On Wed, Mar 14, 2001 at 11:11:47PM +0100, Kevin D. Kissell wrote: > > Don't think of the r8000; the kernel only uses the -mcpu=r8000 option > > because the Nevada CPUs have _somewhat_ similar scheduling properties > > to the R8000. This of it as an independant ISA expension which can > > be used with an arbitrary MIPS processor - even a R3000 processor. > > In the interests of historical accuracy and general pedantry, > let me point out that -mcpu=r8000 is in effect a rather klugy > way of saying "-mips4" to compilers that predate official > MIPS IV ISA support. The R8000 was the first MIPS IV > CPU, followed by the R10000 and the R5000. The "Nevada" > processors added three implementation specific instructions > to the MIPS IV ISA: MAD, MADU, and MUL (targeted multiply). > "Correct" usage would be to enable those three instructions > with a "-mcpu=nevada", or better still, "-mcpu=r5200" (for > consistency), and to enable the rest of the MIPS IV ISA > with "-mips4" instead of the archaic r8000 hack. Your historic facts may be right but the GCC fact aren't. -mcpu=xxx tell GCC to schedule instructions for a certain processor xxx. This does not enable the full use of it's instruction set. Back in time when I choose these options I choose because GCC didn't know -mcpu=r5000 but the R8000 was supported and it was the closest fit. Gcc 1.1.2 knows this option so I just changed all instances of -mcpu=r8000 into -mcpu=r5000. > Note, furthermore, that -mmad needs to be handled with care: > Prior to MIPS standardizing the instruction as part of > MIPS32, there were four or five subtly (or not so subltly) > different definitions of integer multiply-accumulate for MIPS. > Most use the same opcode, but even those can differ in side > effects (is the rd field interpreted, etc.). A R4650 madd operation > will probably behave equivalently on a Nevada CPU, > but certainly not on a Vr41xx part, for example. Unfortunately true but there is a reason that QED's manual marks it as an proprietary extension ... Ralf
