[compiler-rt] r250365 - builtins: Expand out floating point exponents for MSVC

[Yaron Keren via llvm-commits]

The constants could be constructed bit-exact for MSVC using a union and a
cast, nicely wrapped in a template or macro.

I thought something like this could be found in
http://julialang.org/openlibm/ but they actually using the decimal
representation in the C functions.



2015-10-22 18:13 GMT+03:00 Stephen Canon via llvm-commits <
llvm-commits at lists.llvm.org>:

> This is a bit ridiculous, and dangerous too.
>
> Hexadecimal FP constants are an essential tool for numerical library
> work.  This change obfuscates the code.  The comments help somewhat, but
> there’s no way for a casual reader to see that the values are the same.
> They need to either take your word for it, or check each one.  The hex
> constants are clear and unambiguous.
>
> Decimal-binary conversions are hard to get right, so this is also prone to
> introducing bugs (MSVC itself has been documented to have bugs in
> decimal-to-binary conversion routines, the very routines you’re depending
> on here:
> http://www.exploringbinary.com/incorrectly-rounded-conversions-in-visual-c-plus-plus/).
> A bug introduced in that way would likely give people slightly wrong
> answers for a long time before it was noticed.
>
> MSVC support is great, but at some point obfuscating code and risking bugs
> to support a compiler that so willfully refuses to support 25-year-old
> basic engineering tools is ridiculous.  If we really need to avoid
> hexadecimal FP to support MSVC (ugh), *please* specify their representation
> as hexadecimal integer literals rather than their value as decimal
> literals, to avoid any risk of decimal-to-binary conversion producing the
> wrong value with MSVC or some other compiler.
>
> – Steve
>
> > On Oct 15, 2015, at 12:26 AM, Saleem Abdulrasool via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
> >
> > Author: compnerd
> > Date: Wed Oct 14 23:26:19 2015
> > New Revision: 250365
> >
> > URL: http://llvm.org/viewvc/llvm-project?rev=250365&view=rev
> > Log:
> > builtins: Expand out floating point exponents for MSVC
> >
> > MSVC 2013 doesnt support C99 fully, including the hexidecimal floating
> point
> > representation.  Use the expanded value to permit building with it.
> >
> > Patch by Tee Hao Wei!
> >
> > Modified:
> >    compiler-rt/trunk/lib/builtins/floatdidf.c
> >    compiler-rt/trunk/lib/builtins/floatundidf.c
> >
> > Modified: compiler-rt/trunk/lib/builtins/floatdidf.c
> > URL:
> http://llvm.org/viewvc/llvm-project/compiler-rt/trunk/lib/builtins/floatdidf.c?rev=250365&r1=250364&r2=250365&view=diff
> >
> ==============================================================================
> > --- compiler-rt/trunk/lib/builtins/floatdidf.c (original)
> > +++ compiler-rt/trunk/lib/builtins/floatdidf.c Wed Oct 14 23:26:19 2015
> > @@ -32,8 +32,8 @@ ARM_EABI_FNALIAS(l2d, floatdidf)
> > COMPILER_RT_ABI double
> > __floatdidf(di_int a)
> > {
> > -     static const double twop52 = 0x1.0p52;
> > -     static const double twop32 = 0x1.0p32;
> > +     static const double twop52 = 4503599627370496.0; // 0x1.0p52
> > +     static const double twop32 = 4294967296.0; // 0x1.0p32
> >
> >       union { int64_t x; double d; } low = { .d = twop52 };
> >
> >
> > Modified: compiler-rt/trunk/lib/builtins/floatundidf.c
> > URL:
> http://llvm.org/viewvc/llvm-project/compiler-rt/trunk/lib/builtins/floatundidf.c?rev=250365&r1=250364&r2=250365&view=diff
> >
> ==============================================================================
> > --- compiler-rt/trunk/lib/builtins/floatundidf.c (original)
> > +++ compiler-rt/trunk/lib/builtins/floatundidf.c Wed Oct 14 23:26:19 2015
> > @@ -32,9 +32,9 @@ ARM_EABI_FNALIAS(ul2d, floatundidf)
> > COMPILER_RT_ABI double
> > __floatundidf(du_int a)
> > {
> > -     static const double twop52 = 0x1.0p52;
> > -     static const double twop84 = 0x1.0p84;
> > -     static const double twop84_plus_twop52 = 0x1.00000001p84;
> > +     static const double twop52 = 4503599627370496.0; // 0x1.0p52
> > +     static const double twop84 = 19342813113834066795298816.0; //
> 0x1.0p84
> > +     static const double twop84_plus_twop52 =
> 19342813118337666422669312.0; // 0x1.00000001p84
> >
> >       union { uint64_t x; double d; } high = { .d = twop84 };
> >       union { uint64_t x; double d; } low = { .d = twop52 };
> >
> >
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