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Re: macro FIXNUM_OVERFLOW_P in lisp.h is valid ?
From: |
Eli Zaretskii |
Subject: |
Re: macro FIXNUM_OVERFLOW_P in lisp.h is valid ? |
Date: |
Sat, 24 Oct 2009 15:00:03 +0200 |
> Date: Sat, 24 Oct 2009 20:59:47 +0900
> Cc: address@hidden
> From: Toru TSUNEYOSHI <address@hidden>
>
> BTW, how does Emacs on 64-bit platforms eval the following expressions?
>
> (string-to-number "1152921504606846975") ; 2^60 - 1
> => 1.1529215046068467e+018 ; on 32-bit platforms
>
> On 64-bit platforms, string-to-number should return the number as type
> `EMACS_INT' (= `LONG'), I think.
Yes, it should.
> Although, the code of string-to-number (Fstring_to_number) deals with
> the number as type `double' (of variable `v').
Yes, it does. And thus it loses least significant digits:
(string-to-number "1152921504606846975") => 1152921504606846720
> #define make_fixnum_or_float(val) \
> (FIXNUM_OVERFLOW_P (val) \
> ? make_float (val) \
> : make_number ((EMACS_INT)(val)))
>
> In this code, FIXNUM_OVERFLOW_P (val) will returns 0.
> Then Emacs processes `make_number ((EMACS_INT)(val)))'.
> On that time, `val' is casted from type `double' to `EMACS_INT'.
> At last, can string-to-number return all digits of the number
> 1152921504606846975 properly?
>
> I guessed it can't.
> That is the reason why I made my former patch.
OK, but please make your patch compare against MOST_POSITIVE_FIXNUM,
instead of using INT_MAX or LONG_MAX. Also, if you know that the
value does not overflow an EMACS_INT, you can simply call make_number,
instead of make_fixnum_or_float.
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Re: macro FIXNUM_OVERFLOW_P in lisp.h is valid ?, Andreas Schwab, 2009/10/23