Date: Fri, 15 Nov 2013 14:42:25 -0500 From: Rich Felker <dalias@...ifal.cx> To: musl@...ts.openwall.com Subject: Re: question float.h & powerpc On Fri, Nov 15, 2013 at 12:25:07PM -0600, James Gregurich wrote: > Hi. > > I’m puzzled by the long double settings for powerpc. > > bits/float.h has > [...] > > I believe that LDBL_MANT_DIG of 53 is for an 8 byte long double, but > powerpc uses 16 byte long doubles and has a LDBL_MANT_DIG value of > 106. Why is there a discrepancy? I note that the i386 value is > correct at 64. What do I need to do to reconcile this difference to > allow the library’s configure script to complete the long double > test correctly? This is a complex issue. There are multiple powerpc ABIs; at one point in the not-so-distant past, everyone was using long double == double on powerpc. GCC then added an IBM double-double ABI (presumably to match IBM) and glibc adopted it. musl, however, uses the old long double definition. The reasoning behind this is fairly simple: musl aims to always provide IEEE floating point semantics, and internally _depends_ on long double having IEEE semantics. IBM double-double, on the other hand, behaves very badly and does not conform to IEEE requirements. So supporting the double-double version of 128-bit long double was out of the question. The alternative, which was considered, was supporting IEEE quad, which is also 128 bits but has a different representation. Unfortunately, there's essentially no hardware support for this format, so it would end up being soft-float, and this would in turn end up pulling 128-bit (heavy) soft-float code into every single static binary using printf. Aside from that, the feeling of most people who contributed to the decision making at the time was that it only makes sense to have long double larger than double when the _hardware_ actually provides a floating point type larger than IEEE double. Using long double as a slow emulated type was deemed a poor choice. (Note that IBM double-double is also an emulated type, but it's optimized for being easy to emulate at the expense of providing correct floating-point semantics.) In addition to that, I was already diverging musl from the glibc ABI on powerpc by not supporting the old insecure (runtime-generated code in writable memory) PLT format for dynamic linking, and only supporting the new "secure PLT" mode. So being different elsewhere in the ABI was low-cost. GCC has options to easily switch the compiler to using 64-bit long double on powerpc. I'm not sure about clang, though. Rich
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