Re: a way to specily floating-point numbers as bit patters
On Friday, 9 June 2017 at 16:56:46 UTC, ketmar wrote: Petar Kirov [ZombineDev] wrote: Do HexFloats (http://dlang.org/spec/lex#HexFloat) help? hm. i somehow completely missed "%a" format specifier! yeah, "-0x1.6ep-3" did the trick. tnx. i should do my homework *before* posting big rants, lol. There's also FloatRep in std.bitmanip.
Re: a way to specily floating-point numbers as bit patters
On Friday, 9 June 2017 at 17:25:22 UTC, ketmar wrote: it is highly platform-dependent. and both bin->dec, and dec->bin conversion routines can contain errors, btw. so using decimal forms for exact bit-patterns is the last thing i want to do, as i know how fragile they are. Sure. Hex-format is the right choice if you want a fixed bit pattern.
Re: a way to specily floating-point numbers as bit patters
Honey wrote: On Friday, 9 June 2017 at 16:34:28 UTC, ketmar wrote: Try -0.1685f. 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. it is highly platform-dependent. and both bin->dec, and dec->bin conversion routines can contain errors, btw. so using decimal forms for exact bit-patterns is the last thing i want to do, as i know how fragile they are.
Re: a way to specily floating-point numbers as bit patters
Basile B. wrote: On Friday, 9 June 2017 at 17:18:43 UTC, ketmar wrote: Basile B. wrote: enum binFloat = *cast(float*) i was SO sure that this won't work in CTFE that i didn't even tried to do it. "it will fail anyway, there is no reason in trying!" ;-) You can do the arithmetic as well. I don't know why but i supposed that my static asserts were a proof of CTFE-ability. yeah, it is CTFEable. now i recall that such casting for floats was special-cased in interpreter exactly to allow this kind of things. my bad.
Re: a way to specily floating-point numbers as bit patters
On Friday, 9 June 2017 at 17:18:43 UTC, ketmar wrote: Basile B. wrote: enum binFloat = *cast(float*) i was SO sure that this won't work in CTFE that i didn't even tried to do it. "it will fail anyway, there is no reason in trying!" ;-) You can do the arithmetic as well. I don't know why but i supposed that my static asserts were a proof of CTFE-ability.
Re: a way to specily floating-point numbers as bit patters
Basile B. wrote: enum binFloat = *cast(float*) i was SO sure that this won't work in CTFE that i didn't even tried to do it. "it will fail anyway, there is no reason in trying!" ;-)
Re: a way to specily floating-point numbers as bit patters
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*)
}
unittest
{
static assert(binFloat!("+", "1000",
"100") == 3.0f);
static assert(binFloat!("-", "1000",
"100") == -3.0f);
}
Re: a way to specily floating-point numbers as bit patters
Petar Kirov [ZombineDev] wrote: Do HexFloats (http://dlang.org/spec/lex#HexFloat) help? hm. i somehow completely missed "%a" format specifier! yeah, "-0x1.6ep-3" did the trick. tnx. i should do my homework *before* posting big rants, lol.
Re: a way to specily floating-point numbers as bit patters
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
Re: a way to specily floating-point numbers as bit patters
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 `-0.17`, which has
bit-pattern of 0xBE2AAAB7.
now, let's say i want to use this number in my code:
float v = -0.17f;
writefln("%f 0x%08X", v, *cast(uint*));
ps. "-0.17 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)*)([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?
Re: a way to specily floating-point numbers as bit patters
Basile B. wrote: ps. "-0.17 0xBE2AAAC1". it's not the same! (and yes, it matters). -0.17f is not representable as a 32 bit float. The actuall value that's stored is -0.169994592259765625, 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)*)([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!
Re: a way to specily floating-point numbers as bit patters
On Friday, 9 June 2017 at 16:34:28 UTC, ketmar wrote: Try -0.1685f. 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.
Re: a way to specily floating-point numbers as bit patters
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 `-0.17`, which has
bit-pattern of 0xBE2AAAB7.
now, let's say i want to use this number in my code:
float v = -0.17f;
writefln("%f 0x%08X", v, *cast(uint*));
ps. "-0.17 0xBE2AAAC1". it's not the same! (and yes, it
matters).
-0.17f is not representable as a 32 bit float. The actuall
value that's stored is -0.169994592259765625, 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)*)([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.
Re: a way to specily floating-point numbers as bit patters
Honey wrote:
On Friday, 9 June 2017 at 16:07:36 UTC, ketmar wrote:
one of my calculated values is `-0.17`, which has bit-pattern of
0xBE2AAAB7.
now, let's say i want to use this number in my code:
float v = -0.17f;
writefln("%f 0x%08X", v, *cast(uint*));
ps. "-0.17 0xBE2AAAC1". it's not the same! (and yes, it matters).
Try -0.1685f.
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.
Re: a way to specily floating-point numbers as bit patters
On Friday, 9 June 2017 at 16:07:36 UTC, ketmar wrote:
one of my calculated values is `-0.17`, which has
bit-pattern of 0xBE2AAAB7.
now, let's say i want to use this number in my code:
float v = -0.17f;
writefln("%f 0x%08X", v, *cast(uint*));
ps. "-0.17 0xBE2AAAC1". it's not the same! (and yes, it
matters).
Try -0.1685f.
a way to specily floating-point numbers as bit patters
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 `-0.17`, which has bit-pattern of 0xBE2AAAB7.
now, let's say i want to use this number in my code:
float v = -0.17f;
writefln("%f 0x%08X", v, *cast(uint*));
ps. "-0.17 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)*)([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.
