Re: [PATCH][RFC] fix strtod int optimization in non-nearest rounding mode

[Rich Felker <dalias@libc.org>]

On Sun, Sep 04, 2016 at 04:51:03AM +0200, Szabolcs Nagy wrote:
> the mid-sized integer optimization relies on lnz set up properly
> to mark the last non-zero decimal digit, but this was not done
> if the non-zero digit lied outside the KMAX digits of the base
> 10^9 number representation.
> 
> so if the fractional part was a very long list of zeros (>2048*9 on
> x86) followed by non-zero digits then the integer optimization could
> kick in discarding the tiny non-zero fraction which can mean wrong
> result on non-nearest rounding mode.
> 
> strtof, strtod and strtold were all affected.

And *scanf. :-)

> ---
> the dc counter is long long, but lnz is int, so this is
> not correct on 64bit targets where dc can be >INT_MAX,
> probably lnz should be just set to KMAX*9 or similar in
> this case.

The issue is not exclusive to 64-bit targets. You can scanf a >2GB
decimal string on 32-bit too. We could just set lnz to match the
location where the |=1 takes place (on the order of KMAX*9), or we
could just make lnz long long (it's only used one other place). Your
approach is probably nicer though.

Rich