From 68232c03f0d0ffb39b40aa90cbf8d824b668a3a0 Mon Sep 17 00:00:00 2001
From: Alban Gruin <alban.gruin@irit.fr>
Date: Fri, 9 Apr 2021 16:16:17 +0200
Subject: [PATCH] fixup! frontend: disable branch prediction, mkII
---
src/frontend/frontend.sv | 272 +++++++++------------------------------
1 file changed, 61 insertions(+), 211 deletions(-)
diff --git a/src/frontend/frontend.sv b/src/frontend/frontend.sv
index 819a885f..b9104b92 100644
--- a/src/frontend/frontend.sv
+++ b/src/frontend/frontend.sv
@@ -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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