On Fri, 2005-04-15 at 17:49, Dave Korn wrote: > ----Original Message---- > >From: Richard Earnshaw > >Sent: 15 April 2005 17:42 > > > On Fri, 2005-04-15 at 15:50, Robert Dewar wrote: > >> Richard Earnshaw wrote: > >> > >>> Not all environments can change the rounding mode dynamically. For > >>> example, on the FPA co-processor for ARM, rounding is set by the > >>> instruction selected -- so the concept of having an environment variable > >>> to control this is meaningless. > >> > >> Right, such a feature obviously does not apply to hardware that is this > >> far from the IEEE standard (which requires dynamic control). The Alpha > >> architecture has similar problems. > > > > Precisely which statement in IEEE 754-1985 states this? As far as I can > > tell this standard is a mathematical abstraction for mapping floating > > point onto machine types. Chapter 4 describes the required rounding > > modes but definitely makes no statement saying that this has to be > > selectable in a dynamic manner. > > > > R. > > Doesn't the C language spec require the mode to be switchable at runtime? > > In any case, the ARM or Alpha isn't prevented from working in such a > fashion just because the rounding mode is encoded in the instruction; it > just means that fp primitives have to be compiled as intrinsics that test > whatever flag controls the rounding mode and then branch to an fp insn of > the appropriate form.
c90 doesn't require anything; it's all new in c99. C99 has a number of functions and defines a number of macros *iff* the environment supports rounding switching. It certainly doesn't require it. Code that changes the rounding has to be wrapped with certain pragmas to inform the compiler that this might happen: on something like ARM+FPA that might permit dynamic code switching, but I doubt anyone would really want to pay that price. R.
