On the whole, you should not worry about which types are "more efficient" - the CPU architecture and implementation and the compiler between them will generally do a pretty good job. If the function is inlined, the compiler doesn't need to follow the procedure call standard for this value and the uxtb will likely disappear. Only one truncation is needed, at the end - and that it only needed because the procedure call standard requires the spare bits to be zero when returning a value of type unsigned char. The upper bits of the intermediate result in r0 contain garbage in the form of overflowed bits, but the compiler knows that this doesn't affect the bits that are important for the result. the compiler may generate code like this for the core of sum(): ldrb r1, For example, in the following code: extern unsigned char c So the cost is not always incurred.Īs a follow-on to Chris' comment about type conversions often coming for free, it's worth pointing out that compilers also know that there is no need to do type conversions for intermediate results. For example, if you are simply going to store the result of a computation to memory, it may be that the necessary truncation can be carried out (for free) by the appropriate STRB/STRH instruction. In many cases, these can be avoided though. Computation can then be carried out at natural size while memory footprint can be managed as necessary with very little (if any) overhead.Īs Jens points out above (very kind of you to say so, but I'm not sure I'm the world's greatest expert on this!) the extra overhead is required when carrying out computations on sub-word quantities. LDRSB/LDRH/LDRSH carry out the conversion and/or sign extension as part of the load) values can be automatically converted back to byte/halfword on storing (STRB/STRH automatically remove upper bits). Values can be automatically converted from byte/halfword to word on loading (LDRB The ARM architecture is actually very good for this. But there are also good reasons for, in some circumstances, wanting to store variables in the smallest possible container (reduces memory footprint, reduces cache pollution etc.) You are right that there are good reasons for carrying out computations using the natural word size of the machine - as you say, this avoids extra instructions to deal with truncation, rounding, normalization etc. What you have here is the tension between storage size and computation size. Most people assume that a long is 32-bit, but it does not have to be. The reason is that you don't really know the size of a long or short or int or long long. To use those, just include, or compile as C99 or newer. I recommend using uint8_t, uint16_t, uint32_t and uint64_t, plus their signed siblings int8_t, int16_t, int32_t and int64_t instead of unsigned char/short/long/long long, etc. On some architectures (for instance, Cortex-M), this will take extra time. Instead, var1_8 will contain the value 256, so the compiler will need to wrap this to 0 by clearing the upper 24 bits. Var1_8 is loaded into a register and since it's now in a 32-bit register, incrementing it will not wrap it to 0, when the value 255 is incremented. This is something that chrisshore is an exper t on. something will happen, that makes using an 8-bit value slower than using a 32-bit value. But I imagine that the variables do not always contain the same values, so imagining the code. Even if you're reading the values from memory, the speed would be the same. Installation is fast, simple, and free.I agree. Ready to give it a try? Download Grammarly for Microsoft Word on your Mac or your PC today. You’ll see suggestions for making your writing clearer, improving your word choice, and polishing your writing in all kinds of exciting ways-absolutely no copying and pasting required. As you write your document, Grammarly’s comprehensive writing feedback will appear on the right side of the page. Grammarly for Microsoft Word is designed to fit into your writing process. And if you’re a fan of Word Online, we’re happy to tell you that Grammarly now works there, too! We heard loud and clear that you needed a Mac version, so we’ve been hard at work to make that happen. Grammarly for Microsoft Word has been available for some time, but until now, it was only compatible with the Windows version of Word. Great news, Mac users! You can now add Grammarly to Microsoft Word ! That means you can see and apply Grammarly’s suggestions for making your writing stronger and clearer without leaving the document you’re working on. This article was originally published on March 31, 2020.
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