Vdsp(bf561)中的浮点运算（11）:fract16与float的转换[VC/C++编程]

vdsp供应了两个函数用以实现fract16与float之间的彼此转换:

fract16 float_to_fr16 (float _x);
float fr16_to_float (fract16 _x);

看看这两个转换函数到底做了什么.

1.1 float_to_fr16

这个函数的原始代码在Blackfinlibsrclibcruntimefl2fr.asm中,先看看它的注释:

/***************************************************************************
*
* Function:　 FLOAT_TO_FR16 -- Convert a floating-point value to a fract16
*
* Synopsis:
*
*　　　　　　 #include <fract2float_conv.h>
*　　　　　　 fract16 float_to_fr16(float x);
*
* Description:
*
*　　　　　　 The float_to_fr16 function converts a single precision, 32-bit IEEE
*　　　　　　 value into a fract16 number in 1.15 notation. Floating-point values
*　　　　　　 that cannot be converted to fract15 notation are handled as follows:
*
*　　　　　　　　　　 Return 0x7fffffff if x >= 1.0 or　 NaN or +Inf
*　　　　　　　　　　 Return 0x80000000 if x < -1.0 or -NaN or -Inf
*　　　　　　　　　　 Return 0　　　　　　　　　 if fabs(x) < 3.0517578125e-5
*
*　　　　　　 (Note that the IEEE single precision, 32-bit, representation
*　　　　　　 contains 24 bits of precision, made up of a hidden bit and 23
*　　　　　　 bits of mantissa, and thus some precision may be lost by converting
*　　　　　　 a float to a fract16).
*
* Algorithm:
*
*　　　　　　 The traditional　algorithm to convert a floating-point value to 1.15
*　　　　　　 fractional　notation is:
*
*　　　　　　　　　　 (fract16) (x * 32768.0)
*
*　　　　　　 However on Blackfin, floating-point multiplication is relatively
*　　　　　　 slow is emulated in software, and this basic algorithm does not
*　　　　　　 handle out of range results.
*
*　　　　　　 This implementation is based on the support routine that converts
*　　　　　　 a float to fract32, and then converts the fract32 into a fract16
*　　　　　　 by performing an arithmetic right shift by 16 bits. (It is possible
*　　　　　　 to avoid the shift by coding the function to "multiply" the input
*　　　　　　 input argument by 2^15 (rather than 2^31) but this approach can lead
*　　　　　　 to the loss of 1-bit precision when handling negative inputs).
*
*　　　　　　 The following is a C implementation of this function and is about
*　　　　　　 a third slower:

#include <fract2float_conv.h>

extern fract16
float_to_fr16(float x)
{

int temp;
fract32 result;

temp = *(int *)(&x);

if ((temp & 0x7f800000) >= 0x3f800000) {
result = 0x7fffffff;
if (temp < 0)
result = 0x80000000;
} else {
temp = temp + 0x0f800000;
result = *(float *)(&temp);
}

return (result >> 16);

}
*
*　　　　　　 WARNING: This algorithm assumes that the floating-point number
*　　　　　　 representation is conformant with IEEE.
*
* Cycle Counts:
*
*　　　　　　 31 cycles when the result is within range
*　　　　　　 30 cycles when the result is out of range
*　　　　　　 28 cycles when the input is 0.0
*
*　　　　　　 These cycle counts were measured using the BF532 cycle accurate
*　　　　　　 simulator and include the overheads involved in calling the function
*　　　　　　 as well　as the costs associated with argument passing.
*
* Code Size:
*
*　　　　　　 76 bytes
*
* Registers Used:
*
*　　　　　　 R0 - the input argument
*　　　　　　 R1 - various constants
*　　　　　　 R2 - the exponent of the input argument or a shift amount
*　　　　　　 R3 - the mantissa of the input argument
*
* (c) Copyright 2006 Analog Devices, Inc.　 All　rights reserved.
*　　　　 $Revision: 1.3 $
*
***************************************************************************/

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