Dominic Sweetman wrote: > So we're proposing: > > o The register name<->number mapping is that of n64. > > o Calling convention: register-, not slot-based. Each argument is > represented by a register value. Arguments 0-7 travel in registers > a0-7 (or fa0-7 as required for floating point types). If there are > more than eight arguments, further ones are formed as if put in a > register and then saved on the stack into a 64-bit slot (more than 8 > arguments is rare enough that we can afford to standardise on the > big slots). > > o Use floating point registers for double and float arguments, and > integer registers for all integer/pointer values which will > fit. Larger or structured data items are implicitly passed by > reference: to maintain pass-by-value semantics, the compiler uses a > copy-on-write trick if software writes a by-reference argument (or > takes its address). I'm told gcc is happy enough to do that. > > o The return value comes back in two registers, with the second > return-register used only when the return value consists of two > scalars (ie a complex or double-precision number). [Folklore insists > this is essential for Fortran support of complex numbers, and I > don't want to fight folklore]. > > All other non-scalar return values are returned via a pointer > specified by the caller as an implicit first argument. > > o Reserved registers: all the traditional ones. But now: > > - gp will be the GOT pointer in Linux, and should be defined as > saved (ie a function must preserve values in this registers, which > means it will need to save-and-restore the register if it is > written locally). > > - we'll define some other register as a per-thread data pointer. > > Some details are still to be worked out. But do you think this is on > the right lines? And who would like to take an active part in > specifying or reviewing? > All of this sounds good to me. However my current concerns are how to make my code run on a mips32[r2] core with no floating point. We are using several different systems with variations of this cpu type. So for me, making sure that a soft-float variant of the ABI is well specified is also important. I suppose it would be to treat float/double values as appropriate encoding of 32/64 bit integer values. David Daney.
