浅谈vivado HLS从C/C++到硬件描述语言转换

  高层次综合（High Level Synthesis, HLS）是Xilinx公司推出的最新一代的FPGA设计工具，它能让用户通过编写C/C++等高级语言代码实现RTL级的硬件功能。随着这款工具的出现，软硬之间的区别越来越模糊，即使你对于硬件完全不懂，你也能编写出符合工程功能要求的RTL代码。看到HLS工具具有如此神奇的功能，你是否想立即尝试一下呢？

         接下来我们将谈谈HLS相关的简单操作以及C/C++到VHDL的一个转换关系：

         首先确保你已经下好了vivado相关套件，并且HLS的license（官网有试用的，但只能用一个月）已经下到，然后打开桌面上，就可以按照一般的软件建工程一样，这里我们写了一个很简单的函数int andfunction(int array[2],int array1[2]);这里我们注意到函数返回值是整形，同时两个形参是都是数组，这个函数代码如下:

int andfunction(int array[2],int array1[2])
{
	int i,sum;
	sum=0;
	for(i=0;i<2;i++)
	{
		array[i]=i*2;
		array1[i]=array[i]+i;
		sum+=array1[i];
	}
	return sum;
}

       写好了这个函数后，那么我们不妨看看综合后的VHDL是怎样的。点击绿色三角按钮后生成硬件描述语言，solution1->syn->vhdl里可以看到生成的VHDL，代码如下：

-- ==============================================================
-- RTL generated by Vivado(TM) HLS - High-Level Synthesis from C, C++ and SystemC
-- Version: 2012.4
-- Copyright (C) 2012 Xilinx Inc. All rights reserved.
-- 
-- ===========================================================

library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;

entity andfunction is
port (
    ap_clk : IN STD_LOGIC;
    ap_rst : IN STD_LOGIC;
    ap_start : IN STD_LOGIC;
    ap_done : OUT STD_LOGIC;
    ap_idle : OUT STD_LOGIC;
    ap_ready : OUT STD_LOGIC;
    array_r_address0 : OUT STD_LOGIC_VECTOR (0 downto 0);
    array_r_ce0 : OUT STD_LOGIC;
    array_r_we0 : OUT STD_LOGIC;
    array_r_d0 : OUT STD_LOGIC_VECTOR (31 downto 0);
    array1_address0 : OUT STD_LOGIC_VECTOR (0 downto 0);
    array1_ce0 : OUT STD_LOGIC;
    array1_we0 : OUT STD_LOGIC;
    array1_d0 : OUT STD_LOGIC_VECTOR (31 downto 0);
    ap_return : OUT STD_LOGIC_VECTOR (31 downto 0) );
end;


architecture behav of andfunction is 
    attribute CORE_GENERATION_INFO : STRING;
    attribute CORE_GENERATION_INFO of behav : architecture is
    "andfunction,hls_ip_2012_4,{HLS_INPUT_TYPE=c,HLS_INPUT_FLOAT=0,HLS_INPUT_FIXED=0,HLS_INPUT_PART=xc5vlx110tff1136-1,HLS_INPUT_CLOCK=10.000000,HLS_INPUT_ARCH=others,HLS_SYN_CLOCK=3.090000,HLS_SYN_LAT=3,HLS_SYN_TPT=none,HLS_SYN_MEM=0,HLS_SYN_DSP=0,HLS_SYN_FF=3,HLS_SYN_LUT=8}";
    constant ap_const_logic_1 : STD_LOGIC := '1';
    constant ap_const_logic_0 : STD_LOGIC := '0';
    constant ap_ST_st1_fsm_0 : STD_LOGIC_VECTOR (0 downto 0) := "0";
    constant ap_ST_st2_fsm_1 : STD_LOGIC_VECTOR (0 downto 0) := "1";
    constant ap_const_lv2_0 : STD_LOGIC_VECTOR (1 downto 0) := "00";
    constant ap_const_lv1_0 : STD_LOGIC_VECTOR (0 downto 0) := "0";
    constant ap_const_lv2_2 : STD_LOGIC_VECTOR (1 downto 0) := "10";
    constant ap_const_lv2_1 : STD_LOGIC_VECTOR (1 downto 0) := "01";
    constant ap_const_lv32_3 : STD_LOGIC_VECTOR (31 downto 0) := "00000000000000000000000000000011";

    signal ap_CS_fsm : STD_LOGIC_VECTOR (0 downto 0) := "0";
    signal i_1_fu_66_p2 : STD_LOGIC_VECTOR (1 downto 0);
    signal i_reg_42 : STD_LOGIC_VECTOR (1 downto 0);
    signal exitcond1_fu_60_p2 : STD_LOGIC_VECTOR (0 downto 0);
    signal i_cast_fu_54_p1 : STD_LOGIC_VECTOR (31 downto 0);
    signal tmp_fu_72_p2 : STD_LOGIC_VECTOR (1 downto 0);
    signal tmp_1_fu_83_p2 : STD_LOGIC_VECTOR (1 downto 0);
    signal ap_NS_fsm : STD_LOGIC_VECTOR (0 downto 0);

begin

    -- the current state (ap_CS_fsm) of the state machine. --
    ap_CS_fsm_assign_proc : process(ap_clk)
    begin
        if (ap_clk'event and ap_clk =  '1') then
            if (ap_rst = '1') then
                ap_CS_fsm <= ap_ST_st1_fsm_0;
            else
                ap_CS_fsm <= ap_NS_fsm;
            end if;
        end if;
    end process;


    -- ap_reg assign process. --
    ap_reg_proc : process(ap_clk)
    begin
        if (ap_clk'event and ap_clk = '1') then
            if (((ap_ST_st2_fsm_1 = ap_CS_fsm) and (exitcond1_fu_60_p2 = ap_const_lv1_0))) then 
                i_reg_42 <= i_1_fu_66_p2;
            elsif (((ap_ST_st1_fsm_0 = ap_CS_fsm) and not((ap_start = ap_const_logic_0)))) then 
                i_reg_42 <= ap_const_lv2_0;
            end if; 
        end if;
    end process;

    -- the next state (ap_NS_fsm) of the state machine. --
    ap_NS_fsm_assign_proc : process(ap_start, ap_CS_fsm, exitcond1_fu_60_p2)
    begin
        if (((ap_ST_st2_fsm_1 = ap_CS_fsm) and not((exitcond1_fu_60_p2 = ap_const_lv1_0)))) then 
            ap_NS_fsm <= ap_ST_st1_fsm_0;
        elsif ((((ap_ST_st1_fsm_0 = ap_CS_fsm) and not((ap_start = ap_const_logic_0))) or ((ap_ST_st2_fsm_1 = ap_CS_fsm) and (exitcond1_fu_60_p2 = ap_const_lv1_0)))) then 
            ap_NS_fsm <= ap_ST_st2_fsm_1;
        else 
            ap_NS_fsm <= ap_CS_fsm;
        end if; 
    end process;


    -- ap_done assign process. --
    ap_done_assign_proc : process(ap_CS_fsm, exitcond1_fu_60_p2)
    begin
        if (((ap_ST_st2_fsm_1 = ap_CS_fsm) and not((exitcond1_fu_60_p2 = ap_const_lv1_0)))) then 
            ap_done <= ap_const_logic_1;
        else 
            ap_done <= ap_const_logic_0;
        end if; 
    end process;


    -- ap_idle assign process. --
    ap_idle_assign_proc : process(ap_start, ap_CS_fsm)
    begin
        if ((not((ap_const_logic_1 = ap_start)) and (ap_ST_st1_fsm_0 = ap_CS_fsm))) then 
            ap_idle <= ap_const_logic_1;
        else 
            ap_idle <= ap_const_logic_0;
        end if; 
    end process;


    -- ap_ready assign process. --
    ap_ready_assign_proc : process(ap_CS_fsm, exitcond1_fu_60_p2)
    begin
        if (((ap_ST_st2_fsm_1 = ap_CS_fsm) and not((exitcond1_fu_60_p2 = ap_const_lv1_0)))) then 
            ap_ready <= ap_const_logic_1;
        else 
            ap_ready <= ap_const_logic_0;
        end if; 
    end process;

    ap_return <= ap_const_lv32_3;
    array1_address0 <= i_cast_fu_54_p1(1 - 1 downto 0);

    -- array1_ce0 assign process. --
    array1_ce0_assign_proc : process(ap_CS_fsm, exitcond1_fu_60_p2)
    begin
        if (((ap_ST_st2_fsm_1 = ap_CS_fsm) and (exitcond1_fu_60_p2 = ap_const_lv1_0))) then 
            array1_ce0 <= ap_const_logic_1;
        else 
            array1_ce0 <= ap_const_logic_0;
        end if; 
    end process;

    array1_d0 <= std_logic_vector(resize(unsigned(tmp_1_fu_83_p2),32));

    -- array1_we0 assign process. --
    array1_we0_assign_proc : process(ap_CS_fsm, exitcond1_fu_60_p2)
    begin
        if ((((ap_ST_st2_fsm_1 = ap_CS_fsm) and (exitcond1_fu_60_p2 = ap_const_lv1_0)))) then 
            array1_we0 <= ap_const_logic_1;
        else 
            array1_we0 <= ap_const_logic_0;
        end if; 
    end process;

    array_r_address0 <= i_cast_fu_54_p1(1 - 1 downto 0);

    -- array_r_ce0 assign process. --
    array_r_ce0_assign_proc : process(ap_CS_fsm, exitcond1_fu_60_p2)
    begin
        if (((ap_ST_st2_fsm_1 = ap_CS_fsm) and (exitcond1_fu_60_p2 = ap_const_lv1_0))) then 
            array_r_ce0 <= ap_const_logic_1;
        else 
            array_r_ce0 <= ap_const_logic_0;
        end if; 
    end process;

    array_r_d0 <= std_logic_vector(resize(unsigned(tmp_fu_72_p2),32));

    -- array_r_we0 assign process. --
    array_r_we0_assign_proc : process(ap_CS_fsm, exitcond1_fu_60_p2)
    begin
        if ((((ap_ST_st2_fsm_1 = ap_CS_fsm) and (exitcond1_fu_60_p2 = ap_const_lv1_0)))) then 
            array_r_we0 <= ap_const_logic_1;
        else 
            array_r_we0 <= ap_const_logic_0;
        end if; 
    end process;

    exitcond1_fu_60_p2 <= "1" when (i_reg_42 = ap_const_lv2_2) else "0";
    i_1_fu_66_p2 <= std_logic_vector(unsigned(i_reg_42) + unsigned(ap_const_lv2_1));
    i_cast_fu_54_p1 <= std_logic_vector(resize(unsigned(i_reg_42),32));
    tmp_1_fu_83_p2 <= std_logic_vector(unsigned(tmp_fu_72_p2) + unsigned(i_reg_42));
    tmp_fu_72_p2 <= std_logic_vector(shift_left(unsigned(i_reg_42),to_integer(unsigned('0' & ap_const_lv2_1(2-1 downto 0)))));
end behav;

看到这么长一大串代码后，你或许会说看起来太复杂来人，还不如自己写；一两个简单的可以自己写，成千上万个呢？这里我们不去关注architecture里面具体的实现过程（这里面包含很多优化），这里我们仅仅讨论从C/C++到VHDL的entity的关系。

        看到VHDL后，你也许第一眼就看到了VHDL中的entity了，但是你不一定理解其管脚到底指代什么，下面给出一张int andfunction(int array[2],int array1[2])这个函数的硬件结构图

硬件引脚的具体含义是（这部分是参考的http://xilinx.eetop.cn/viewnews-1483）：

ap_clk：设计的时钟信号

ap_rst：设计的复位信号

ap_start：开始计算的开始信号

ap_done：计算结束和输出就绪的完成信号

ap_idle：表示实体（设计）空闲的空闲信号

ap_ready：表示设计为新输入数据做好准备，与ap_idle 配合使用

ap_return：设计的返回值

name_address：存储器的读地址 （name指代array或array1,因为它们都是数组，在硬件中综合成了寄存器）

name_ce0：存储器的芯片使能

name_we0：存储器的写使能

name_do0：存储器的写数据

ap_return: 函数返回值端口

分析上面的硬件管脚，其中ap_clk、ap_rst、ap_start、ap_done、ap_idle、ap_ready为大多数综合后硬件默认必有的，因为这些引脚便于处理器对其进行控制；name_address0、name_ce0、name_we0、name_do0这些引脚是为了获取所使用到寄存器的状态，在这里我们应当注意：数组开辟的空间尽量不要大，过大容易造成硬件资源不足，无法开辟满足要求的寄存器。

今天就对HLS从C/C++到VHDL的转换大致说到这里，由于刚接触这一块，难免有所纰漏，欢迎大家指出！