On Mon, 14 Aug 2023 at 10:58, Paul Dreik via Libstdc++ < libstd...@gcc.gnu.org> wrote:
> The patch below fixes an issue with the fix already committed for > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110860 which unfortunately > was not sufficient for small magnitude floating point values. > > With the patch in place, the code now survives the fuzzing I used to > find the problem in the first place. Tested on amd64. > > I prepared the patch using git show, which should include the signoff as > instructed per the DCO. > I couldn't apply the patch directly from the email, but I'm not sure where it got mangled. I just applied it by hand instead. Pushed to trunk, thanks for the patch! I'll push it to gcc-13 shortly too. > > Thanks, Paul > > ------------------------------------------------------------------------ > commit 848b8d948787495e64ed9c55d681eccf730b74fb > Author: Paul Dreik <gccpatc...@pauldreik.se> > Date: Mon Aug 14 11:52:30 2023 +0200 > > libstdc++: Avoid problematic use of log10 in std::format [PR110860] > > If abs(__v) is smaller than one, the result will be on the > form 0.xxxxx. It is only if the magnitude is large that more digits > are needed before the decimal dot. > > This uses frexp instead of log10 which should be less expensive > and have sufficient precision for the desired purpose. > > It removes the problematic cases where log10 will be negative or not > fit in an int. > > Signed-off-by: Paul Dreik <gccpatc...@pauldreik.se> > > diff --git a/libstdc++-v3/include/std/format > b/libstdc++-v3/include/std/format > index f4520ff3f..729e3d4b9 100644 > --- a/libstdc++-v3/include/std/format > +++ b/libstdc++-v3/include/std/format > @@ -1490,14 +1490,22 @@ namespace __format > // If the buffer is too small it's probably because of a > large > // precision, or a very large value in fixed format. > size_t __guess = 8 + __prec; > - if (__fmt == chars_format::fixed && __v != 0) // +ddd.prec > + if (__fmt == chars_format::fixed) // +ddd.prec > { > - if constexpr (is_same_v<_Fp, float>) > - __guess += __builtin_log10f(__v < 0.0f ? -__v : __v); > - else if constexpr (is_same_v<_Fp, double>) > - __guess += __builtin_log10(__v < 0.0 ? -__v : __v); > - else if constexpr (is_same_v<_Fp, long double>) > - __guess += __builtin_log10l(__v < 0.0l ? -__v : __v); > + if constexpr (is_same_v<_Fp, float> || is_same_v<_Fp, > double> || > is_same_v<_Fp, long double>) > + { > + // the number of digits to the left of the decimal > point > + // is floor(log10(max(abs(__v),1)))+1 > + int __exp{}; > + if constexpr (is_same_v<_Fp, float>) > + __builtin_frexpf(__v, &__exp); > + else if constexpr (is_same_v<_Fp, double>) > + __builtin_frexp(__v, &__exp); > + else if constexpr (is_same_v<_Fp, long double>) > + __builtin_frexpl(__v, &__exp); > + if (__exp>0) > + __guess += 1U + __exp * 4004U / 13301U; // > log10(2) approx. > + } > else > __guess += numeric_limits<_Fp>::max_exponent10; > } >
