Thanks for the detailed explanation. I now realize that there are many different values that are similarly affected.
Cheers, Rob On Wed, May 8, 2024 at 1:09 AM LIU Hao <lh_mo...@126.com> wrote: > 在 2024-05-07 19:05, sisyphus 写道: > > Although I often test 32-bit gcc on Windows, I don't actually rely on > those > > compilers - so I don't personally need a fix. > > I just wanted to mention this issue in case it's in the "public > interest". > > It looks like this is caused by `-fexcess-precision=standard`, as in GCC > manual: > > -fexcess-precision=style > > This option allows further control over excess precision on machines > where floating-point operations occur in a format with more precision > or range than the IEEE standard and interchange floating-point types. > By default, -fexcessprecision=fast is in effect; this means that > operations may be carried out in a wider precision than the types > specified in the source if that would result in faster code, and it is > unpredictable when rounding to the types specified in the source code > takes place. When compiling C or C++, if -fexcessprecision=standard is > specified then excess precision follows the rules specified in ISO C99 > or C++; in particular, both casts and assignments cause values to be > rounded to their semantic types (whereas -ffloat-store only affects > assignments). This option is enabled by default for C or C++ if a > strict conformance option such as -std=c99 or -std=c++17 is used. > -ffast-math enables -fexcess-precision=fast by default regardless of > whether a strict conformance option is used. > > -fexcess-precision=standard is not implemented for languages other > than C or C++. On the x86, it has no effect if -mfpmath=sse or > -mfpmath=sse+387 is specified; in the former case, IEEE semantics > apply without excess precision, and in the latter, rounding is > unpredictable. > > > By default the x87 FPU is used for floating-point arithmetic, so > `1.7976931348623157e+308` is more > precise than `double` and does not equal `0x1.fffffffffffffp+1023` due to > the extra significant bits. > > x86-64 is not subject to this, as SSE registers are exact `double` and > there is no excess precision. > > > -- > Best regards, > LIU Hao > > _______________________________________________ Mingw-w64-public mailing list Mingw-w64-public@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/mingw-w64-public
