On Thu, Nov 25, 2010 at 10:15 PM, Michael Hope <michael.h...@linaro.org> wrote: > On Fri, Nov 26, 2010 at 12:00 AM, Dave Martin <dave.mar...@linaro.org> wrote: >> Hi, >> >> On Wed, Nov 24, 2010 at 9:49 PM, Michael Hope <michael.h...@linaro.org> >> wrote: >>> It's a bit of a newbie question, but I've been wondering if you can >>> intermix hard float VFPv3-D16 code with VFPv3-D32 code. You can as: >>> >>> According to the ABI: >>> * d0-d15 are used for floating point parameters, no matter if you are >>> D16 or D32 >>> * d0-d15 are not preserved across function calls >>> * d16-d31 must be preserved across function calls >> >> No, I don't think that's correct - see the procedure call standard >> section 5.1.2.1 >> "VFP register usage conventions (VFP v2, v3 and the Advanced SIMD Extension)" >> >> It's not too hard to misread ... my understanding is as follows--- >> >> * d0-d7 (s0-s15; q0-q3) are not callee-saved and are the only regs >> used for parameter and return value exchange in the standard ABI >> variants >> * d8-d15 (s16-s31; q4-q7) are _callee-saved_ >> * d16-d31 (q8-q15) are _not callee-saved_ > > Ah, I got the s* and d* registers mixed up. So if you have a function > which takes doubles, the first eight parameters go in registers. If > the function takes floats, then the first sixteen go in registers. > D8-D15 are preserved across calls, D16+ aren't.
yup >> So basically, D32 code just gets to use d16-d31 for extra scratchpad >> bandwidth _in between_ external function call sites. (Of course, >> compiler-generated or hand-written code can relax the rules locally in >> some circumstances, just as for the integer ABI) > > I think the conclusion is the same: you can intermix VFP-D16 and > VFP-D32 code as D16 code doesn't use D16-D31 and D32 code doesn't > expect D16-D31 to be preserved across function calls. Yes--- agreed (actually, I misread your first paragraph and thought you were arguing that they _couldn't_ be intermixed) The ABI was deliberately specified that way to allow that AFAIK--- to VFPv3-D32 is just another "extension" which islands of code can optionally use or not use; just like v5E, v6 SIMD or NEON. Cheers ---Dave _______________________________________________ linaro-toolchain mailing list linaro-toolchain@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-toolchain
