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| David Brown <david.brown@hesbynett.no>: May 12 10:29AM +0200 On 11/05/17 21:45, Jerry Stuckle wrote: >> On 11/05/17 15:59, Jerry Stuckle wrote: >>> On 5/11/2017 2:27 AM, Gareth Owen wrote: >>>> David Brown <david.brown@hesbynett.no> writes: <snipping the bits we agree on> >> of it all without an inordinate amount of time and effort. > In practice, yes. There are limits to both registers and pipelines > which a good programmer can handle. The same with scheduling. It gets /really/ fun on a processor like MIPS, with branch delay slots to consider! > But I > disagree that a small change in one part of the assembly will have any > noticeable effect on unrelated code. "/will/ have a noticeable effect" implies that is always or usually the case - I said "/can/", because it is something that /can/ happen. I don't think it is common, but it is absolutely possible. One example situation is where you are trying to keep all your relevant data in registers. Changes in one part of the function can mean you need different priorities in a different part - either earlier or later. It might not mean /big/ changes, but it will mean changes. In C, you can make another local variable when you want it - in assembly, if you run out of registers you need to make changes. Another case is when you have limited branch or conditional execution capability. Some processors have support for small and fast versions that are limited in size. For example, the ARM Thumb-2 instruction set has an "if-the-else" construct that supports up to four conditional instructions. Make a change that goes over that limit of four, and you have to re-structure the code to use conditional branches. This kind of thing is usually more relevant on RISC cpus than x86 (especially in 32-bit mode) where your register usage is already very limited in flexibility. All this might not have a noticeable effect on code performance - but it does have an effect on the time and effort spent. >> the time on the tedious detail, and has a high chance of getting at >> least something wrong. > In some cases, no amount of effort is "too much". True. > code. Data collection and analysis of large scientific projects such as > the VLA (large array of radio telescopes), the NSF's LIGO (gravitational > wave detector) and CERT's Large Hadron Detector are just three examples. No, those are /not/ good examples. It is true that minimising latencies here can be very important - but it is not the case that it is done by using assembly. For these sorts of systems, coding is done carefully in C or C++ for maintainability, correctness, flexibility, and lower development costs. When the smallest latencies are needed (and the smallest variation in latencies, which is usually more important), they switch to programmable logic. Assembly gives very little benefit over C or C++ in terms of speed, no benefit in terms of latency variation (it is still subject to caches, interrupts, etc.), and much higher development costs and risks. Programmable logic lets them get latency variation down to a single clock cycle - it is worth the cost. On the other hand, these same groups need to do massive analysis of the data afterwards - passing it through filters, fourier transforms, etc., that need as high throughput as possible but which can tolerate variations in timings. This sort of thing is done on arrays of commodity processors (or perhaps graphics processors). Here it can be worth making the kernels of these filters in assembly - you know the target processor, and saving 2% average time will save 2% hardware budget and 2% electricity costs. It is absolutely worth the effort going for assembly there. > It is perfectly possible to write clean and maintainable assembly code - > and make it the fastest possible result. It's done every day at each of > the above projects. Possible? Yes. But it is a rare thing - especially on "big" processors. It is a different matter for small microcontrollers, where it is far easier to track exactly what you are doing because there is a small instruction set, no scheduling or multiple issues, very clear and simple pipelines, no caches, etc. >> unrealistic to think that it /would/ be done, except in extreme cases. > As I said - there are many instances in the scientific world where that > is necessary. And there are programmers who do it. I agree that it /is/ done, I merely say that it is very rarely worth the effort. Massive scientific processing is an example where it sometimes /is/ worth the effort. |
| asetofsymbols@gmail.com: May 12 10:06AM -0700 Woman and computer it will be 3% |
| fl <rxjwg98@gmail.com>: May 12 07:07AM -0700 On Thursday, May 11, 2017 at 3:13:40 PM UTC-4, Alf P. Steinbach wrote: > > { *this = v; } > Cheers & hth., > - Alf Thank you for your help. When I run below code, I find one calls assignment while the other calls copy constructor. It is obvious it is caused by the template parameter <W>. It is still a surprise to me. I thought both should call copy constructor. Can you explain it to me? Thanks, ////////////////// const sc_biguint<4> atomwrj1(2); sc_biguint<3> atomwrj0=atomwrj1; // Thus calls assignment constructor sc_biguint<4> atomwrj2=atomwrj1; // This calls copy constructor // copy constructor sc_biguint( const sc_biguint<W>& v ) : sc_unsigned( W ) { *this = v; } // assignment constructor sc_biguint( const sc_unsigned& v ) : sc_unsigned( W ) { *this = v; } |
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