[ruby-core:54575] [ruby-trunk - Bug #8299] Minor error in float parsing

Issue #8299 has been updated by naruse (Yui NARUSE).


phasis68 (Heesob Park) wrote:
> 2013/4/25 naruse (Yui NARUSE) <[e-mail]>:
>  >
>  > Issue #8299 has been updated by naruse (Yui NARUSE).
>  >
>  >
>  > phasis68 (Heesob Park) wrote:
>  >> 2013/4/25 naruse (Yui NARUSE) <[e-mail]>:
>  >>  > Therefore use SSE2 rather than such workaround.
>  >>
>  >>  I'm not sure why you think SSE2 can fix this issue.
>  >>
>  >>  The ruby_strtod function used in converting string value to double
>  >>  value requires double-precision (53-bit) rounding precision but
>  >>  mingw32 gcc 4.5.2 have default 64-bit precision which higher than
>  >>  other compilers.
>  >>
>  >>  So the patch lowers precision from 64 bit to 53 bit.
>  >
>  > double arithmetics with SSE2 is double-precision.
>  > see also gcc's -mfpmath=sse option
>  > http://gcc.gnu.org/onlinedocs/gcc-4.0.0/gcc/i386-and-x86_002d64-Options.html#index-march-959
>  
>  I agree that SSE2 is the better solution for modern OS and modern compiler.
>  
>  But SSE2 has more restrictions than x87.
>  SSE2 is not supported on the following environment.
>     Microsoft Visual C++ Compiler prior to Visual Studio .NET 2003.

Use newer compiler.

>     AMD CPUs prior to Athlon 64, including all Socket A-based CPUs
>     Intel CPUs prior to Pentium 4

Use -mfpmath=sse,387.
As I wrote before, if you want to get the same result with x87 FPU _control87(_PC_53, _MCW_PC) is not sufficient.
It needs to handle 15 bit exponent.
If you want to do that, it is as hard as implementing strictfp of Java on x87.
see also http://math.nist.gov/javanumerics/reports/jgfnwg-01.html
http://www.shudo.net/java-grandprix99/strictfp/#JGNWG98-2 (Japanese)
----------------------------------------
Bug #8299: Minor error in float parsing
https://bugs.ruby-lang.org/issues/8299#change-38900

Author: bobjalex (Bob Alexander)
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 
ruby -v: trunk
Backport: 


I encountered a float that either parses [slightly] differently (or converts to string differently) in Ruby than it does in Python or Java. This looks like a  Ruby bug since the result "looks" incorrect.

It is easily reproduced by entering the magic number (-1.1505945E-5) into irb. It behaves the same in 2.0 and 1.9. I'm using Windows.

Below is an irb session that demonstrates. Also included are JRuby and Python trials that show better behavior.

This issue is not causing me any problems, but just in case someone there is interested in looking into it...

Bob

>ruby -v
ruby 2.0.0p0 (2013-02-24) [i386-mingw32]

>irb
irb(main):001:0> RUBY_VERSION
=> "2.0.0"
irb(main):002:0> -1.1505945E-5
=> -1.1505945000000001e-05

>ruby19 -v
ruby 1.9.3p392 (2013-02-22) [i386-mingw32]

>irb19
irb(main):001:0> RUBY_VERSION
=> "1.9.3"
irb(main):002:0> -1.1505945E-5
=> -1.1505945000000001e-05
irb(main):002:0>

>jirb
irb(main):001:0> -1.1505945E-5
=> -1.1505945e-05

>python
Python 2.7.4rc1 (default, Mar 24 2013, 14:34:32) [MSC v.1500 32 bit (Intel)] on
win32
Type "help", "copyright", "credits" or "license" for more information.
>>> -1.1505945E-5
-1.1505945e-05
>>> repr(-1.1505945E-5)
'-1.1505945e-05'




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