On Thu, Aug 19, 2004 at 03:17:10PM +0100, Dominic Sweetman wrote: > > Jun Sun, > > > I am looking into porting NPTL to MIPS. Just curious if > > anybody has tried this before. > > > > I notice there was a discussion about the ABI extension > > for TLS (thread local storage) support. Before that support > > becomes a reality it seems one can still use NPTL with > > the help of additional system calls. > > Well, this is an area of substantial interest to MIPS Technologies. > We are working on our multi-threading extension to the MIPS > architecture, and one of our longer-term aims is to achieve really > good NPTL performance. > > As you said, this motivates a revision of the ABI. Any incompatible > change to the ABI is painful, since everything has to be recompiled; > so our reasoning at the moment is that we might as well be radical... > > MIPS Technologies would write up the definition; make and circulate > changes to the compiler, binutils and debugger; and make and circulate > supporting changes to the kernel and glibc. We're aware this is only > a part of the problem, so we do need to figure out what will attract > community approval. > > A few years ago I was volubly arguing in favour of keeping o32 until > we could move to n32/n64. But times change: SGI compatibility > no longer seems important, and Linux on 32-bit MIPS CPUs needs > long-term support. > > In many ways it's easier to get away from the ingenious but > troublesome stack-structure-based calling convention. The "stack > structure" trick was invented so that o32 could work without function > prototypes - but function prototypes are now required, and something > with fixed-size arguments is simpler. Something like the old > Cygnus/WRS EABI proposal, in fact... > > 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? > I am not against any of those. However, from NPTL implementation point of view I really just care about the per-thread register. What is the motivation of other changes? Taking this chance to make things all right in one shot? Jun
