[cfe-dev] APFloat::toString() handling of MS FLT_MAX and DBL_MAX

[Jordan Rose]

I just hit this myself (months later!) trying to round-trip floats through strings. I don't think the problem is printed values exceeding their maximums so much as that APFloat::toString thinks the natural precision for DBL_MAX is one digit shorter than it actually is. I'll take a look, but someone with more floating-point experience might be able to get to the bottom of this sooner.

Jordan


On Apr 16, 2013, at 8:54 , Will Wilson <will at indefiant.com> wrote:

> Sorry, I'm not being clear enough. It's nothing to do with runtime libs. Rather it's the result of printing out the expanded literal value directly from the AST. For instance:
> Clang parses the DBL_MAX macro into the AST (defined as 1.7976931348623158e+308)
> Pretty printing out the FloatingLiteral results in: 1.797693134862316E+308
> Feeding these literals back to MSVC results in: error C2177: constant too big
> So my question is: Would it be sensible to try and ensure the printed values never exceed their maximums?
> 
> Cheers,
> Will.
> 
> 
> On 16 April 2013 00:56, David Blaikie <dblaikie at gmail.com> wrote:
> 
> On Apr 16, 2013 8:01 AM, "Will Wilson" <will at indefiant.com> wrote:
> >
> > Hi All,
> >
> > I'm rewriting MS C++ code via clang and have an issue involving the printed representations of FLT_MAX and DBL_MAX. In both cases the printed literals exceed the inputs.
> >
> > From MS float.h:
> >
> > #define DBL_MAX         1.7976931348623158e+308
> > // Becomes 1.797693134862316E+308
> >
> > #define FLT_MAX         3.402823466e+38F
> > // Becomes 3.4028235E+38F
> >
> > Rewriting the values leads to compilation errors due to the literals exceeding their maximums. This doesn't altogether surprise me given the nature of FP arithmetic and string conversion but I was wondering if this could/should be treated as a bug given the significance of these values?
> 
> How are you printing these values out? Libstdc++ or libc++? Does the behavior reproduce under GCC too? I could guess/assume a bug in the standard library implementation you are using, but that's just a guess
> 
> >
> > - Will.
> >
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> 
> 
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