Hi Carl! I very much think that 80 bits should be the default for long doubles, even if Microsoft does differently.
On Mon, Nov 24, 2014 at 3:57 AM, Carl Kleffner <cmkleff...@gmail.com> wrote: > I don't see the point, why computation accuracy has to be set to 0x37f per > default. Usually computations are done with single or double precision > today. Using 80bit long double precision is outdated 80-bit long doubles are hardly outdated. Many people use them and rely on them. > and non portable. That's quite a specious argument, given x86 (and other) support for 80-bit precision dating back to the 8087. Sure, hardware support for 80-bit doubles is not portable across all platforms (nor is support for 64-bit doubles, nor for 32-bit doubles, etc., etc.), but that's no reason to make mingw-w64 run suboptimally on very widespread hardware. The article by Kahan that Yaron linked to makes a very good argument (in the context of java) that exact compatibility / portability is a bad idea (and an illusory goal in any event). (He also has a few choice words about Microsoft's bad choice on this issue, as well he should.) > It is > just fine to have the possibilty to use 80bit longs doubles if needed (not > possible with MSVC). Nevertheless I suggest double computation precision > should be the default and extended double computation precision an option. > Silently change the FPU settings can cause bad side effects (see the link on > my first posting). Although polemical in tone, Kahan's explains things very well. Not using extended precision is sufficiently worse than using it that not doing so can be considered wrong. So, if Microsoft had a sqrt function that gave sqrt(16) == 5, would you ask that mingw-w64 do the same in the name of compatibility with Microsoft? On common modern hardware, float is 32 bits, double is 64 bits, and common modern hardware supports 80-bit doubles. long double is a different type from double. Why force long double and double to be the same 64 bits, when 80 bits is supported by hardware? You want 64 bits, use double. I want 80 bits, I'll use long double. Why make it hard (or impossible) for me to use 80-bit long doubles just because you want 64-bit doubles even if you accidentally type "long double"? Read Kahan. He is absolutely an expert in this kind of thing (and has been making these kind of arguments since before I was born, which is saying something). Mingw-w64 has the choice of doing what Microsoft does, or following Kahan's recommendation. Given that choice, it's Kahan, hands down. Trust me -- if you become an expert on numerical analysis and floating-point computation, you will agree with Kahan. But you don't need to become an expert -- just follow the experts' advice. (And if the experts' advice differs from what Microsoft and python and java, etc., etc. do, follow the experts' advice anyways.) > > 2014-11-23 17:39 GMT+01:00 Yaron Keren <yaron.ke...@gmail.com>: >> >> It is a real incompatibility between VC and mingw. Visual C++ dropped >> support for 80 bit long doubles on x87 many years ago, in VC long doubles >> are 64 bit, which is why they use 0x27F. mingw still supports 80 long >> doubles on x87. As long this is the case, it is incosistent for mingw to set >> computation accuracy to 0x27f. >> >> The 'long double' size incompatibility requires mingw to implement various >> long-double functions including printf and scanf, independently of MSVCRT. >> >> So the decision is whether long doubles should be 64 or 80 bits. The >> control word is simply the result of this decision. >> >> BTW. see Kahan about long doubles (in Java, though), >> http://www.cs.berkeley.edu/~wkahan/JAVAhurt.pdf Thank you Yaron -- this is an excellent reference. It's well worth reading (as is most anything by Kahan). Happy Floating-Point Hacking! K. Frank ------------------------------------------------------------------------------ Download BIRT iHub F-Type - The Free Enterprise-Grade BIRT Server from Actuate! Instantly Supercharge Your Business Reports and Dashboards with Interactivity, Sharing, Native Excel Exports, App Integration & more Get technology previously reserved for billion-dollar corporations, FREE http://pubads.g.doubleclick.net/gampad/clk?id=157005751&iu=/4140/ostg.clktrk _______________________________________________ Mingw-w64-public mailing list Mingw-w64-public@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/mingw-w64-public
