Developer Guide

Intel oneAPI FPGA Handbook

ID 785441
Date 2/07/2024
Public

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Document Table of Contents

Advantages and Limitations of Variable Precision Data Types

Advantages

The variable precision data types have the following advantages over the use of standard C/C++ data types:

  • You can achieve narrower data paths and processing elements for various operations in the circuit.
  • The data types ensure that all operations are carried out in a size guaranteed not to lose any data. However, you can still lose data if you store data in a location where the data type is too narrow in size.

Limitations

AC Data Types

The AC data types have the following limitations:

  • Multipliers are limited to generating 512-bit results.
  • Dividers for ac_int data types are limited to a maximum of 64-bit unsigned or 63-bit signed.
  • You must initialize all AC data type variables before accessing them. Accessing the AC data types before initialization is an undefined behavior and leads to unexpected results. Assigning each bit explicitly using the [] operator or set_slc function does not count as initializing an AC data type. The default constructor of AC data types does not initialize the variable; other constructors initialize them.
  • Dividers for ac_fixed data types are limited to a maximum of 64-bits (unsigned or signed).
  • Creation of ac_fixed variables larger than 32 bits are supported only with the use of the bit_fill utility function.

    For example:

    // Creating an ac_fixed with value set to 4294967298, which is larger than 2^32.
      
    // Unsupported
    ac_fixed<64, 64, false> v1 = ac_fixed<64, 64, false>(4294967298);
    
    // Supported
    // 4294967298 is 0b100000000000000000000000000000010 in binary
    // Express that as two 32-bit numbers and use the bit_fill utility function.
    const int vec_inp[2] = {0x00000001, 0x00000002};
    ac_fixed<64, 64, false> bit_fill_res;
    bit_fill_res.bit_fill<2>(vec_inp);
  • The AC data types are not supported on the Red Hat Enterprise Linux* (RHEL) 7 operating system for emulation due to a bug in the glibc version bundled with RHEL 7.
  • You cannot template the ac_complex data type with the ap_float data type.
  • When using the bit_fill_hex() function inside a kernel, pass the input string to the kernel through a char buffer and not as a string buffer. In addition, hardware and simulation compile flows do not support using a string literal or passing the string directly to the function. The following are the supported and unsupported code patterns:

    Supported Patterns

    // Supported Pattern 1: Passing string as a char sycl::buffer to the kernel 
    ac_int<140, false> supported_example1(queue &q) {
      ac_int<140, false> a; 
      std::string hex_string{"0x177632EE7E265080BD54FF0CE7EF42C12"}; 
      constexpr int N = 36; // size of hex_string 
    
      buffer<ac_int<140, false>, 1> inp1(&a, 1); 
      // Note: the N + 1 ensures that the null byte  
      //terminating the char array buffer is copied
      buffer<char, 1> inp2(hex_string.c_str(), range<1>(N + 1)); 
      
      q.submit([&](handler &h) { 
        accessor x(inp1, h, read_write);
        accessor y(inp2, h, read_only); 
        h.single_task<class D>([=] { 
          x[0].bit_fill_hex(&y[0]);
        }); 
      }); 
      q.wait(); 
      return a; 
    }
    // Supported Pattern 2: Create a char array with the string literal. 
    ac_int<140, false> supported_example2(queue &q) {
      ac_int<140, false> a; 
      buffer<ac_int<140, false>, 1> inp1(&a, 1); 
    
      q.submit([&](handler &h) { 
        accessor x(inp1, h, read_write); 
        h.single_task<class D>([=] {
          char str[36] = "0x177632EE7E265080BD54FF0CE7EF42C12"; 
          x[0].bit_fill_hex(str);
        }); 
      }); 
      q.wait(); 
      return a; 
    }

    Unsupported Patterns

    // Unsupported Pattern 1 – Using a string Literal, will result in compilation error 
    ac_int<140, false> unsupported_example1(queue& q) {
      { 
        ac_int<140, false> a; 
        buffer<ac_int<140, false>, 1> a_buff(&a, 1); 
       
        q.submit([&](handler &h) { 
          accessor a_acc {a_buff, h, write_only, no_init}; 
          h.single_task<class A>([=]() { 
            a_acc[0].bit_fill_hex("1141e98e8c51b7ac7ad387d7f8ee4f1b9"); 
          }); 
        }); 
        q.wait_and_throw(); 
        return a; 
      } 
    }
    // Unsupported Pattern 2 – Passing the string to the kernel in a string sycl::buffer 
    ac_int<140, false> unsupported_example2(queue& q) {
      {
        std::string str{"1141e98e8c51b7ac7ad387d7f8ee4f1b9"}; 
        ac_int<140, false> a; 
        buffer<std::string, 1> str_buff(&str, 1); 
        buffer<ac_int<140, false>, 1> a_buff(&a, 1); 
        
        q.submit([&](handler &h) {
          accessor str_acc {str_buff, h, read_only}; 
          accessor a_acc {a_buff, h, write_only, no_init}; 
      
          h.single_task<class B>([=]() { 
            a_acc[0].bit_fill_hex(str_acc[0].c_str());
          }); 
        }); 
        q.wait_and_throw(); 
        return a; 
      } 
    }
ap_float Data Type

The ap_float data type has the following limitations:

  • While the floating-point optimization of converting into constants is performed for float and double data types, it is not performed for the ap_float data type.
  • A limited set of math functions is supported. For details, see Math Functions Supported by ap_float Data Type.
  • Constant initialization works only with the round-towards-zero (RZERO) rounding mode.
  • For emulation, the ap_float math library is not supported on the Red Hat Enterprise Linux* (RHEL) 7 operating system.
  • When computing A^B using ap_float's ihc_pown function, if B is an unsigned type T of size N bits and is equal to the maximum unsigned value, redefine B to be of size N+1 bits. Otherwise, results will be incorrect. For example:
    // Sample Code: 
    ap_float<8, 7> a = 2; 
    ac_int<4, false> b = 15; // max value that this ac_int can hold 
    … = ihc_pown(a , b); // !!! Will produce incorrect result 
    // Workaround: 
    ap_float<8, 7> a = 2;
    ac_int<5, false> b = 15; // Workaround 
    … = ihc_pown(a , b); // Will produce correct result