From: Heesob Park <phasis@...>
Date: 2013-04-25T11:09:11+09:00
Subject: [ruby-core:54561] Re: [ruby-trunk - Bug #8299] Minor error in float parsing

2013/4/25 naruse (Yui NARUSE) <naruse@airemix.jp>:
>
> Issue #8299 has been updated by naruse (Yui NARUSE).
>
>
> phasis68 (Heesob Park) wrote:
>> I found the issue is due to the difference of the floating point precision.
>>
>> Here is a patch for this issue:
>>
>>     diff --git a/numeric.c b/numeric.c.new
>>     index 97ba104..c548c0f 100644
>>     --- a/numeric.c
>>     +++ b/numeric.c.new
>>     @@ -3767,6 +3767,9 @@ Init_Numeric(void)
>>          _control87(MCW_EM, MCW_EM);
>>          _control87(_control87(0,0),0x1FFF);
>>      #endif
>>     +#if defined(__MINGW32__)
>>     +    _control87(_PC_53, _MCW_PC);
>>     +#endif
>>          id_coerce = rb_intern("coerce");
>>          id_to_i = rb_intern("to_i");
>>          id_eq = rb_intern("==");
>
> It specify fraction as 53 bit (other IEEE 754 FPU's 52 bit with economized form), but its internal exponent is still 15 bit.
> For example following result should be still different (I don't tested on mingw32).
>
>   [0x0008008000000000,0x3ff0000000000001].pack("Q2").unpack("d2").inject(&:*)
>
> 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.

Regards,
Park Heesob