E203数据冲突处理OITF

皋陶 · 发表于 2021-3-6 16:02:39

有人预言,RISC-V或将是继Intel和Arm之后的第三大主流处理器体系。欢迎访问全球首家只专注于RISC-V单片机行业应用的中文网站

您需要登录才可以下载或查看，没有帐号？立即注册

x

本帖最后由皋陶于 2021-3-6 16:04 编辑

流水线的数据冲突分为三类：WAR，RAW，WAW

参考链接：3.3 流水线相关冲突及解决办法_v3.0 https://wenku.baidu.com/view/e066926d48d7c1c708a14508.html

WAR: write after read 相关性，又称先读后写相关性。比如下面的指令序列，第一条指令会读取x4，第二条指令会写x4。在流水线中，如果第二条指令比第一条指令先写x4，则第一条指令就会读出错误的值。

add x5, x4,x6

add x4, x3, x2

WAW: write after write 相关性，又称先写后写相关性。比如下面的指令序列，两条指令都会写x5。在流水线中，如果第二条指令比第一条指令先写x5，就会引起逻辑错误。

add x5, x4,x6

add x5, x3, x2

RAW:read after write相关性，又称先写后读相关性。比如下面指令序列，如果第二条指令，在第一条指令写x5之前，第二条指令先读x4，就会引起逻辑错误。

add x5, x4,x6

add x4, x5, x2

由于蜂鸟E200系列是按序派遣，按顺序写回的微架构，在指令派遣时候就已经从通用寄存器数组中读取了源操作数。后续执行的指令写回regfile的操作不可能影响到前面指令的读取，所以不可能发生WAR相关性造成的数据冲突。

正在派遣的指令处在流水线的第二级，假设之前派遣的指令是单周期指令，则前序指令肯定已经完成了执行且将结果写回了Regfile。因此正在派遣的指令不可能会发生RAW数据冲突。但是假设之前派遣的指令是多周期指令(长指令)，由于指令需要多个周期才能写回结果。因此正在派遣的指令可能会产生前序相关的RAW相关性。

正在派遣的指令处在流水线的第二级，假设之前派遣的指令是单周期指令，则前序指令肯定已经完成了执行且将结果写回了Regfile。因此正在派遣的指令不可能会发生WAW数据冲突。但是假设之前派遣的指令是多周期指令(长指令)，由于指令需要多个周期才能写回结果。因此正在派遣的指令可能会产生前序相关的WAW相关性。

为了能检测出长指令的RAW和WAW相关性，蜂鸟E200使用了一个outstanding instruction track fifo(OITF)模块。在流水线的派遣(Dispatch)点，每一次派遣一个长指令，则会在OITF中分配一个表项(Entry)，在这个表项中会存储该长指令的结果寄存器索引。在流水线的写回(Write-back)点，每次按顺序写回一个长指令之后，就会将此指令在OITF中的表项移除。

每条指令派遣时，都会将本指令的源操作数和目的操作数寄存器索引和OITF中的各个表项进行比对，从而判断本指令是否与已经被派遣出，且尚未写回的长指令产生RAW和WAW相关性。如果产生相关性，则stall住当前指令的派遣。如果没有RAW和WAW相关性，且该指令为多周期长指令，把该指令写入OITF，如果OITF是full，则仍要stall住管线，等待OITF释放空间后，再写入并派遣。

在writeback模块，会进行长指令写回仲裁，长指令写回regfile后，会释放OITF中相应的表项。

OITF代码如下，如果fifo full，则dis_ready=0, 与dispatch模块握手失败，不会发送新的dispatch进来。如果不为空，会发送新的指令进来进行判断。

`include "e203_defines.v"
module e203_exu_oitf (
output dis_ready,
input dis_ena, //dispatch a long instruction enable signal
input ret_ena, //write back a long instruction enable signal
output [`E203_ITAG_WIDTH-1:0] dis_ptr, //write pointer
output [`E203_ITAG_WIDTH-1:0] ret_ptr, //read pointer
output [`E203_RFIDX_WIDTH-1:0] ret_rdidx,
output ret_rdwen,
output ret_rdfpu,
output [`E203_PC_SIZE-1:0] ret_pc,
input disp_i_rs1en, // enable if current dispatch instruction fetch first source operand
input disp_i_rs2en, // ...
input disp_i_rs3en, // ...
input disp_i_rdwen, // enable if current dispatch instruction write back to register
input disp_i_rs1fpu, // enable if current dispath instruction need to read float gpr
input disp_i_rs2fpu, //...
input disp_i_rs3fpu, //...
input disp_i_rdfpu, //enable if current dipatch instruction need to write back to float register files.
//register index
input [`E203_RFIDX_WIDTH-1:0] disp_i_rs1idx,
input [`E203_RFIDX_WIDTH-1:0] disp_i_rs2idx,
input [`E203_RFIDX_WIDTH-1:0] disp_i_rs3idx,
input [`E203_RFIDX_WIDTH-1:0] disp_i_rdidx,
input [`E203_PC_SIZE -1:0] disp_i_pc, //pc of current dispatch instruciotn
output oitfrd_match_disprs1, //dispatch instruction rs1 is same as any item of result register in oitf
output oitfrd_match_disprs2, //...
output oitfrd_match_disprs3, //...
output oitfrd_match_disprd, //dispatch instruction rd is same as any item of result register in oitf.
//if empty, no conflict
output oitf_empty,
input clk,
input rst_n
);
wire [`E203_OITF_DEPTH-1:0] vld_set;
wire [`E203_OITF_DEPTH-1:0] vld_clr;
wire [`E203_OITF_DEPTH-1:0] vld_ena;
wire [`E203_OITF_DEPTH-1:0] vld_nxt;
wire [`E203_OITF_DEPTH-1:0] vld_r; //if it is valid signal in all item
wire [`E203_OITF_DEPTH-1:0] rdwen_r;// if it is write back register in all item
wire [`E203_OITF_DEPTH-1:0] rdfpu_r; //result register in all item if are float
wire [`E203_RFIDX_WIDTH-1:0] rdidx_r[`E203_OITF_DEPTH-1:0]; //register index in all items
// The PC here is to be used at wback stage to track out the
// PC of exception of long-pipe instruction
wire [`E203_PC_SIZE-1:0] pc_r[`E203_OITF_DEPTH-1:0];
wire alc_ptr_ena = dis_ena; //dispatch a long instruction enable signal, as write pointer enable signal
wire ret_ptr_ena = ret_ena; //write back a long instruction enable signal, as read pointer enable signal
wire oitf_full ;
wire [`E203_ITAG_WIDTH-1:0] alc_ptr_r; //write pointer, long instruction dispatch
wire [`E203_ITAG_WIDTH-1:0] ret_ptr_r; //read pointer, long instruction write back
generate
if(`E203_OITF_DEPTH > 1) begin: depth_gt1//{
//extra mark bit for write full
wire alc_ptr_flg_r;
wire alc_ptr_flg_nxt = ~alc_ptr_flg_r;
wire alc_ptr_flg_ena = (alc_ptr_r == ($unsigned(`E203_OITF_DEPTH-1))) & alc_ptr_ena;
sirv_gnrl_dfflr #(1) alc_ptr_flg_dfflrs(alc_ptr_flg_ena, alc_ptr_flg_nxt, alc_ptr_flg_r, clk, rst_n);
wire [`E203_ITAG_WIDTH-1:0] alc_ptr_nxt;
//if write to fifo depth, write ptr = 0,otherwise write ptr = write ptr + 1
assign alc_ptr_nxt = alc_ptr_flg_ena ? `E203_ITAG_WIDTH'b0 : (alc_ptr_r + 1'b1);
sirv_gnrl_dfflr #(`E203_ITAG_WIDTH) alc_ptr_dfflrs(alc_ptr_ena, alc_ptr_nxt, alc_ptr_r, clk, rst_n);
//extra mark bit for read empty
wire ret_ptr_flg_r;
wire ret_ptr_flg_nxt = ~ret_ptr_flg_r;
wire ret_ptr_flg_ena = (ret_ptr_r == ($unsigned(`E203_OITF_DEPTH-1))) & ret_ptr_ena;
sirv_gnrl_dfflr #(1) ret_ptr_flg_dfflrs(ret_ptr_flg_ena, ret_ptr_flg_nxt, ret_ptr_flg_r, clk, rst_n);
wire [`E203_ITAG_WIDTH-1:0] ret_ptr_nxt;
//if read to fifo depth, read ptr = 0, otherwise read prt = read prt + 1
assign ret_ptr_nxt = ret_ptr_flg_ena ? `E203_ITAG_WIDTH'b0 : (ret_ptr_r + 1'b1);
sirv_gnrl_dfflr #(`E203_ITAG_WIDTH) ret_ptr_dfflrs(ret_ptr_ena, ret_ptr_nxt, ret_ptr_r, clk, rst_n);
//empty, full mark
assign oitf_empty = (ret_ptr_r == alc_ptr_r) & (ret_ptr_flg_r == alc_ptr_flg_r);
assign oitf_full = (ret_ptr_r == alc_ptr_r) & (~(ret_ptr_flg_r == alc_ptr_flg_r));
end//}
else begin: depth_eq1//}{
assign alc_ptr_r =1'b0;
assign ret_ptr_r =1'b0;
assign oitf_empty = ~vld_r[0];
assign oitf_full = vld_r[0];
end//}
endgenerate//}
assign ret_ptr = ret_ptr_r;
assign dis_ptr = alc_ptr_r;
////
//// // If the OITF is not full, or it is under retiring, then it is ready to accept new dispatch
//// assign dis_ready = (~oitf_full) | ret_ena;
// To cut down the loop between ALU write-back valid --> oitf_ret_ena --> oitf_ready ---> dispatch_ready --- > alu_i_valid
// we exclude the ret_ena from the ready signal
assign dis_ready = (~oitf_full);
wire [`E203_OITF_DEPTH-1:0] rd_match_rs1idx;
wire [`E203_OITF_DEPTH-1:0] rd_match_rs2idx;
wire [`E203_OITF_DEPTH-1:0] rd_match_rs3idx;
wire [`E203_OITF_DEPTH-1:0] rd_match_rdidx;
genvar i;
generate //{
for (i=0; i<`E203_OITF_DEPTH; i=i+1) begin:oitf_entries//{
//every time, assign a item and write pointer same as current i, then
//valid set is high
assign vld_set[i] = alc_ptr_ena & (alc_ptr_r == i);
//every time, assign a item and read pointer same as current i, then
//valid clr is high
assign vld_clr[i] = ret_ptr_ena & (ret_ptr_r == i);
assign vld_ena[i] = vld_set[i] | vld_clr[i];
assign vld_nxt[i] = vld_set[i] | (~vld_clr[i]);
sirv_gnrl_dfflr #(1) vld_dfflrs(vld_ena[i], vld_nxt[i], vld_r[i], clk, rst_n);
//Payload only set, no need to clear
sirv_gnrl_dffl #(`E203_RFIDX_WIDTH) rdidx_dfflrs(vld_set[i], disp_i_rdidx, rdidx_r[i], clk);
sirv_gnrl_dffl #(`E203_PC_SIZE ) pc_dfflrs (vld_set[i], disp_i_pc , pc_r[i] , clk);
sirv_gnrl_dffl #(1) rdwen_dfflrs(vld_set[i], disp_i_rdwen, rdwen_r[i], clk);
sirv_gnrl_dffl #(1) rdfpu_dfflrs(vld_set[i], disp_i_rdfpu, rdfpu_r[i], clk);
//compare dispatch source operand with result register in fifo
assign rd_match_rs1idx[i] = vld_r[i] & rdwen_r[i] & disp_i_rs1en & (rdfpu_r[i] == disp_i_rs1fpu) & (rdidx_r[i] == disp_i_rs1idx);
assign rd_match_rs2idx[i] = vld_r[i] & rdwen_r[i] & disp_i_rs2en & (rdfpu_r[i] == disp_i_rs2fpu) & (rdidx_r[i] == disp_i_rs2idx);
assign rd_match_rs3idx[i] = vld_r[i] & rdwen_r[i] & disp_i_rs3en & (rdfpu_r[i] == disp_i_rs3fpu) & (rdidx_r[i] == disp_i_rs3idx);
assign rd_match_rdidx [i] = vld_r[i] & rdwen_r[i] & disp_i_rdwen & (rdfpu_r[i] == disp_i_rdfpu ) & (rdidx_r[i] == disp_i_rdidx );
end//}
endgenerate//}
//rs1 in fifo, so RAW relative
assign oitfrd_match_disprs1 = |rd_match_rs1idx;
//rs2 in fifo, so RAW relative
assign oitfrd_match_disprs2 = |rd_match_rs2idx;
//rs3 in fifo, so RAW relative
assign oitfrd_match_disprs3 = |rd_match_rs3idx;
//rd in fifo, so WAW relative
assign oitfrd_match_disprd = |rd_match_rdidx ;
assign ret_rdidx = rdidx_r[ret_ptr];
assign ret_pc = pc_r [ret_ptr];
assign ret_rdwen = rdwen_r[ret_ptr];
assign ret_rdfpu = rdfpu_r[ret_ptr];
endmodule

复制代码

完

RISC-V单片机技术交流群

E203数据冲突处理OITF

有人预言,RISC-V或将是继Intel和Arm之后的第三大主流处理器体系。欢迎访问全球首家只专注于RISC-V单片机行业应用的中文网站

相关帖子

RISC-V单片机中文网 /2

RISC-V单片机技术交流群

E203数据冲突处理OITF

有人预言,RISC-V或将是继Intel和Arm之后的第三大主流处理器体系。欢迎访问全球首家只专注于RISC-V单片机行业应用的中文网站

相关帖子

RISC-V单片机中文网 /2 document.write(unescape("%3Cspan id='cnzz_stat_icon_1278870330'%3E%3C/span%3E%3Cscript src='https://s4.cnzz.com/z_stat.php%3Fid%3D1278870330%26show%3Dpic1' type='text/javascript'%3E%3C/script%3E"));

RISC-V单片机中文网 /2