From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <490AD875.2040101@domain.hid> Date: Fri, 31 Oct 2008 11:05:41 +0100 From: Gilles Chanteperdrix MIME-Version: 1.0 References: <49076E08.5060708@domain.hid> <49077D56.8040908@domain.hid> <49098653.7060709@domain.hid> <490ABF6D.5090806@domain.hid> In-Reply-To: <490ABF6D.5090806@domain.hid> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-core] llimd. List-Id: "Xenomai life and development \(bug reports, patches, discussions\)" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: Xenomai core Jan Kiszka wrote: > Gilles Chanteperdrix wrote: >> Jan Kiszka wrote: >>> Gilles Chanteperdrix wrote: >>>> Hi Jan, >>>> >>>> I see that the implementation of rthal_llmulshft seems to account for >>>> the first argument sign. Does it work ? Namely, in the generic >>>> implementation will __rthal_u96shift propagate the sign bit ? >>> Yes, this works (given there is no overflow, of course). If you consider >>> a high word of 0xfffffff0 and a (right) shift of 8, we effectively cut >>> off all the leading 1s: high << (32-8) = 0xf0000000. But this only works >>> because we replace a right shift with a left shift (plus some OR'ing >>> later on). If we had to do a real right shift, we would also have to >>> take signed vs. unsigned into account (ie. shift in zeros or the sign >>> bit from the left?). >>> >>>> If yes, do you see a way llimd could be made to work the same way ? This >>>> way we would avoid inline ullimd twice in llimd code. >>> As the basic building block here is a multiplication, we cannot get >>> around telling apart signed from unsigned (or converting signed into >>> unsigned): the underlying multiplication logic is different. >>> >>> But what about this approach: >>> >>> static inline __attribute__((__const__)) long long >>> __rthal_generic_llimd (long long op, unsigned m, unsigned d) >>> { >>> int signed = 0; >>> long long ret; >>> >>> if (op < 0LL) { >>> op = -op; >>> signed = 1; >>> } >>> ret = __rthal_generic_ullimd(op, m, d); >>> return signed ? -ret : ret; >>> } >>> >>> However, I guess writing this in assembly for archs that suffer should >>> be more efficient. >> Hi Jan, >> >> You may have noticed that we played a bit with arithmetic operations >> (namely, we use an llimd without division to make the reverse of >> llmulshft), and it pays off on slow machines, such as ARM, where the >> division is done in software. >> >> At this chance, I looked at the code generated by this soluion, and I am >> not sure that it is better: on ARM, and I suspect this is true on other >> architectures, the operations needed to negate a long long clobbers the >> code conditions, which means we can not make these operations >> conditionals without a conditional jump, so the hand-coded assembler is >> not better than what the compiler does: it uses two conditional jumps >> whereas the original solution uses only one. Of course we could set sign >> to -1 or 1, and multiply by sign at the end, but the multiplication is >> probably even heavier than conditional jump. > > Yes, on the archs that matter here (32-bit). > >> So, would you have any idea of a better solution ? > > In an assembly version, one could save 'sign' in form of a jump target > that should be taken after __rthal_generic_ullimd (ie. jump to the > negation, or jump over it). Specifically when that address is kept in a > register, I think smart branch prediction units will be able to do the > right forecast. Good idea, there is even a gcc extension which allows to do this in the generic section: static inline __attribute__((__const__)) long long __rthal_generic_llimd (long long op, unsigned m, unsigned d) { void *epilogue; long long ret; if (op < 0LL) { op = -op; epilogue = &&ret_neg; } else epilogue = &&ret_unchanged; ret = __rthal_generic_ullimd(op, m, d); goto *epilogue; ret_unchanged: return ret; ret_neg: return -ret; } -- Gilles.