X64 stack alignment We’re almost finished with the dry material I promise. Nov 11, 2015 · x87 instructions can do 64-bit loads / stores (including fild / fistp of 64-bit integer data), and Pentium had that integrated. For information about the stack layout, see x64 stack usage. Sep 15, 2024 · The compiler only have to respect the stack alignment of the calling convention and make enough space for the shadow area and the parameters. main: [stack is initially misaligned or aligned Where at least one function parameter of type __m256 is transferred on the stack, Unix systems (32 and 64 bit) align the parameter by 32 and the called function can rely on the stack being aligned by 32 before the call (i. This avoids the run-time failures seen on 32-bit systems when a gcc compiled function is called by one compiled by another compiler. 4. x64 register usage Feb 28, 2010 · By maintaining a known stack alignment at the entry of functions, the compiler can safely use the more efficientmovdqa to save the nonvolatile registers rather than using the unaligned movdqu version. Aug 19, 2022 · Normally you wouldn't sub/add RSP around every call (which the code in the question was doing before an edit optimized it). it will try to align to 16-byte boundaries. Notably, this means that the alignment requirement for a char[20] array is the same as the requirement for a plain char. This arises due to something called stack alignment. global vars or locals on the stack), modern compilers in 32-bit mode do choose to align int64_t to an 8-byte boundary for efficiency (so it can be loaded / copied with MMX or SSE2 64-bit loads, or x87 fild to do int64_t -> double conversion). Sep 6, 2011 · In this article I will examine the stack frame layout of the newer 64-bit version of the x86 architecture, x64 [1]. x86, C++, gcc and memory alignment. Sep 23, 2009 · I'm not sure if the x86 architecture supports 8-byte alignment even though they support a 64-bit environment. By adjusting rsp after each push, the stack alignment issue was For details on stack allocation, alignment, function types and stack frames on x64, see x64 stack usage. When ARMv8. thanks for the reply and counter tests. According to MSDN, the /Zp command defaults to 8, which means 64-bit alignment boundaries are used. From Windows Data Alignment on IPF, x86, and x64 archive:. 00000000, then another thread might in theory read say either 0 or 1FFFFFFFF, especially IF SAY the value STRADDLED A CACHE-LINE boundary. When building for a target with strict alignment requirements (ARM for example), GCC will report locations that might lead May 12, 2017 · Aligning the stack may be beneficial any time there are data objects that exceed the default stack alignment of the system. Nov 20, 2020 · Obviously, in this case the compiler is not told to care for a 16-byte stack alignment. Feb 20, 2016 · a struct is aligned to the same alignment as needed for the largest element type within the structure. main: [stack is initially misaligned or aligned to 8] sub rsp,8 ;align the stack to 16 [stack now is aligned to 16] call A_FUNCTION ;misalignment due to internal push add rsp,8 ret A_FUNCTION: [stack is misaligned, inherited from main] sub rsp,8 ;align the stack to 16 On Windows, the malloc function is documented to return memory aligned on a 8-byte boundary for 32-bit applications, and on a 16-byte boundary for 64-bit applications. Sep 30, 2012 · This is what the common Windows x64 stack frame looks like without optimizations. Feb 14, 2022 · In x64 assembly, the stack frame, according to Microsoft, should be 16-byte aligned. Now, I'm struggling to come up with a way of doing it beyond this code (which I didn't invent, but I can't for the life of me remember where I found it. 7 describes as the following. The ABI specifies that before calling a C function I must align the stack to 16 bytes. 1. In x64 calling conventions, it is vital that the stack remains 16-byte aligned at the point of function calls like ExitProcess. g. (Not necessary in the downwards order. So, if you use push & pop always in pairs (push rax, push rdi . In this article, I will examine the stack frame layout of the newer 64-bit version of the x86 architecture, x64. Apr 24, 2021 · Recent versions of GCC (4. For programming 64 bit SSE examples the author puts align 16 to a particular point in the code. That is to say that if you are to push only 1 8-byte value onto the stack, you should padd it by adding the other 8 bytes. (My first x64 compiler used its own call convention, and didn't bother with stack alignment. On x64 hardware, alignment faults are disabled by default and the hardware similarly fixes the fault. They just benefit from not splitting across a cache-line boundary. Unwindability Jul 30, 2013 · So if you make use of the stack to store function variables you need to make sure of the stack alignment prior to calling another method ( in addition to ensuring your own variables are aligned properly on the stack ). Windows uses a somewhat different ABI Oct 31, 2019 · In the prologue, when using stack frames (explained later), rbp is modified, so before rbp is used in a stack frame, it is pushed onto the stack to preserve it when returning. ret is basically pop rip, which is why you have to restoring the stack to its original value before you can Mar 1, 2019 · - in 64-bit code, you have more chances to misalign stack because it must be aligned to 16 bytes while the standard push reg changes the stack by 8 bytes. so. Jan 4, 2010 · Sometimes the alignment (for whatever reason?) is so bad that access to the double is more than 50x slower than its fastest access. Nov 10, 2014 · Overall, there are a number of things to consider for alignment: First, according to the Wikipedia page on Data Structure Alignment, Embarcadero might be a bit of an exception if it aligns all objects to 8-byte boundaries. Jun 8, 2012 · Given a structure definition like struct foo { int a, b, c; }; What's the best (simplest, most reliable and portable) way to specify that it should always be aligned to a 64-bit address, even Mar 24, 2010 · The alignment rules are platform-specific (i. It is starting on an even addresses with 16, 32 or 64 bit of data following. Yet some __declspec(align(16)) arrays on the stack didn't even get a warning, and I am sure it must be pushing and popping the __m128s (I recall working out 12 registers were required on x64, and even then it moved some to the stack it didn't need for a bit and did its own thing anyway). 7 Stack alignment on exception entry May 17, 2022 · Alignment above 16 bytes must be done manually. Aug 18, 2022 · Shadow space + stack alignment Most resources I found seem to completely neglect this when doing simple "hello world" programs. 04 Feb 2010, 08:19 Madis731 Nov 15, 2024 · In x86 this is SOOOO much easier since we don’t have to account for stack alignment. Such code will very likely trigger some fault, when the called code will try to use that alignment assumption to its own advantage and the rsp will be IIRC, stack alignment is when variables are placed on the stack "aligned" to a particular number of bytes. the code for assembly listing for main looks like such. In Windown x64 the stack is aligned on a 16 byte boundary so all the compilers made for windows need to follow that convention. And it doesn't make any sense for different Windows versions running on same x64 platforms requiring different stack alignment. Windows uses a somewhat different ABI, and I will mention it briefly in the end. A small issue you may get when pwning on 64-bit systems is that your exploit works perfectly locally but fails remotely - or even fails when you try to use the provided LIBC version rather than your local one. 64-bit mode: always aligned by 16: Both x86-64 System V and Windows x64 ABIs require RSP%16 == 0 before a call, and thus guarantee RSP % 16 == 8 on function entry. Oct 6, 2023 · alignment는 performance를 위한 권장사항이라고 한다. It doesn't have to be the exact, minimum, amount that satisfies these conditions. The former method of achieving the alignment seems to be incorrect though: Jan 5, 2023 · The same MS docs you already found, and/or google, e. Aug 2, 2021 · A fundamental alignment is an alignment that's less than or equal to the largest alignment that's supported by the implementation without an alignment specification. Even Jul 4, 2016 · Stack alignment in x64 assembly. Not actually necessary to properly run 64-bit code, but the alignment guarantee ensures that the compiler can safely emit SSE instructions. I have always assumed that for 32-bit applications, the MSVC compiler will use 32-bit boundaries. Oct 28, 2016 · x86 stack must be aligned on 4 byte only (generic register size). (You break that alignment with a 16-bit (2-byte) push, this is bad). This allocates enough space for the Shadow Space plus the fifth parameter to WriteFile. com x64 shadow space. It improves speed measurably. In a test case that involves 2 different kinds of UB (strict aliasing violation Feb 26, 2017 · To make it more clear; 1. I think I understand now. 2. (In Visual C++, this is the alignment that's required for a double, or 8 bytes. Responsibility of stack alignment in 32-bit regards to the stack alignment, that is by sub-bing the RSP by 8 upon entry. Dec 20, 2017 · As you say, apparently ml64 (Microsoft Macro Assembler (x64)) doesn't let you change the alignment of the . X64 programming in assembly is more complex than 32-bit programming when it comes to calling conventions. Aug 31, 2020 · A separate comment is that you have a lot of instructions like mov $7, %rdi where you operate on 64-bit registers. Each function is responsible for creating and destroying its own stack frame. Also Cygwin's 64-bit version of GCC 4. The values stored in registers are transient and cannot be reconstructed when moving up the call stack. 5. 1 4-1) 프로그램과 자료 구조(특히 stack)의 성능을 위해서 alignment를 수행해야 한다고 나와있다. If we were to include other variables such as code alignment and arrangement, it would be a completely different discussions, no longer related to stack alignment which is a dynamic entity of a x64 Interrupt stack alignment My understanding is: when an ISR, written in assembly, calls into a high level function it works because there is an even number of general registers that are pushed to the stack, so it's still aligned to 16 bytes, as required by the SysV ABI. Apr 15, 2013 · The MSDN page here includes the following relevant information about your question "why not make the default alignment 8 for x64?":. Difference in data alignment in struct vs Mar 1, 2010 · In particular, 16-byte stack alignment avoids the need to insert conditional code to align SSE objects, both when allocating stack, and when entering SSE loops. Computation instructions which use a memory operand that may not be aligned to a 16-byte boundary must be replaced with an unaligned 128-bit load (MOVDQU) followed by the same computation operation that uses instead register operands. That was the short answer ;-) Moral of this story: don't use invoke in 64-bit code: the only way for code to be efficient is to reserve shadow space once that multiple calls use, but an invoke macro or built-in statement can't assume anything about surrounding code. 64-bit systems will align long int and double to 8-byte boundaries. in x64 - this is caller responsibility for stack align on 16*x before call – RbMm Jan 5, 2022 · In any of these, using frame pointers or not within a function isn't part of the ABI, unless required for stack unwinding. x86 Assembly , stack push instruction. text section, so you can't have anything in it with an alignment bigger than 16 bytes. For example, on 32/64bit Linux, and 64bit Windows, the default stack alignment is 16 bytes, while 32bit Windows is 4 bytes. When the stack is misaligned, it means we start trying to read variables from the middle of that 16 byte window and usually end up with a segmentation May 12, 2015 · It is meant to be used to make debugging x64 easier. This causes the compiler to dynamically align the stack to meet your specifications. For more information, see SP alignment checking on page D1-2333. On 64 bit Windows, stack alignment on a 16 byte boundary is required before calling all except a leaf function. I should set up some shadow space in main and then in the write method I should reserve another 32 byte shadow space for the WinAPI calls and can use the shadow space in main via an offset from rbp. See Why does the x86-64 / AMD64 System V ABI mandate a 16 byte stack alignment? As far as I know, none of these variables would require data alignment. From what I know, since the first field of this struct ie. Therefore, the result of ESP modulo 16 has to be zero prior to a function call. However calling any external, ABI-compliant functions via an FFI needed to go through a special routine which fixed up the stack properly and stuck the first four arguments into the right The code is required to align stack to 16 bytes before call, but the machine code can easily break that rule by doing something non-conforming, like pushing word (2B = 16b) value on to the stack. You could push the value 2x or, more preferrably, just move the stack pointer. So if you are using a 16 bit stack alignment, each variable on the stack is going to start from a byte that is a multiple of 2 bytes from the current stack pointer within a function. FFFFFFFF and 0x00000001. On Linux in x64, the calling convention states that the first few parameters are passed by register. This would make it safe to use that struct in 32-bit system. Understanding pushing onto the stack (x86 IA32 assembly) 4. Jan 27, 2011 · I've never heard about such a thing as specific stack alignment. 5. Assembly/Compiler Coding Rule 55. (AT&T syntax calls this movabs). code ImageUint8ToFloat_ proc frame _CreateFrame U2F_,0,64 ; helper macros to create prolog _SaveXmmRegs xmm10,xmm11,xmm12,xmm13 ; helper macros to create prolog _EndProlog ; helper macros to create prolog Jun 25, 2019 · In order to check alignment of an address, follow this simple rule; Since, byte is the smallest unit to work with memory access A 64 bit address has 8 bytes. Most important implementation detail of the x64 ABI is that the stack must always be aligned to 16. Mar 6, 2012 · I'm writing interrupt handling routines for x86_64. double requires 8-byte alignment and SSE extensions require 16-byte alignment. On the Intel Itanium architecture, however, if an alignment fault occurs while 64-bit kernel-mode code is running, the hardware raises an exception. Feb 20, 2023 · Today, we will discuss the details of stack frame layout in x64 and understand how it differs from that of x86. According to "ARM®v7-M Architecture Reference Manual [ARM DDI 0403E. Your structure's alignment is 4 because nothing within it requires 8-byte alignment. May 10, 2010 · The HeapAlloc function does not specify the alignment guarantees in the MSDN page, but I'm inclined to think that it should have the same guarantees of GlobalAlloc, which is guaranteed to return memory 8-byte aligned (although relying on undocumented features is evil); after all, it's explicitly said that Global/LocalAlloc are just wrappers around HeapAlloc (although they may discard the first Feb 2, 2022 · So, once said this, the padding is computed based on the alignment requirements of the next field, based on the alignment required by the field (if it is an array the alignment required is the same as for the individual cell type of the array, and if it is a simple type it is the size of the type itself, and for structures the base type should Nov 1, 2020 · Stack alignment in x64 assembly. b (ID120114)]", B1. In the called function, the stack is 8 mod 16. It's not your code that needs stack alignment. Jun 1, 2018 · So sub rsp, 28h allocates 0x28 bytes of stack space (and aligns the stack by 16 bytes, because it was 16-byte aligned before call in your caller pushed a return address. 2 can give variables 32-byte alignment on the stack. , you have to align differently on e. The focus will be on Linux and other OSes following the official System V AMD64 ABI. The heaviest difficulty under win x64 asm coding is keeping stack alignment at dqword (align 16). Feb 25, 2023 · Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Aug 7, 2009 · With a 64-bit value you need an x64 machine to give you atomic read-and-write between threads. See this SO question: natural alignment is important for performance, and is required on the x64 architecture (so it's not just PRE-x86 systems, but POST-x86 ones too -- x64 may still be a bit of a niche case but it's growing in popularity after all;-); that may be why Microsoft documents it as required (hard to find docs on whether MS has Aug 15, 2019 · you showed that, because of 16 byte alignment ABI requirement of the stack, the stack would look like 4 64-bit shadow values followed by a 64-bit padding followed by the 64-bit return value as the last thing on the stack because before the function is invoked. However, dynamically adjusting the stack at run time may cause slower execution of your application. for site:stackoverflow. Aug 1, 2018 · The OP might be interested to know that if you pass so many args that the calling convention doesn't have registers for them, the Windows x64 calling convention (like x86-64 System V) will pass them in 8-byte stack slots. Recall that the first 4 parameters are passed in registers. If the bus is 64 bits wide, the system is generally designed to access memory at multiples of eight bytes (64 bits). x86-64 Linux No seg fault on stack Jun 19, 2015 · The problem is not intrinsic to 64-bit Windows. GCC applies this even to local array, but I guess LLVM feels that's overly intrusive into a function's private stack layout, since it's not externally observable except in cases like yours where you pass the address to another function. e. mov qword ptr[rsp + 24], r8 Jan 13, 2014 · Is the stack alignment for the stdcall calling convention always 4 bytes, or is it 4 for a 32 bit machine and 8 for a 64 bit machine? What is the stack alignment size for cdecl? I am using Microsoft Visual Studio 2010. It's the Win API functions such as MessageBox. The article claims that GCC, VC++ and Borland's computer don't align data at 8 bytes unless it's a double or long long. e. a PowerPC CPU than on a x86_64 CPU), and they are implementation-defined, meaning your compiler can do whatever works (and might change that behaviour with different command-line options or after a version update). Here’s something I put together to make more sense of it: 64 byte alignment (w/ padding) Aug 21, 2024 · Shadow Space Allocation: I allocated shadow space (32 bytes) for every function, as required by the Windows x64 calling convention. My program also seems to run just fine when removing all the sub rsp / add rsp statements. Thus, it would seem that 8 byte alignment would be the requirement. 3. that you want to use in your function) has to be an odd multiple of 8. 9. Oct 22, 2015 · Right. If you break into the debugger and inspect the call stack for a thread, you won't be able to see any parameters passed to functions. 1-VHE is implemented, and the Nov 28, 2009 · But that doesn't change between 64-bit and 32-bit compilers. Apr 4, 2017 · Hi nidud. No instructions require 32-bit alignment, though. The problem lies in fact that at time of performing CALL instruction the stack has to be 16-byte aligned. and . If pushing the value of a 64-bit register to the stack (e. (Odd because call itself pushes an 8-byte return address, so on function entry, RSP%16 == 8 is If you changed n to 2, it would only allocate 8 bytes on the stack. A wrong alignment may not immediately cause a problem, but it may cause a crash whenever some function tries to use an SSE instruction with a memory operand on Sep 16, 2015 · As I understand the x64 calling convention in Windows (based on this and this): The first 4 arguments are passed in registers, although 32 bytes of shadow size is reserved in the stack. So, Get Beginning x64 Assembly Programming: From Novice to AVX Professional now with the O’Reilly learning platform. FWIW, you can't use inline assembly in Visual C++ ARM either for the same reason. Despite what Kai Tietz said in the bug report you linked, Microsoft's x64 ABI does allow a compiler to give variables a greater than 16-byte alignment on the stack. Nov 1, 2024 · Part 1 - x64 Essentials: Stack Alignment. Stack alignment on x86. Why there's the "default" 8 bytes and then 24=8+16 bytes is because the stack already contains 8 bytes for leave and ret, so the compiled code must adjust the stack first by 8 bytes to get it aligned Feb 5, 2022 · There is no single thing that makes a computer a “32-bit system” or a “64-bit system. 14. ANyway, it's strange the 16-byte requirement on 64-bit when the stack alignment should be the WORD-SIZE for that platform (8 bytes), like it's 4-byte for the 32-bit platform. However, since you take its address, the compiler will store it somewhere on the stack after the function is called. Writing applications that use the latest processor instructions introduces some new constraints and issues. Most of the time, 8 bytes works fine; this happens pretty often when working with ROP chains. This alignment must be preserved when calling functions. The x86_64 ISA specifies that on entry to an ISR, my stack is 8 byte aligned. – paxdiablo Commented Nov 2, 2011 at 8:23 Nov 19, 2011 · I think I understand memory alignment, but what confuses me is that the address of a pointer on some systems is going to be in virtual memory, right? So most of the checking/ensuring of alignment I have seen seem to just use the pointer address. ) – I'm curious to see if my 64-bit application suffers from alignment faults. By maintaining a known stack alignment at the entry of functions, the compiler can safely use the more efficientmovdqa to save the nonvo Stack 16 byte alignment. Finally, the alignment requirement of the struct as a whole is the maximum of the alignment requirements of each of its elements. If say you read the value from another thread when it's say incrementing between 0x00000000. (Although in Win32 IIRC ESP alignment is only guaranteed to be 4 bytes, so odd/even numbers of pushes are irrelevant. Feb 20, 2023 · Here is a summary of the important points we have to remember when we talk about x64 stack frames: x64 omits Frame pointer usage and relies on RSP (Stack pointer) alone for stack operations; This just so happens to align the stack because of the return address already on the stack (40 + 8 = 48). so usually not need do special tasks for stack align. AND. I was actually trying to eliminate other variables and focused solely on the performance of function prologs/epilogs vs plain fastcall, arranged as-is. From the MSDN documentation: __m128 types, arrays and strings are never passed by immediate value but rather a pointer is passed to memory allocated by the caller. Nov 23, 2015 · The System V AMD64 ABI requires 16-byte stack alignment. This would be better to write as mov $7, %edi. 4 Alignment. In Windows, an application program that generates an alignment fault will raise an exception, EXCEPTION_DATATYPE_MISALIGNMENT. Running the code on a 64 bit machine cuts down the issue, but I think it was still alternating between two timing (of which I could get similar results by changing the double to a float on a 32 bit machine) Apr 28, 2013 · You can pass as many 128 bit SSE intrinsic parameters as you like under x64. So a store from RAX can use a 64-bit absolute address, but a store from RBX would have to truncate the address. x64 datatype alignment requirements Stack: Correct stack alignment on x64 systems means that the stack frame must be aligned on a 16 byte boundary. When a Win API function is called, it must see the current stack (Top Of Stack) be aligned to 16 byte boundary. Shadow space example / "Hello world" in x64 assembly for Windows - Shadow space / Stack alignment / What is the 'shadow space' in x64 assembly? – Feb 17, 2016 · The issue with hot loop instruction alignment exists also on gcc 5. Sep 15, 2023 · From this Goasm Document:. The processor can do 32-bit load/store operations on any 32-bit boundary, but a 64-bit load or store requires 64-bit alignment. The x64 ABI was designed with these types in mind. Align data, paying attention to data layout and stack alignment Alignment and forwarding problems are among the most common sources of large delays on processors based on Intel NetBurst microarchitecture. Jun 4, 2021 · One reason to require alignment is to avoid splitting accesses across memory transfers. The reason of 8-byte alignment of Cortex-M7 would be guessed as that the internal AXI bus width is 64 bit. For more information about structure layout and alignment, see x64 type and storage layout. 0. Jun 22, 2019 · Well they will always be at least 16 and 12 on any system that requires alignment for 32 bit and 64 bit integers, that's for sure. On most 64-bit systems, the standard C primitive types will have an alignment requirement equal to their size. GNU Assembler, function call using stack, seg fault. Feb 6, 2015 · Oh, I just realized why this works. The shadow space cascades downward through method calls. However, there's a simple workaround for this problem and that's to use PECOFF's grouped sections feature. ) push rsp push [rsp] Nov 20, 2009 · you can with some processors (the nehalem can do this), but previously all memory access was aligned on a 64-bit (or 32-bit) line, because the bus is 64 bits wide, you had to fetch 64 bit at a time, and it was significantly easier to fetch these in aligned 'chunks' of 64 bits. A_FUNCTION: [stack is misaligned or aligned to 8] sub rsp,8 ;align the stack to 16 [stack now is aligned to 16] … add rsp,8 ret If you look carefully, both main and A_FUNCTION are actually doing the same exact thing in regards to the stack alignment, that is by sub-bing the RSP by 8 upon entry. Additionally, if you're using GCC, I suggest you enable -Wcast-align warnings. The 16 bytes is not a natural alignment for x64 - most stack operations work in 8-byte increments, so they naturally maintain an 8 byte alignment. 5 and later) for Linux x64 require the stack to be aligned on a 16-byte boundary when calling functions. Apr 21, 2022 · If you require more strict alignment, use __declspec(align(N)) on your variable declarations. if the largest element is an int and on a 32bit architecture (or forcing compiling using 32bit instruction) will be on a 4byte boundary. In any case, ESP (or RSP or just SP, depending on the address size) is incremented or decremented by 2 (for 16 bit operations), 4 (for 32 bit operations) or 8 (for 64 bit operations). Nov 14, 2016 · This is x64 code, note the usage of the rsp register. Something I am aware of, but honestly haven’t fully explored, is that the stack is on a 16-byte alignment. ) Especially if it makes stack alignment work out nicely by allowing an odd number of total pushes, if there's no sub rsp, n to reserve more stack space. difference betweem MSVC and ML64: MSVC use /Zp16 as default for x64 and ARM64 but ML64 not. And obviously the compiler allocates more stack than necessary in this case. 6, the modified get_it and the python3 script I wrote - After more research apparently this problem is called stack alignment. If the D-bit is 1, then the opposite is true. Is this because of "stack alignment"? Yes. 3 days ago · x64 stack defaults to 64-bit width so it is easy to keep the stack 8-byte aligned. Notes: Nov 2, 2022 · The C/C++ headers in the Windows SDK assume the platform's default alignment is used. This didn’t resolve the issue. Every time you push something on the stack, the stack pointer will decrease by 8 bytes, and every time you pop something from the stack, the stack pointer will increase by 8 bytes. ) Nov 7, 2020 · The choice of both Windows x64 and x86-64 System V to maintain 16-byte stack alignment is pretty good, and allows aligned spill/reload of XMM registers, and more efficient auto-vectorization of legacy-SSE loops over local arrays or single objects. Because rax can use the 64-bit absolute address mov moffs8/16/32/64 encoding that's only available for al/ax/eax/rax. In this article, we explained the concept of parameter homing, volatile and non Basically what it boils down to is that you need to move the stack pointer RSP 32 bytes before doing a call (keep in mind 16 byte alignment of the stack). “PUSH RBX”) decreases the stack by 8 bytes, we can realign the stack by pushing another 64-bit register (e. I have just told the compiler to generate an 8-byte and a 16-byte alignment for the stack; all other compiler options (and of course the source code) were the same. You'll notice that you decrease rsp by an odd number of quad words after that. The compiler typically takes care of this for you - by placing member variables of class/struct instances on natural boundaries for the size of the variable. Basically you have to make sure the total stack pointer movement is a multiple of 16, including the return address. Alignment of data concerns all kinds of variables: • Dynamically allocated variables • Members of a data structure Like you said, MacOS X has a 16 byte stack alignment, which means that the machine expects each variable on the stack to start on a byte that is a multiple of 16 from the current stack pointer. Recall that every write to a 32-bit register will zero the upper half of the corresponding 64-bit register, so the effect is the same as long as your constant is unsigned 32 bits, and the encoding of mov $7, %edi is one byte shorter as Jan 13, 2022 · The GNU documentation states that malloc is aligned to 16 byte multiples on 64 bit systems. Nov 23, 2023 · See Why does the x86-64 / AMD64 System V ABI mandate a 16 byte stack alignment? for more details about some things I mention in this answer. (4 byte minimum alignment for stack args would have been a valid choice, too. For some reason though, the asm version provides a bit faster speed. Mar 1, 2010 · In particular, 16-byte stack alignment avoids the need to insert conditional code to align SSE objects, both when allocating stack, and when entering SSE loops. You'd sub rsp, 40 at the top of a function to reserve shadow space and align. gcc x86 Windows stack alignment. Apr 30, 2019 · Outside of structs (e. x86 code uses the esp register. Let’s take a look at Microsoft’s HeapAlloc function (basically malloc) as an example of how this would work. The stack will always be maintained 16-byte aligned, except within the prolog (for example, after the return address is pushed), and except where indicated in Function Types for a certain class of frame functions. The example in the question is just that, an example. Prolog and epilog Every function that allocates stack space, calls other functions, saves nonvolatile registers, or uses exception handling must have a prolog whose address limits are described in the unwind data associated with the The article concluded with a useful diagram presenting the stack frame layout of a typical function call. According to the ABI, the int variables require 4 byte alignment, and the void * requires 8 byte alignment. . I have found an interesting article about automatic stack alignment on GoAsm site. 2. In x64, because of stack alignment needs, we require some padding here and there. Mar 28, 2012 · Stack Overflow for Teams Where developers & technologists share private knowledge with Two 32-bit members would give you 64-bit alignment as a matter of course Feb 20, 2010 · pushes 16 bits onto the stack. In code that targets 64-bit platforms, it's 16 bytes. 13. Since 16 bytes is a common alignment size for XMM operations, this value should work for most code. 1 The focus will be on Linux and other OSes following the official System V AMD64 ABI. If that sounds like a foreign language to you don’t fret, it’s fairly straight forward albeit somewhat tedius to implement. You can also use: #pragma align 8 (variable) to tell the compiler how you want a global or static variable aligned. Not true. Even when not building a stack frame, rbp is the ideal candidate to align the stack because it is not used for argument passing to a function. Alignment requirements are platform-specific, but typically a primitive type's minimum alignment is the same as its size. The stack grows upwards, and the numbers in the block are the sizes that it can be: The space between the dotted lines is the stack frame, the memory a function owns on the stack. 66 ff /6 pushes 32 bits (or 64 if it's a 64-bit segment). It worked fine when calling functions within the same language. At first, we must align stack at exe entry point. Intrinsics are a lot easier, and portable between x86 and x64 if you avoid use of __m64. Also, semi-related: x86-64 System V requires that global arrays of 16 bytes and larger be aligned by 16. Apr 12, 2012 · Now I want to understand the alignment of the struct __wait_queue_head on a 64-bit Linux machine following the LP64 standard. I could test and confirm that __attribute__ works well on gcc 5. 10. Memory alignment today and 20 years ago. The overall stack must be 16-byte aligned (although individual arguments don't have to be). Dec 4, 2021 · Adding a copy of my x64 libc. The alignment requirement of the element is the alignment requirement of the element's base type. The default value for n is 4 i. (considering, 1 byte = 8bit) Therefore, Log2(n) = Log2(8) = 3 (to know the power) Where, n is number of bytes. Nov 7, 2012 · There's a difference between 32/64 bit systems: 32-bit systems will still align 8-byte variables to 32-bit boundaries. – Arne J Apr 15, 2015 · The key thing to remember is that writing properly formed x64 assembly is a real pain with all the required rules for stack unwinding. Nov 2, 2011 · A 64-bit int is going to have more stringent alignment requirements that a 8-bit char (or even an array of a billion 8-bit char elements). Data: If not - it would seem it is up to the user to ensure alignment, in which case any time we use an std::atomic<T> larger than one byte, it would seem we'd have to use std::aligned_storage to ensure it is properly aligned, which (A) seems cumbersome, and (B) is something I've never actually seen done in practice or in any examples/tutorials. The main function aligns the stack by subtracting 40 bytes from rsp, and each I really doubt different stack alignment would be used in different versions of a same Distro. Feb 17, 2022 · An operating system for a machine using an x64 CPU and such a strange hardware would probably use a calling convention with a 48-byte stack alignment to ensure that local variables are both 16-byte aligned (for SSE operations) and 3-byte aligned (for the graphics DMA). When set to 1, if a load or store instruction executed at EL0 uses the SP as the base address and the SP is not aligned to a 16-byte boundary, then a SP alignment fault exception is generated. Apr 25, 2021 · (In Windows x64, RDI and RSI are call-preserved registers, unlike x86-64 System V where they're call-clobbered arg-passing registers. E. So can Pentium's lock cmpxchg8b which is very slow if it crosses a cache-line boundary. B1. After all, 4-byte alignment on a 64-bit platform wouldn't harm anything. Argument passing will be Mar 5, 2010 · x64 compilers can assume the presence of SSE registers, which on Windows have a calling convention associated with them (XMM6-15 are nonvolatile, aka callee-save). Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. Sep 21, 2016 · I am reading Modern x86 Assembly language book from Apress. pop rdi, pop rax), then the stack will never be misaligned. So in your case, both a and b are passed by register rather than on the stack. If there is alignment requirements for the CPU, alignment is done on all kinds of data memory, no matter if it is stored on the stack or elsewhere. spinlock_t lock is an unsigned int, which occupies 4 bytes on a 64-bit machine, this struct should begin at a 4-byte aligned address. Arguments that are 1, 2, 4, or 8 bytes can go on the stack. The fun stuff is just around the corner. The coding to achieve automatic stack alignment and to adjust the stack for the Windows x64 FASTCALL calling convention is as follows (which one is used depends on the number of parameters) Aug 5, 2016 · Different types can have different alignment requirements, the size of a type must be a multiple of it's alignment requirement. Dec 29, 2020 · Stack alignment in x64 assembly. Jan 24, 2024 · It's not just "isn't necessary aligned" - it's actually mandatory for the stack to not be aligned at the start and the end of the function - and it must be misaligned by exactly 8 bytes. Jun 17, 2010 · However if you are writing code in assembler, or writing hooks using dynamically created machine language, you need to be aware of the 16 byte stack alignment requirement. gcc documentation points it in its documentation for -mpreferred-stack-boundary option:-mpreferred-stack-boundary=num. This makes pure asm programming (without macros) quite difficult and requies new coding style. On Linux, it only said that it returns "memory that is suitably aligned for any kind of variable", but it probably is also 8 and 16. ” Computers have several features that affect how much data it processes in various operations: width of the memory bus, width of processor registers, widths of data operands supported by instructions, width of the address space. 6. this controlled by /Zp (Struct Member Alignment) option in both msvc and ML/ML64 ( The alignment can be 1, 2, 4, 8 or 16) look also How align works with data packing. Dec 30, 2024 · A breakthrough was reached with the realization that the stack pointer (rsp) was going out of alignment every time something was pushed onto the stack. sub rsp, 8 fixes that, preserving the alignment. Don't know any hardware that doesn't require that myself. They could have not made this mandatory and instead obliged every SSE user to manually align the stack to 16 bytes at a performance penalty, but decided that mandating a stack alignment makes more sense. May 9, 2006 · I work on macros to support FASTCALL calling convention in 64-bit MASM. When the processor wants to read some memory, say from a 64-bit address, it sends only the first 61 bits to the memory device(s). May 19, 2023 · The AMD64 SysV ABI also requires 16-byte alignment for arrays themselves if they're 16 bytes or larger, or VLAs. Why is this? If my understanding is correct, registers and all instructions operate on values that are a maximum of 8 bytes wide. Nov 21, 2017 · According to the stack alignment requirements of the System V ABI, the stack must be aligned by 16 bytes before every call instruction (the stack boundary is 16 bytes by default when not changed with the option -mpreferred-stack-boundary). Windows x64 also requires 16-byte stack alignment before a call, presumably for similar motivations as x86-64 System V. Aug 3, 2021 · SA0, bit [4] SP Alignment check enable for EL0. Let’s discuss the 16 byte stack alignment convention. Apr 19, 2012 · Intel's official optimization guide has a chapter on converting from MMX commands to SSE where they state the fallowing statment:. the stack pointer is 32 minus the word size modulo 32 at the function entry). Seems to be a duplicate of Why does the x86-64 / AMD64 System V ABI mandate a 16 byte stack alignment?, although the code example is mostly unrelated to alignment, just some version of GCC's choice of where to put things in the red zone with optimization disabled. Intel® 64 and IA-32 Architectures Software Developer Manuals (Vol. For example: struct Test { char foo; int bar; }; Nov 11, 2014 · The reason you have to subtract 8 is that call itself places 8 bytes of return address on the stack, thereby violating this constraint. Nov 5, 2010 · The hardware fixes the fault as described in the previous paragraph. Is it not possible that the physical memory address will not be aligned? Feb 28, 2021 · Furthermore, one may think about solving this issue manually with preserving the functionality of the program. 😺 Okay moving on. Stack Alignment: I ensured that the stack is aligned properly before making any function calls. Why push first decreases the Aug 7, 2012 · Once enable it's working a lot like ARM alignment settings in /proc/cpu/alignment, see answer How to trap unaligned memory access? for examples. To maintain 16-byte stack alignment before a call, the total amount of stack allocation (including for your local vars, and pushes of call-preserved registers like RBP, RBX, etc. Thus the stack pointer needs to be moved by multiple of 16 + 8 to leave room for the return Jun 3, 2024 · The stack frame is 64 bytes. ) To enable use of SSE instructions with stack memory, the stack has to be aligned to 16 bytes. RCX) just after the “PUSH RBX”: Feb 18, 2020 · In general, you rarely have to worry about optimizing or controlling the padding of data structures to get more harmonious memory alignment. I need to align my stack pointer to 16 bytes therefore. If it doesn't align the stack on a 16 byte boundary, then just round it upward until you hit a multiple of 16. dcrk safaa hpoh kihe vpmfy bbyivx jieylsw ozqbnq eioc wohzi

X64 stack alignment. Stack alignment on x86.