Jump to page: 1 2
Thread overview
a way to specily floating-point numbers as bit patters
Jun 09, 2017
ketmar
Jun 09, 2017
Honey
Jun 09, 2017
ketmar
Jun 09, 2017
Honey
Jun 09, 2017
Honey
Jun 09, 2017
ketmar
Jun 09, 2017
Honey
Jun 09, 2017
Basile B.
Jun 09, 2017
ketmar
Jun 09, 2017
Basile B.
Jun 09, 2017
ketmar
Jun 09, 2017
Basile B.
Jun 09, 2017
ketmar
Jun 09, 2017
ketmar
Jun 09, 2017
Nicholas Wilson
June 09, 2017
let's say that i have precomputed some `float`-typed tables, and now i want to use 'em in my code. for example, a table for Lagrange series. the table itself is like 10 numbers, but the code calculating it rather big, and it depends of architecture (so it can yield different results on different arch). yet i want the same IEEE numbers everywhere.

the code problem is that i really can't express *exactly* *the* *same* IEEE float with D. let me illustrate this.

one of my calculated values is `-0.166667`, which has bit-pattern of 0xBE2AAAB7.
now, let's say i want to use this number in my code:

	float v = -0.166667f;
	writefln("%f 0x%08X", v, *cast(uint*)&v);

oooops. "-0.166667 0xBE2AAAC1". it's not the same! (and yes, it matters).

and this means that i can't inline my calculated values! each time i want to get floating value with a known bit-pattern, i have to store it as uint, and resort to ugly hack: `*cast(immutable(float)*)(&mytable[2])`.

and i can't do this trick in CTFE, as such pointer tricks aren't permitted.

i tried different workarounds, but they're all ugly. it would be very nice to have a way to define IEEE floating point numbers as bit-patterns in the language itself. what do you think?

yes, i know that floating numbers has to be loaded from memory anyway, so my trick is not really worse than specifying the constant directly. but it is still dirty trick, and i cannot use it in @safe code too, so i have to mark my code as @trusted, which is not the same at all. there probably may be other trick such this for @safe code, but... you got the idea, i think.
June 09, 2017
On Friday, 9 June 2017 at 16:07:36 UTC, ketmar wrote:
> one of my calculated values is `-0.166667`, which has bit-pattern of 0xBE2AAAB7.
> now, let's say i want to use this number in my code:
>
> 	float v = -0.166667f;
> 	writefln("%f 0x%08X", v, *cast(uint*)&v);
>
> oooops. "-0.166667 0xBE2AAAC1". it's not the same! (and yes, it matters).

Try -0.16666685f.
June 09, 2017
Honey wrote:

> On Friday, 9 June 2017 at 16:07:36 UTC, ketmar wrote:
>> one of my calculated values is `-0.166667`, which has bit-pattern of 0xBE2AAAB7.
>> now, let's say i want to use this number in my code:
>>
>> 	float v = -0.166667f;
>> 	writefln("%f 0x%08X", v, *cast(uint*)&v);
>>
>> oooops. "-0.166667 0xBE2AAAC1". it's not the same! (and yes, it matters).
>
> Try -0.16666685f.

it doesn't matter if i can find the decimal representation for the given bit pattern or not. the whole post is about removing the need to rely on lossy binary->decimal->binary conversions.
June 09, 2017
On Friday, 9 June 2017 at 16:07:36 UTC, ketmar wrote:
> let's say that i have precomputed some `float`-typed tables, and now i want to use 'em in my code. for example, a table for Lagrange series. the table itself is like 10 numbers, but the code calculating it rather big, and it depends of architecture (so it can yield different results on different arch). yet i want the same IEEE numbers everywhere.
>
> the code problem is that i really can't express *exactly* *the* *same* IEEE float with D. let me illustrate this.
>
> one of my calculated values is `-0.166667`, which has bit-pattern of 0xBE2AAAB7.
> now, let's say i want to use this number in my code:
>
> 	float v = -0.166667f;
> 	writefln("%f 0x%08X", v, *cast(uint*)&v);
>
> oooops. "-0.166667 0xBE2AAAC1". it's not the same! (and yes, it matters).

-0.166667f is not representable as a 32 bit float. The actuall value that's stored is -0.16666699945926666259765625, hence the difference. See https://www.h-schmidt.net/FloatConverter/IEEE754.html and enter your value in the field labeled "You entered".

> and this means that i can't inline my calculated values! each time i want to get floating value with a known bit-pattern, i have to store it as uint, and resort to ugly hack: `*cast(immutable(float)*)(&mytable[2])`.
>
> and i can't do this trick in CTFE, as such pointer tricks aren't permitted.
>
> i tried different workarounds, but they're all ugly. it would be very nice to have a way to define IEEE floating point numbers as bit-patterns in the language itself. what do you think?

Yes, easy to do, a template alà octal or hexString.

> yes, i know that floating numbers has to be loaded from memory anyway, so my trick is not really worse than specifying the constant directly. but it is still dirty trick, and i cannot use it in @safe code too, so i have to mark my code as @trusted, which is not the same at all. there probably may be other trick such this for @safe code, but... you got the idea, i think.


June 09, 2017
On Friday, 9 June 2017 at 16:34:28 UTC, ketmar wrote:
>> Try -0.16666685f.
>
> it doesn't matter if i can find the decimal representation for the given bit pattern or not. the whole post is about removing the need to rely on lossy binary->decimal->binary conversions.

Lossless turn-around is guaranteed if you are using sufficiently many digits. In case of IEEE-754 single precision it's 8 significant decimal digits.
June 09, 2017
Basile B. wrote:

>> oooops. "-0.166667 0xBE2AAAC1". it's not the same! (and yes, it matters).
>
> -0.166667f is not representable as a 32 bit float. The actuall value that's stored is -0.16666699945926666259765625, hence the difference. See https://www.h-schmidt.net/FloatConverter/IEEE754.html and enter your value in the field labeled "You entered".

i'm completely aware about floating point values representation, and problems with bin->dec->bin conversions of floats. the post is about avoiding those conversions at all.

>> and this means that i can't inline my calculated values! each time i want to get floating value with a known bit-pattern, i have to store it as uint, and resort to ugly hack: `*cast(immutable(float)*)(&mytable[2])`.
>>
>> and i can't do this trick in CTFE, as such pointer tricks aren't permitted.
>>
>> i tried different workarounds, but they're all ugly. it would be very nice to have a way to define IEEE floating point numbers as bit-patterns in the language itself. what do you think?
>
> Yes, easy to do, a template alà octal or hexString.

can you show it, please? remember, CTFE-able!
June 09, 2017
On Friday, 9 June 2017 at 16:07:36 UTC, ketmar wrote:
> let's say that i have precomputed some `float`-typed tables, and now i want to use 'em in my code. for example, a table for Lagrange series. the table itself is like 10 numbers, but the code calculating it rather big, and it depends of architecture (so it can yield different results on different arch). yet i want the same IEEE numbers everywhere.
>
> the code problem is that i really can't express *exactly* *the* *same* IEEE float with D. let me illustrate this.
>
> one of my calculated values is `-0.166667`, which has bit-pattern of 0xBE2AAAB7.
> now, let's say i want to use this number in my code:
>
> 	float v = -0.166667f;
> 	writefln("%f 0x%08X", v, *cast(uint*)&v);
>
> oooops. "-0.166667 0xBE2AAAC1". it's not the same! (and yes, it matters).
>
> and this means that i can't inline my calculated values! each time i want to get floating value with a known bit-pattern, i have to store it as uint, and resort to ugly hack: `*cast(immutable(float)*)(&mytable[2])`.
>
> and i can't do this trick in CTFE, as such pointer tricks aren't permitted.
>
> i tried different workarounds, but they're all ugly. it would be very nice to have a way to define IEEE floating point numbers as bit-patterns in the language itself. what do you think?
>
> yes, i know that floating numbers has to be loaded from memory anyway, so my trick is not really worse than specifying the constant directly. but it is still dirty trick, and i cannot use it in @safe code too, so i have to mark my code as @trusted, which is not the same at all. there probably may be other trick such this for @safe code, but... you got the idea, i think.

Do HexFloats (http://dlang.org/spec/lex#HexFloat) help?
June 09, 2017
On Friday, 9 June 2017 at 16:41:01 UTC, Honey wrote:
> Lossless turn-around is guaranteed if you are using sufficiently many digits. In case of IEEE-754 single precision it's 8 significant decimal digits.

s/turn-around/recovery/g
s/8/9/g

:-P


June 09, 2017
Petar Kirov [ZombineDev] wrote:

> Do HexFloats (http://dlang.org/spec/lex#HexFloat) help?

hm. i somehow completely missed "%a" format specifier! yeah, "-0x1.55556ep-3" did the trick.

tnx. i should do my homework *before* posting big rants, lol.
June 09, 2017
On Friday, 9 June 2017 at 16:42:31 UTC, ketmar wrote:
> Basile B. wrote:
>> Yes, easy to do, a template alà octal or hexString.
>
> can you show it, please? remember, CTFE-able!

Sure, here's a dirty draft:

template binFloat(string sign, string exp, string mant)
{
    enum s = sign == "+" ? "0" : "1";
    const b = mixin("0b" ~ s ~ exp ~ mant);
    pragma(msg,"0b" ~ s ~ exp ~ mant);
    enum binFloat = *cast(float*) &b;
}

unittest
{
    static assert(binFloat!("+", "10000000", "10000000000000000000000") == 3.0f);
    static assert(binFloat!("-", "10000000", "10000000000000000000000") == -3.0f);
}
« First   ‹ Prev
1 2