On Tue, 23 Sep 2014, Uros Bizjak wrote: > Hello! > > Joseph, is there any support for underflow control in soft-fp library? > >From a private correspondence with FX about implementing gfortran IEEE > support for extended modes, soft-fp that implements 128bit support on > x86 could read this setting from FPU control registers and handle > denormals accordingly.
My current series of soft-fp patches pending review on libc-alpha includes one for control of handling of *input* subnormals <https://sourceware.org/ml/libc-alpha/2014-09/msg00488.html>, because that feature is in the kernel version (and David Miller volunteered to help get the Linux kernel using the current version of soft-fp, given the features from the kernel version added to it <https://sourceware.org/ml/libc-alpha/2013-10/msg00347.html>). As in the kernel version, that implementation sets the denormal operand exception in this case, although it appears SSE does not do that (so further conditioning would need to be added for this to be an accurate x86 emulation; different architectures do different things in this case). Neither version has anything to control underflow on *output*. A flush-to-zero mode as on x86 appears to be different in directed rounding modes from abruptUnderflow as defined in IEEE 754-2008 (flush-to-zero always produces zero, abruptUnderflow can produce the smallest normal result depending on the rounding mode). Such flush-to-zero or abruptUnderflow control could of course be implemented; there might be a need for further conditions on whether the underflow and inexact exception flags are set on flushing to zero (again, there are architecture variations). (Note that such underflow control on output, at least in the architecture manuals I checked, means underflow when the architecture-specific tininess condition is met - not when the rounding result ends up subnormal - so it's not possible to implement it with a late check of the rounded result; soft-fp would need to check at the point where it decides whether the result is tiny.) I don't know whether you'd want flush-to-zero emulating architecture semantics, abruptUnderflow, or both. (When used on x86 for operations involving binary128, there's also the point that only SSE not x87 has such modes, so you'd need to consider when it's correct to apply them to binary128 operations - is it appropriate for conversions between TFmode and XFmode or not? Of course applying to all operations is easiest and avoids needing the soft-fp conditionals to depend on the operation / types involved, or having different sfp-machine.h definitions used in different source files.) -- Joseph S. Myers jos...@codesourcery.com
