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. > 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. -- Michael _______________________________________________ linaro-toolchain mailing list linaro-toolchain@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-toolchain
