check if address is 16 byte aligned check if address is 16 byte aligned

How do I connect these two faces together? I am using icc 15.0.2 which is compatible togcc 4.4.7. For instance (ad & 0x7) == 0 checks if ad is a multiple of 8. This operation masks the higher bits of the memory address, except the last 4, like so. For what it's worth, here's a quick stab at an implementation of aligned_storage based on gcc's __attribute__(__aligned__, directive: A quick test program to show how to use this: Of course, in real use you'd wrap up/hide most of the ugliness I've shown here. How can I explicitly free memory in Python? uint64_t can be used more safely, additionally, the padding can be hidden away by using a bit field: I don't think you can assure 64 bit alignment this way on a 32 bit architecture @Aconcagua: indeed. Why restrict?, looks like it doesn't do anything when there is only one pointer? How Do I check a Memory address is 32 bit aligned in C. How to check if a pointer points to a properly aligned memory location? Find centralized, trusted content and collaborate around the technologies you use most. To my knowledge a common SSE-optimized function would look like this: However, how do I correctly determine if the memory ptr points to is aligned by e.g. Connect and share knowledge within a single location that is structured and easy to search. What you are doing later is printing an address of every next element of type float in your array. I'll try it. The typical use case will be 64-bit platform and pointer heavy data structures, giving me three tag bits, but I want to make sure the code still works if compiled 32-bit. // and use this pointer to read or write data into array, // dellocate memory original "array", NOT alignedArray. EDIT: Sorry I misread. Browse other questions tagged. Follow Up: struct sockaddr storage initialization by network format-string, Minimising the environmental effects of my dyson brain, Acidity of alcohols and basicity of amines. How to use this macro to test if memory is aligned? How to show that an expression of a finite type must be one of the finitely many possible values? Hence. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Why should C++ programmers minimize use of 'new'? Therefore, you need to append 15 bytes extra when allocating memory. How can I measure the actual memory usage of an application or process? An access at address 1 would grab the last half of the first 16 bit object and concatenate it with the first half of the second 16 bit object resulting in incorrect information. This is what libraries like Botan and Crypto++ do for algorithms which use SSE, Altivec and friends. Because I'm planning to use low order bits of pointers as tag bits. rev2023.3.3.43278. Connect and share knowledge within a single location that is structured and easy to search. Depending on the situation, people could use padding, unions, etc. I think that was corrected before gcc 4.4.7, which has become outdated . Because 16-byte aligned address must be divisible by 16, the least significant digit in hex number should be 0 all the time. I have to work with the Intel icc compiler. reserved memory is 0x20 to 0xE0. random-name, not sure but I think it might be more efficient to simply handle the first few 'unaligned' elements separately like you do with the last few. Making statements based on opinion; back them up with references or personal experience. In this context, a byte is the smallest unit of memory access, i.e. Thanks for contributing an answer to Stack Overflow! By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. On the other hand, if you ask for the 8 bytes beginning at address 8, then only a single fetch is needed. some compilers provide directives to make a structure aligned with n bytes, for VC, it is #prgama pack(8), and for gcc, it is __attribute__((aligned(8))). Also, my sizeof trick is quite limited, it doesn't help at all if your structure has 4 ints instead of only 3, whereas the same thing with alignof does. This memory access can be aligned or unaligned, and it all depends on the address of the variable pointed by the data pointer. So the function is doing a right thing. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Connect and share knowledge within a single location that is structured and easy to search. Data thats aligned on a 16 byte boundary will have a memory address thats an even number strictly speaking, a multiple of two. How do I set, clear, and toggle a single bit? The cast to void * (or, equivalenty, char *) is necessary because the standard only guarantees an invertible conversion to uintptr_t for void *. Lets illustrate using pointers to the addresses 16 (0x10) and 92 (0x5C). rev2023.3.3.43278. Checkweigher user's manual STX: Start byte, 02H State 1: 20H State 2: 20H State 3: 20H Mark: 1 byte When a new value sampled, this byte adds 1, this byte cycles from 31H to 39H. CPU will handle misaligned data properly, so you do not need to align the address explicitly. When you print using printf, it knows how to process through it's primitive type (float). Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Time arrow with "current position" evolving with overlay number. To take into account this issue, the C standard has alignment . If an address is aligned to 16 bytes, is it also aligned to 8 bytes? Hughie Campbell. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. What happens if address is not 16 byte aligned? Minimising the environmental effects of my dyson brain. What is a word for the arcane equivalent of a monastery? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Thanks for contributing an answer to Stack Overflow! Why are Suriname, Belize, and Guinea-Bissau classified as "Small Island Developing States"? Alignment helps the CPU fetch data from memory in an efficient manner: less cache miss/flush, less bus transactions etc. As pointed out in the comments below, there are better solutions if you are willing to include a header A pointer p is aligned on a 16-byte boundary iff ((unsigned long)p & 15) == 0. 16 Bytes? So what is happening? You should always use the and operation. So, a total of 12 bytes of memory is . If the address is 16 byte aligned, these must be zero. If the address is 16 byte aligned, these must be zero. When you do &A[1] you are telling the compiller to add one position to a float pointer. GCC has __attribute__((aligned(8))), and other compilers may also have equivalents, which you can detect using preprocessor directives. Or if your algorithm is idempotent (like. ARMv5 and earlier For word transfers, you must ensure that addresses are 4-byte aligned. Find centralized, trusted content and collaborate around the technologies you use most. Intel does not provide its own C or C++ runtime libraries so the version of malloc you link in should be the same as GNU's. GCC implements taking the address of a nested function using a technique -called @dfn{trampolines}. If my system has a bus 32-bits wide, given an address how can i know if its aligned or unaligned? A place where magic is studied and practiced? What does alignment to 16-byte boundary mean . When working with SIMD intrinsics, it helps to have a thorough understanding of computer memory. A memory address a, is said to be n-byte aligned when a is a multiple of n bytes (where n is a power of 2). I always like checking my input, so hence the compile time assertion. But as said, it has not much to do with alignments. However, if you are developing a library you can't. For example, on a 32-bit machine, a data structure containing a 16-bit value followed by a 32-bit value could have 16 bits of padding between the 16-bit value and the 32-bit value to align the 32-bit value on a 32-bit boundary. Can you just 'and' the ptr with 0x03 (aligned on 4s), 0x07 (aligned on 8s) or 0x0f (aligned on 16s) to see if any of the lowest bits are set? E.g. And using the intrinsics to load data from unaligned memory into the SSE registers seems to be horrible slow (Even slower than regular C code). It is assistant for sampling values. Do I need a thermal expansion tank if I already have a pressure tank? I'm using C++11 with GCC 4.5.2, and hoping to also support Clang. Then operate on the 16-byte aligned buffer without the need to fixup leading or tail elements. And you'd have to pass a 64-bit aligned type to. Where does this (supposedly) Gibson quote come from? 0x000AE430 Redoing the align environment with a specific formatting, Time arrow with "current position" evolving with overlay number, How to handle a hobby that makes income in US. there is a memory which can take addresses 0x00 to 0x100 except the reserved memory. Why is there a voltage on my HDMI and coaxial cables? rev2023.3.3.43278. In practice, the compiler probably assigns memory for it, which would be 8-byte aligned. Where does this (supposedly) Gibson quote come from? No, you can't. To check if an address is 64 bits aligned, you just have to check if its 3 least significant bits are null. It will remove the false positives, but still leave you with some conforming implementations on which the union fails to create the alignment you want, and hence fails to compile. We simply mask the upper portion of the address, and check if the lower 4 bits are zero. The memory will have these 8 byte units at address 0, 8, 16, 24, 32, 40 etc. Most SSE instructions that include 128-bit memory references will generate a "general protection fault" if the address is not 16-byte-aligned. Now, the char variable requires 1 byte but memory will be accessed in word size of 4 bytes so 3 bytes of padding is added again. most compilers, including the Intel compiler will vectorize the code even though v is not 32-byte aligned (I assume that you CPU has 256 bit vector length which is the case of modern Intel CPU). EDIT: casting to long is a cheap way to protect oneself against the most likely possibility of int and pointers being different sizes nowadays. The nature of simulating nature: A Q&A with IBM Quantum researcher Dr. Jamie We've added a "Necessary cookies only" option to the cookie consent popup. Notice the lower 4 bits are always 0. In conclusion: Always use void * to get implementation-independant behaviour. The following system parameters can be set. Using the GNU Compiler Collection (GCC) Specifying Attributes of Variables aligned (alignment) This attribute specifies a minimum alignment for the variable or structure field, measured in bytes. You should use __attribute__((aligned(8)). Why is there a voltage on my HDMI and coaxial cables? We simply mask the upper portion of the address, and check if the lower 4 bits are zero. A limit involving the quotient of two sums. So, except for the the very beginning and the very end of the loop, your code will get vectorized. Find centralized, trusted content and collaborate around the technologies you use most. SSE support is a deliberate feature of memory allocator. Intel Advisor is the only profiler that I know that can do those things. Is this homework? What is the difference between #include and #include "filename"? If you preorder a special airline meal (e.g. Notice the lower 4 bits are always 0. A memory access is said to be aligned when the data being accessed is n bytes long and the datum address is n-byte aligned. // because in worst case, the data can be misaligned upto 15 bytes. I get a memory corruption error when I try to use _aligned_attribute (which is suitable for gcc alone I think). Staging Ground Beta 1 Recap, and Reviewers needed for Beta 2, How to allocate and free aligned memory in C. How to make tr1::array allocate aligned memory? Making statements based on opinion; back them up with references or personal experience. It's portable to the two compilers in question. What should I know about memory alignment in SIMD? Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. I didn't check the align() routine, as this memory problem needed to be addressed. @MarkYisri: yes, I expect that in practice, every implementation that supports SSE2 instructions provides an implementation-specific guarantee that'll work :-), -1 Doesn't answer the question. . It means the lower three bits to be zero, in order to follow the alignment rule. It is also useful to add one more directive into the code before the loop: #pragma vector aligned Is a collection of years plural or singular? Some compilers align data structures so that if you read an object using 4 bytes, its memory address is divisible by 4. Secondly, there's posix_memalign to be sure. With modern CPU, most likely, you won't feel il (maybe a few percent slower, but it will be most likely in the noise of a basic timer measurement). Or, you can manually align address like this; Because 16-byte aligned address must be divisible by 16, the least significant digit in hex number should be 0 all the time. You may re-send via your accident in butte, mt today; ramy abbas issa net worth; check if address is 16 byte aligned "If you requested a byte at address "9" do we need to care about alignment at byte level? Thanks. If you requested a byte at address "9", the CPU would actually ask the memory for the block of bytes beginning at address 8, and load the second one into your register (discarding the others). But a more straight-forward test would be to do a MOD with the desired alignment value, and compare to zero. How do I connect these two faces together? The Disney original film Chip 'n Dale: Rescue Rangers seemingly managed to pull off a trifecta with a reboot of the Rescue Rangers franchise that won over fans of the original series, young . aligned_alloc(64, sizeof(foo) will return 0xed2040. How can I explain to my manager that a project he wishes to undertake cannot be performed by the team? Is it possible to create a concave light? Each byte is 8 bits, so to align on a 16 byte boundary, you need to align to each set of two bytes. Is it possible to rotate a window 90 degrees if it has the same length and width? Just because you are using the memalign routine, you are putting it into a float type. address should not take reserved memory. @Hasturkun Division/modulo over signed integers are not compiled in bitwise tricks in C99 (some stupid round-towards-zero stuff), and it's a smart compiler indeed that will recognize that the result of the modulo is being compared to zero (in which case the bitwise stuff works again).