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932db14619
* rename files and modules to cve2 Signed-off-by: Szymon Bieganski <szymon.bieganski@oss.nxp.com> * updated tb files Signed-off-by: Szymon Bieganski <szymon.bieganski@oss.nxp.com> * remaining references to ibex: gitignore, examples, etc. Signed-off-by: Szymon Bieganski <szymon.bieganski@oss.nxp.com> Signed-off-by: Szymon Bieganski <szymon.bieganski@oss.nxp.com> Co-authored-by: Szymon Bieganski <szymon.bieganski@oss.nxp.com>
100 lines
3.3 KiB
Systemverilog
100 lines
3.3 KiB
Systemverilog
// Copyright lowRISC contributors.
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// Licensed under the Apache License, Version 2.0, see LICENSE for details.
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// SPDX-License-Identifier: Apache-2.0
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/**
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* Branch Predictor
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*
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* This implements static branch prediction. It takes an instruction and its PC and determines if
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* it's a branch or a jump and calculates its target. For jumps it will always predict taken. For
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* branches it will predict taken if the PC offset is negative.
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*
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* This handles both compressed and uncompressed instructions. Compressed instructions must be in
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* the lower 16-bits of instr.
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*
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* The predictor is entirely combinational but takes clk/rst_n signals for use by assertions.
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*/
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`include "prim_assert.sv"
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module cve2_branch_predict (
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input logic clk_i,
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input logic rst_ni,
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// Instruction from fetch stage
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input logic [31:0] fetch_rdata_i,
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input logic [31:0] fetch_pc_i,
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input logic fetch_valid_i,
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// Prediction for supplied instruction
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output logic predict_branch_taken_o,
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output logic [31:0] predict_branch_pc_o
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);
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import cve2_pkg::*;
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logic [31:0] imm_j_type;
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logic [31:0] imm_b_type;
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logic [31:0] imm_cj_type;
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logic [31:0] imm_cb_type;
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logic [31:0] branch_imm;
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logic [31:0] instr;
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logic instr_j;
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logic instr_b;
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logic instr_cj;
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logic instr_cb;
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logic instr_b_taken;
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// Provide short internal name for fetch_rdata_i due to reduce line wrapping
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assign instr = fetch_rdata_i;
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// Extract and sign-extend to 32-bit the various immediates that may be used to calculate the
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// target
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// Uncompressed immediates
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assign imm_j_type = { {12{instr[31]}}, instr[19:12], instr[20], instr[30:21], 1'b0 };
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assign imm_b_type = { {19{instr[31]}}, instr[31], instr[7], instr[30:25], instr[11:8], 1'b0 };
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// Compressed immediates
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assign imm_cj_type = { {20{instr[12]}}, instr[12], instr[8], instr[10:9], instr[6], instr[7],
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instr[2], instr[11], instr[5:3], 1'b0 };
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assign imm_cb_type = { {23{instr[12]}}, instr[12], instr[6:5], instr[2], instr[11:10],
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instr[4:3], 1'b0};
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// Determine if the instruction is a branch or a jump
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// Uncompressed branch/jump
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assign instr_b = opcode_e'(instr[6:0]) == OPCODE_BRANCH;
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assign instr_j = opcode_e'(instr[6:0]) == OPCODE_JAL;
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// Compressed branch/jump
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assign instr_cb = (instr[1:0] == 2'b01) & ((instr[15:13] == 3'b110) | (instr[15:13] == 3'b111));
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assign instr_cj = (instr[1:0] == 2'b01) & ((instr[15:13] == 3'b101) | (instr[15:13] == 3'b001));
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// Select out the branch offset for target calculation based upon the instruction type
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always_comb begin
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branch_imm = imm_b_type;
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unique case (1'b1)
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instr_j : branch_imm = imm_j_type;
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instr_b : branch_imm = imm_b_type;
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instr_cj : branch_imm = imm_cj_type;
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instr_cb : branch_imm = imm_cb_type;
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default : ;
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endcase
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end
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`ASSERT_IF(BranchInsTypeOneHot, $onehot0({instr_j, instr_b, instr_cj, instr_cb}), fetch_valid_i)
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// Determine branch prediction, taken if offset is negative
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assign instr_b_taken = (instr_b & imm_b_type[31]) | (instr_cb & imm_cb_type[31]);
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// Always predict jumps taken otherwise take prediction from `instr_b_taken`
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assign predict_branch_taken_o = fetch_valid_i & (instr_j | instr_cj | instr_b_taken);
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// Calculate target
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assign predict_branch_pc_o = fetch_pc_i + branch_imm;
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endmodule
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