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digitalmars.D - Less-than-optimal decimal real literal conversion to x86 extended

reply pineapple <meapineapple gmail.com> writes:
I'm not sure whether this should be counted as a bug, but I ran 
into it and thought it deserved mentioning. I've been testing 
this with DMD on Windows.

I wrote this function to support parsing of strings as floating 
point values:
https://github.com/pineapplemachine/mach.d/blob/master/mach/math/floats/inject.d#L28

It is a direct copy of the relevant parts of strtod as 
implemented here:
https://opensource.apple.com/source/tcl/tcl-10/tcl/compat/strtod.c

When writing tests for this code I assumed that the result of 
expressions like `float x = some_literal` would be at least as 
accurate as the values returned by my code, and so I wrote tests 
which asserted that the literals and outputted values should be 
exactly equal.

Take for example 0.0005, which is the input that caused me quite 
a lot of trouble. When producing floats and doubles, the literals 
and the function outputs are exactly equal, e.g. 
`myfunc!double(5, -4) == 5e-4`. This is not the case for reals.

When you write `real x = 0.0005;` x in fact represets a value of 
about 
0.000500000000000000000032187251995663412884596255025826394. This 
is a about 3.2 * 10^-23 more than 0.0005.

The output of my function in this case was about 
0.000499999999999999999979247692792269641692826098733348771. This 
is a about 2.1 * 10^-23 less than 0.0005.

In this case, at least, the output of the function I wrote 
produced a more accurate value than the compiler did. Would it be 
possible and/or desirable to make DMD use a more accurate 
string-to-float algorithm?
Jan 21
parent pineapple <meapineapple gmail.com> writes:
On Sunday, 22 January 2017 at 01:54:59 UTC, pineapple wrote:
 When you write `real x = 0.0005;` x in fact represets a value 
 of about 
 0.000500000000000000000032187251995663412884596255025826394. 
 This is a about 3.2 * 10^-23 more than 0.0005.

 The output of my function in this case was about 
 0.000499999999999999999979247692792269641692826098733348771. 
 This is a about 2.1 * 10^-23 less than 0.0005.
I might have added, for clarity, that there are no representable values in between these two, 0.0005 or otherwise.
Jan 21