_FETCH-1:0] btb_prediction;
bht_prediction_t [INSTR_PER_FETCH-1:0] bht_prediction_shifted;
btb_prediction_t [INSTR_PER_FETCH-1:0] btb_prediction_shifted;
ras_t ras_predict;
logic [INSTR_PER_FETCH-1:0] instruction_valid;
// branch-predict update
logic is_mispredict;
logic ras_push, ras_pop;
logic [riscv::VLEN-1:0] ras_update;
// Instruction FIFO
logic [riscv::VLEN-1:0] predict_address;
......@@ -111,7 +101,7 @@ module frontend import ariane_pkg::*; #(
logic [ariane_pkg::INSTR_PER_FETCH-1:0] taken_rvc_cf;
logic stalling_d, stalling_q;
logic previous_is_branch;
logic bp_valid;
logic serving_unaligned;
// Re-align instructions
......@@ -127,25 +117,8 @@ module frontend import ariane_pkg::*; #(
.addr_o ( addr ),
.instr_o ( instr )
);
// --------------------
// Branch Prediction
// --------------------
// select the right branch prediction result
// in case we are serving an unaligned instruction in instr[0] we need to take
// the prediction we saved from the previous fetch
assign bht_prediction_shifted[0] = (serving_unaligned) ? bht_q : bht_prediction[0];
assign btb_prediction_shifted[0] = (serving_unaligned) ? btb_q : btb_prediction[0];
// for all other predictions we can use the generated address to index
// into the branch prediction data structures
for (genvar i = 1; i < INSTR_PER_FETCH; i++) begin : gen_prediction_address
assign bht_prediction_shifted[i] = bht_prediction[addr[i][$clog2(INSTR_PER_FETCH):1]];
assign btb_prediction_shifted[i] = btb_prediction[addr[i][$clog2(INSTR_PER_FETCH):1]];
end
// for the return address stack it doens't matter as we have the
// address of the call/return already
logic bp_valid;
assign branch_speculation_o = bp_valid;
assign branch_speculation_o = 1'b0;
logic [INSTR_PER_FETCH-1:0] is_branch;
logic [INSTR_PER_FETCH-1:0] is_call;
......@@ -166,121 +139,48 @@ module frontend import ariane_pkg::*; #(
assign is_jalr[i] = instruction_valid[i] & ~is_return[i] & ~is_call[i] & (rvi_jalr[i] | rvc_jalr[i] | rvc_jr[i]);
end
logic should_stall;
assign should_stall = (stalling_q | (|is_branch) | (|is_return) | (|is_jalr)) & (!resolved_branch_i.valid);
// taken/not taken
always_comb begin
stalling_d = stalling_q;
taken_rvi_cf = '0;
taken_rvc_cf = '0;
predict_address = '0;
for (int i = 0; i < INSTR_PER_FETCH; i++) cf_type[i] = ariane_pkg::NoCF;
ras_push = 1'b0;
ras_pop = 1'b0;
ras_update = '0;
// lower most prediction gets precedence
for (int unsigned i = INSTR_PER_FETCH - 1; i >= 0 ; i--) begin
// Direct jumps are always predicted correctly since we have their
// target address.
unique case ({is_branch[i], is_return[i], is_jump[i], is_jalr[i]})
4'b0000:; // regular instruction e.g.: no branch
// unconditional jump to register, we need the BTB to resolve this
4'b0001: begin
ras_pop = 1'b0;
ras_push = 1'b0;
if (btb_prediction_shifted[i].valid & enable_bp_i) begin
predict_address = btb_prediction_shifted[i].target_address;
cf_type[i] = ariane_pkg::JumpR;
end
end
// its an unconditional jump to an immediate
4'b0010: begin
ras_pop = 1'b0;
ras_push = 1'b0;
taken_rvi_cf[i] = rvi_jump[i];
taken_rvc_cf[i] = rvc_jump[i];
cf_type[i] = ariane_pkg::Jump;
end
// return
4'b0100: begin
// make sure to only alter the RAS if we actually consumed the instruction
ras_push = 1'b0;
ras_pop = ras_predict.valid & instr_queue_consumed[i];
if (enable_bp_i) begin
predict_address = ras_predict.ra;
cf_type[i] = ariane_pkg::Return;
end
end
// branch prediction
4'b1000: begin
ras_pop = 1'b0;
ras_push = 1'b0;
if (enable_bp_i) begin
// if we have a valid dynamic prediction use it
if (bht_prediction_shifted[i].valid) begin
taken_rvi_cf[i] = rvi_branch[i] & bht_prediction_shifted[i].taken;
taken_rvc_cf[i] = rvc_branch[i] & bht_prediction_shifted[i].taken;
// otherwise default to static prediction
end else begin
// set if immediate is negative - static prediction
taken_rvi_cf[i] = rvi_branch[i] & rvi_imm[i][riscv::VLEN-1];
taken_rvc_cf[i] = rvc_branch[i] & rvc_imm[i][riscv::VLEN-1];
end
if (taken_rvi_cf[i] || taken_rvc_cf[i]) cf_type[i] = ariane_pkg::Branch;
end
end
default:;
// default: $error("Decoded more than one control flow");
endcase
// if this instruction, in addition, is a call, save the resulting address
// but only if we actually consumed the address
if (is_call[i]) begin
ras_push = instr_queue_consumed[i];
ras_update = addr[i] + (rvc_call[i] ? 2 : 4);
end
// calculate the jump target address
if (taken_rvc_cf[i] || taken_rvi_cf[i]) begin
if (rvi_jump[i] || rvc_jump[i] || enable_bp_i) begin
predict_address = addr[i] + (taken_rvc_cf[i] ? rvc_imm[i] : rvi_imm[i]);
end
end
end
end // always_comb
// or reduce struct
always_comb begin
bp_valid = 1'b0;
instruction_really_valid = '0;
previous_is_branch = 1'b0;
stalling_d = stalling_q;
if (!stalling_q) begin
// BP cannot be valid if we have a return instruction and the RAS is not giving a valid address
// Check that we encountered a control flow and that for a return the RAS
// contains a valid prediction.
for (int unsigned i = 0; i < INSTR_PER_FETCH; i++) begin
bp_valid |= ((cf_type[i] != NoCF & cf_type[i] != Return) | ((cf_type[i] == Return) & ras_predict.valid));
if (enable_bp_i) begin
instruction_really_valid[i] = instruction_valid[i];
end else begin
instruction_really_valid[i] = instruction_valid[i] && !(previous_is_branch);
end
if (!should_stall) begin
stalling_d = 1'b0;
previous_is_branch |= !(cf_type[i] inside {NoCF, Jump});
for (int unsigned i = 0; i < INSTR_PER_FETCH; i++) begin
unique case ({is_branch[i] | is_return[i] | is_jalr[i], is_jump[i]})
2'b00:;
2'b01: begin
if (!(|taken_rvi_cf) && !(|taken_rvc_cf))
predict_address = addr[i] + (taken_rvc_cf[i] ? rvc_imm[i] : rvi_imm[i]);
taken_rvi_cf[i] = rvi_jump[i];
taken_rvc_cf[i] = rvc_jump[i];
cf_type[i] = ariane_pkg::Jump;
bp_valid = 1'b1;
end
2'b10: begin
taken_rvi_cf[i] = 1'b1;
taken_rvc_cf[i] = 1'b1;
bp_valid = 1'b1;
stalling_d = 1'b1;
end
default:;
endcase
end
stalling_d |= previous_is_branch;
end else if (resolved_branch_i.valid) begin
stalling_d = 1'b0;
end
end
assign is_mispredict = resolved_branch_i.valid & resolved_branch_i.is_mispredict;
// Cache interface
assign icache_dreq_o.req = instr_queue_ready;
assign icache_dreq_o.req = instr_queue_ready & (!should_stall);
assign if_ready = icache_dreq_i.ready & instr_queue_ready;
// We need to flush the cache pipeline if:
// 1. We mispredicted
......@@ -291,21 +191,6 @@ module frontend import ariane_pkg::*; #(
// also if we killed the first stage we also need to kill the second stage (inclusive flush)
assign icache_dreq_o.kill_s2 = icache_dreq_o.kill_s1 | bp_valid;
// Update Control Flow Predictions
bht_update_t bht_update;
btb_update_t btb_update;
assign bht_update.valid = resolved_branch_i.valid
& (resolved_branch_i.cf_type == ariane_pkg::Branch);
assign bht_update.pc = resolved_branch_i.pc;
assign bht_update.taken = resolved_branch_i.is_taken;
// only update mispredicted branches e.g. no returns from the RAS
assign btb_update.valid = resolved_branch_i.valid
& resolved_branch_i.is_mispredict
& (resolved_branch_i.cf_type == ariane_pkg::JumpR);
assign btb_update.pc = resolved_branch_i.pc;
assign btb_update.target_address = resolved_branch_i.target_address;
// -------------------
// Next PC
// -------------------
......@@ -334,33 +219,36 @@ module frontend import ariane_pkg::*; #(
// keep stable by default
npc_d = npc_q;
end
// 0. Branch Prediction
if (bp_valid) begin
fetch_address = predict_address;
npc_d = predict_address;
if (!should_stall) begin
// 0. Branch Prediction
if (bp_valid) begin
fetch_address = predict_address;
npc_d = predict_address;
end
// 1. Default assignment
if (if_ready) npc_d = {fetch_address[riscv::VLEN-1:2], 2'b0} + 'h4;
// 2. Replay instruction fetch
if (replay) npc_d = replay_addr;
// 3. Control flow change request
if (is_mispredict) npc_d = resolved_branch_i.target_address;
// 4. Return from environment call
if (eret_i) npc_d = epc_i;
// 5. Exception/Interrupt
if (ex_valid_i) npc_d = trap_vector_base_i;
// 6. Pipeline Flush because of CSR side effects
// On a pipeline flush start fetching from the next address
// of the instruction in the commit stage
// we came here from a flush request of a CSR instruction or AMO,
// as CSR or AMO instructions do not exist in a compressed form
// we can unconditionally do PC + 4 here
// TODO(zarubaf) This adder can at least be merged with the one in the csr_regfile stage
if (set_pc_commit_i) npc_d = pc_commit_i + {{riscv::VLEN-3{1'b0}}, 3'b100};
// 7. Debug
// enter debug on a hard-coded base-address
if (set_debug_pc_i) npc_d = ArianeCfg.DmBaseAddress[riscv::VLEN-1:0] + dm::HaltAddress[riscv::VLEN-1:0];
icache_dreq_o.vaddr = fetch_address;
end
// 1. Default assignment
if (if_ready) npc_d = {fetch_address[riscv::VLEN-1:2], 2'b0} + 'h4;
// 2. Replay instruction fetch
if (replay) npc_d = replay_addr;
// 3. Control flow change request
if (is_mispredict) npc_d = resolved_branch_i.target_address;
// 4. Return from environment call
if (eret_i) npc_d = epc_i;
// 5. Exception/Interrupt
if (ex_valid_i) npc_d = trap_vector_base_i;
// 6. Pipeline Flush because of CSR side effects
// On a pipeline flush start fetching from the next address
// of the instruction in the commit stage
// we came here from a flush request of a CSR instruction or AMO,
// as CSR or AMO instructions do not exist in a compressed form
// we can unconditionally do PC + 4 here
// TODO(zarubaf) This adder can at least be merged with the one in the csr_regfile stage
if (set_pc_commit_i) npc_d = pc_commit_i + {{riscv::VLEN-3{1'b0}}, 3'b100};
// 7. Debug
// enter debug on a hard-coded base-address
if (set_debug_pc_i) npc_d = ArianeCfg.DmBaseAddress[riscv::VLEN-1:0] + dm::HaltAddress[riscv::VLEN-1:0];
icache_dreq_o.vaddr = fetch_address;
end
logic [FETCH_WIDTH-1:0] icache_data;
......@@ -392,49 +280,11 @@ module frontend import ariane_pkg::*; #(
icache_ex_valid_q <= ariane_pkg::FE_INSTR_ACCESS_FAULT;
end else icache_ex_valid_q <= ariane_pkg::FE_NONE;
// save the uppermost prediction
btb_q <= btb_prediction[INSTR_PER_FETCH-1];
bht_q <= bht_prediction[INSTR_PER_FETCH-1];
stalling_q <= stalling_d;
end
stalling_q <= stalling_d;
end
end
ras #(
.DEPTH ( ArianeCfg.RASDepth )
) i_ras (
.clk_i,
.rst_ni,
.flush_i( flush_bp_i ),
.push_i ( ras_push ),
.pop_i ( ras_pop ),
.data_i ( ras_update ),
.data_o ( ras_predict )
);
btb #(
.NR_ENTRIES ( ArianeCfg.BTBEntries )
) i_btb (
.clk_i,
.rst_ni,
.flush_i ( flush_bp_i ),
.debug_mode_i,
.vpc_i ( icache_vaddr_q ),
.btb_update_i ( btb_update ),
.btb_prediction_o ( btb_prediction )
);
bht #(
.NR_ENTRIES ( ArianeCfg.BHTEntries )
) i_bht (
.clk_i,
.rst_ni,
.flush_i ( flush_bp_i ),
.debug_mode_i,
.vpc_i ( icache_vaddr_q ),
.bht_update_i ( bht_update ),
.bht_prediction_o ( bht_prediction )
);
// we need to inspect up to INSTR_PER_FETCH instructions for branches
// and jumps
for (genvar i = 0; i < INSTR_PER_FETCH; i++) begin : gen_instr_scan
......@@ -466,7 +316,7 @@ module frontend import ariane_pkg::*; #(
.exception_addr_i ( icache_vaddr_q ),
.predict_address_i ( predict_address ),
.cf_type_i ( cf_type ),
.valid_i ( instruction_really_valid ), // from re-aligner
.valid_i ( instruction_valid ), // from re-aligner
.consumed_o ( instr_queue_consumed ),
.ready_o ( instr_queue_ready ),
.replay_o ( replay ),
......
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