linux / linux / kernel / git / torvalds / linux / 44035ec2fde1114254ee465f9ba3bb246b0b6283 / . / Documentation / arm / nwfpe / todo.rst

TODO LIST | |

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:: | |

POW{cond}<S|D|E>{P,M,Z} Fd, Fn, <Fm,#value> - power | |

RPW{cond}<S|D|E>{P,M,Z} Fd, Fn, <Fm,#value> - reverse power | |

POL{cond}<S|D|E>{P,M,Z} Fd, Fn, <Fm,#value> - polar angle (arctan2) | |

LOG{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - logarithm to base 10 | |

LGN{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - logarithm to base e | |

EXP{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - exponent | |

SIN{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - sine | |

COS{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - cosine | |

TAN{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - tangent | |

ASN{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - arcsine | |

ACS{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - arccosine | |

ATN{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - arctangent | |

These are not implemented. They are not currently issued by the compiler, | |

and are handled by routines in libc. These are not implemented by the FPA11 | |

hardware, but are handled by the floating point support code. They should | |

be implemented in future versions. | |

There are a couple of ways to approach the implementation of these. One | |

method would be to use accurate table methods for these routines. I have | |

a couple of papers by S. Gal from IBM's research labs in Haifa, Israel that | |

seem to promise extreme accuracy (in the order of 99.8%) and reasonable speed. | |

These methods are used in GLIBC for some of the transcendental functions. | |

Another approach, which I know little about is CORDIC. This stands for | |

Coordinate Rotation Digital Computer, and is a method of computing | |

transcendental functions using mostly shifts and adds and a few | |

multiplications and divisions. The ARM excels at shifts and adds, | |

so such a method could be promising, but requires more research to | |

determine if it is feasible. | |

Rounding Methods | |

---------------- | |

The IEEE standard defines 4 rounding modes. Round to nearest is the | |

default, but rounding to + or - infinity or round to zero are also allowed. | |

Many architectures allow the rounding mode to be specified by modifying bits | |

in a control register. Not so with the ARM FPA11 architecture. To change | |

the rounding mode one must specify it with each instruction. | |

This has made porting some benchmarks difficult. It is possible to | |

introduce such a capability into the emulator. The FPCR contains | |

bits describing the rounding mode. The emulator could be altered to | |

examine a flag, which if set forced it to ignore the rounding mode in | |

the instruction, and use the mode specified in the bits in the FPCR. | |

This would require a method of getting/setting the flag, and the bits | |

in the FPCR. This requires a kernel call in ArmLinux, as WFC/RFC are | |

supervisor only instructions. If anyone has any ideas or comments I | |

would like to hear them. | |

NOTE: | |

pulled out from some docs on ARM floating point, specifically | |

for the Acorn FPE, but not limited to it: | |

The floating point control register (FPCR) may only be present in some | |

implementations: it is there to control the hardware in an implementation- | |

specific manner, for example to disable the floating point system. The user | |

mode of the ARM is not permitted to use this register (since the right is | |

reserved to alter it between implementations) and the WFC and RFC | |

instructions will trap if tried in user mode. | |

Hence, the answer is yes, you could do this, but then you will run a high | |

risk of becoming isolated if and when hardware FP emulation comes out | |

-- Russell. |