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Message-ID: <20140404205808.GP26358@brightrain.aerifal.cx>
Date: Fri, 4 Apr 2014 16:58:08 -0400
From: Rich Felker <dalias@...ifal.cx>
To: musl@...ts.openwall.com
Subject: Re: printf issues
On Fri, Apr 04, 2014 at 01:42:30PM -0400, Morten Welinder wrote:
> > I _would_ like this code to be easily adaptable for use outside libc
> > if somebody wants it
>
> FYI, I have been doing just that for Gnumeric in a variant that always
> rounds ties away from zero. Two changes would help with making
> the code fit seamlessly into other environments.
>
> 1. Make "i" in fmt_fp unsigned. It's used in connection with
> unsigned values only.
I don't object, but what's the motivation?
> 2. Make "char *s" used to hold "NAN" etc. "const char *s".
Indeed, this is harmless. I assume it's to satisfy that ugly option
that changes the type of string literals from char[] to const char[]?
BTW that option actually just got a lot more problematic with C11; now
that C has _Generic, it potentially changes the semantics of a program
rather than just helping generate warnings.
> Neither of these should make any difference in what the function
> actually does.
>
> I have run tens of millions random numbers through this function
> looking for differences between it and glibc. The extra 0s from "%g"
> is the only problem observed.
Nice. I was actually thinking of some numerical tests we could run on
huge random samples, for instance using using theorems about
properties of the digits of the decimal expansion (e.g. n%3==0 iff the
sum of the digits mod 3==0; this works for diadic-rational 'multiples
of 3' too).
> It looks like the LDBL_EPSILON version could be used in
>
> roundl.c
> modfl.c
> ceill.c
> floorl.c
>
> in the definition of TOINT instead of enumerating choices for
> LDBL_MANT_DIG. It's basically the same thing going on
> there.
Indeed, this would be a welcome change.
> While I was looking for that, I noticed that this modfl fallback looks
> problematic. Even if long double and double are the same thing
> under the hood, I don't think you can cast pointers like that and
> assume it works. It needs a temporary.
>
> #if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
> long double modfl(long double x, long double *iptr)
> {
> return modf(x, (double *)iptr);
> }
Agreed. This is UB (an aliasing violation) and should be fixed even if
it makes the function a few bytes larger/a few cycles slower.
Rich
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