x64 calling convention stack alignment

x86's call-stack doesn't fit that model: all normal calling conventions leave RSP unmodified, or pop the args with ret 16; they don't pop the return address, push an array, then push rcx / ret. The calling convention is based on the x86 fastcall and involves passing the first four parameters in registers rather than on the stack. Stack Alignment: x64 looks for the stack pointer to be 16 byte aligned. itself satisfy calling convention ABI. those variables that you declare at the start of a procedure by using the LOCAL directive. Always pass -mno-red-zone to kernel code (even support libraries such as libc's embedded in the kernel) if interrupts don't respect the red zone. If the return value is an integer/struct/union whose size is less than or equal than 64 bits, . Holds call stack size size and alignment, local stack size and alignment, and various attributes that describe how prolog and epilog should be constructed. Some notes about my Windows Kernel journey. In x86, argument values are pushed to the stack in reverse order. Creating Alignment Chaining To overcome that problem, each and every functions or modules wishing to communicate with the API must chain their stack together to maintain the aligned ecosystem. 2. Here's a sample: ), and how the tasks of preparing for a function call and . . •Enable code re-use (e.g. So the underlying issue is that gcc uses "aligned" vector-instructions for "unaligned" temporary stack variables. The callee does not clean the stack. A nice diagram of the stack and where 16-byte alignment occurs appears here in the YASM manual, 15.2 win64 Structured Exception Handling, as well as other information on x64 calling convention. As we noted, the x64 calling convention is caller-clean, which means that the space for parameters gets reused from function call to function call. 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). I came here from riscv/riscv-isa-manual#44. Functions that allocate stack space, call other functions, save non-volatile registers or use exception handling are called " frame functions "; other functions are called " leaf functions ".. Frame functions use an area on the stack called a " stack frame " and have a defined prologue in which this is set up. But if that's the case, they are misnamed. The assumption that stack follows vector alignment is a short-cut in gcc, and of course just in 1.1. on the stack). A calling convention describes the interface between a caller and a function: . In the standard RISC-V calling convention, the stack grows downward and the stack pointer is Many SSE instructions that read data from memory, require data to be aligned on a 16-byte boundary, otherwise a fault is granted. Either caller or callee clean-up thiscall. Recent versions of GCC (4.5 and later) for Linux x64 require the stack to be aligned on a 16-byte boundary when calling functions. . It just means you must provide 32 bytes on the stack before calling. For clarity's sake, the stack frames showed above were produced without omitting the frame pointer. x64 Calling Conventions. Additional space can also be allocated for local variables and a frame pointer register can be . Implementations with ISA extensions might require extended calling conventions. An Interesting Bit • Because of 2 & 4 byte instruction alignment, 18.1 C Datatypes and Alignment . This content is specific to C++; it does not apply to DPC++. 被调用者应保存调用者的哪个寄存器. Also on x64 the stack must be 16 byte aligned, this means that sometimes you need to allocate even more space to preserve the alignment. 1. x86-64 calling conventions. Text is available under the Creative Commons Attribution-ShareAlike License 3. . If you're going to program in assembly . The call pushes %rip, so the stack is 16-byte aligned again if the callee pushes %rbp. To correct your code, insert add rsp, 8 before and spl, -16, for example. the ARM64EC ABI follows x64 software conventions including calling convention, stack usage, and data alignment, making ARM64EC and x64 interoperable. This calling convention has a great deal in common with the x64 calling convention that Win64 uses. C enums have integer type-kind. The Shadow space (also sometimes called Spill space or Home space) is 32 bytes above the return address which the called function owns (and can use as scratch space), below stack args if any.The caller has to reserve space for their callee's shadow space before running a call instruction.. This will specify a 64 bit code, with a . Microsoft x64 The x64 Application Binary Interface [ 1] (ABI) uses a four-register fast-call calling convention by default. All other registers need to be preserved by the caller (e.g. Cannot be specified on a function with variable arguments. Note that in the x64 calling convention, arguments after the 4th are pushed on the stack, which are on top of this shadow space (pushed before the 32 bytes). The Windows x64 ABI First of all, there is now one and only one OS specified calling convention. . On x64 it must also allocate space for parameters of the called functions. Browsing the web I found documentation for the x64 calling convention, but can find nothing about 32 bits. Return value. Since the "call" instruction pushes an 8-byte return address, this means that every non-leaf function is going to adjust the stack by a value of the form 16n+8 in order to restore 16-byte alignment. Calling convention that specifies the arguments are passed on the stack. Here are some of the basic characteristics of this calling convention: 1. In fact, aside from the x64-specific parts of the x64 calling convention, you can think of the x64 calling convention as a logical extension of __fastcall that is designed to take advantage of the expanded register set available with x64 processors. Align the stack pointer to 16 bytes prior to calling a foreign API; Restore the stack pointer; Return to the caller; Now to call this function from the C/C++ code, we have to make sure that it is defined as PUBLIC in the ASM code, and also declare its prototype in the C/C++ code: gcc adheres to this recommendation and by default omits the frame pointer on x64, when compiling with optimizations. FuncFrame doesn't know anything about function's arguments or return values, . The C++ compilers from Intel, Symantec, Digital Mars and Codeplay are all designed to be binary compatible with Microsoft's C++ compiler, despite the fact that Microsoft has refused to publish important details. Firstly the 64-bit architecture has more registers (eight more general-purpose registers r8 - r15 ). x86-64 calling conventions are quite confusing for me, to say the least. All other part of MS' ABI might, or might not be compatible to that one of gcc (and llvm?). " from "Unwindability" clause on Overview of x64 Calling . Calling Conventions Computer Systems Section 3.7. These are the rules for how registers should be used and how stack frames should be laid out in memory. Understanding Calling Conventions. However, there are a couple of things that help to reduce the stack consumption. But this code is working only in registers, and code alignment issues are the same in 32 and 64 bit mode. x64 Calling Convention: At this point you should be aware of x64 calling convention [ Refer ]. Microsoft x64; 1.2. . Since the "call" instruction pushes an 8-byte return address, this means that every non-leaf function is going to adjust the stack by a value of the form 16n+8 in order to restore 16-byte alignment. This is not (to my knwoledge) a hardware requirement but a software one. this is due to a calling convention in x64 which requires the stack to be 16 bytes aligned before any call instruction. This calling convention is used for calling C++ non-static member functions. __regcall . iOS diverges from Procedure Call Standard for the ARM 64-bit Architecture in several ways, as described here. When a function in a Windows x64 binary is called, the stack frame is used in the following manner: First four integer arguments are passed to RCX, RDX, R8 and R9 registers accordingly (green) Arguments 5, 6, and further are pushed on to the stack (blue) Return address to the caller's next instruction is pushed is found at RSP + 0x0 (yellow) C SIMD types have the vector type-kind. Apps built as ARM64EC may contain x64 code but do not have to, since ARM64EC is its own complete, first-class ABI for Windows. Currently using this 64-bit MASM code to call a C runtime function such as memcmp().I recall this convention was from a GoAsm article on optimizations.. memcmp PROTO;:QWORD,:QWORD,:QWORD PUSH RSP PUSH QWORD PTR [RSP] AND SPL, 0F 0h MOV R8,R11 MOV RDX,R10 MOV RCX,RAX SUB RSP, 32 CALL memcmp LEA RSP,[RSP+ 40] POP RSP This calling convention is used for calling C++ non-static member functions. The assumption that stack follows vector alignment is a short-cut in gcc, and of course just in 调用函数时如何为任务 . The code takes the same shape, uses mostly the same registers, but the `and rsp, -32` is just either not there or placed wrong. (0x28) bytes on the stack (32 bytes for the 4 registers and 8 bytes to align the stack from previous usage - the return RIP address pushed on the stack) for this purpose. Divergences from the Generic Procedure Call Standard. First thing first, the entry point must initiate such aligned ecosystem(to 16) because everybody else will depend on it and will follow suit. If you must define these, I suggest calling them arg*. To understand the C calling convention, you should ﬁrst make sure that you fully understand the push, pop, call, and ret instructions - these will be the basis for most of the rules. Differences in various implementations include where parameters, return values, return addresses and scope links are placed (registers, stack or memory etc. Another difference that will need to be addressed in 64-bit is the calling convention that is used. The stack must be kept 16-byte aligned. The Microsoft x64 calling convention is followed on Windows and pre-boot UEFI (for long mode on x86-64). In computer science, a calling convention is an implementation-level (low-level) scheme for how subroutines receive parameters from their caller and how they return a result. x64 Calling Convention shows how the stack is typically used in function calls. A function calling this needs to have at least 32 bytes of stack to store the parameters and align the stack upon call. Introduction. The main differences in assembly programming are the use of new 64-bit registers and the fact that x64 editions of Windows use a different calling convention for the Windows API. It is meant to be used to make debugging x64 easier. At least 4 * 8 = 32 bytes must be allocated for this purpose. Another important difference is the way functions are called, the calling convention. It uses registers RCX, RDX, R8, R9 for the first four integer or pointer arguments (in that order), and XMM0, XMM1, XMM2, XMM3 are used for floating point arguments. these 64-bit integer types . Let's take a look at Microsoft's HeapAlloc function (basically malloc) as an example of how this would work. See, for example, FDBG sources or look inside some system . The current RISC-V ISA 2.1 requires 16-byte alignment on hardware floating point implementations. The x64 Application Binary Interface (ABI) uses a four-register fast-call calling convention by default. 本條目描述 x86 架构微處理器的调用约定。. At this moment, as we have pushed 7 registers ( each one is of size 8 byte) , the stack should go misaligned ( 7 * 8 = 56 bytes which is not multiple of 16 ). In MASM, the ALIGN directive does not align local (or stack) variables, i.e. Answer: there are a number of conventions that are followed regarding calls. "Stdcall" tells the compiler that the caller of the function will pass all parameters on the stack in a right-to-left sequence and that . CONTEXT Structure Alignment. 2 Upgrading to 64-bit calling conventions 2.1 Step 1: passing some arguments via registers All this messing about with the stack pointer is expensive, since in general most functions don't have many arguments, and on 64-bit architectures we have enough registers available that many functions might be able to get away without using the stack . It is the caller's job to clean the stack. Also on x64 the stack must be 16 byte aligned, this means that sometimes you need to allocate even more space to preserve the alignment. " from "Unwindability" clause on Overview of x64 Calling . Contribute to v1k1ngfr/winkernel development by creating an account on GitHub. functions, libraries) •Reduce chance for mistakes Warning: There is no one true MIPS calling convention. 64 Bit Linux machine use AMD 64 System V Application Binary Interface for their calling conventions. 極微参数或复杂参数独立部分的分配顺序. On x86 "sub esp, n" is used to allocated stack space for local variables. Because of the different calling conventions (Microsoft x64 vs stdcall) and number of parameters for each API, the actual call to an API is made from seperate pieces of code we refer to as "dispatchers". To review, when we call a function, we have to choose some registers to use for arguments, at least one to use for return value, some to be caller-saved (available for temporary use by the function) and some to be callee-saved.Our choices for these were selected so as to align with the standard Unix C ABI calling convention, so with a bit more work, our functions . The x64 (64bit) architecture designed by AMD is based on Intel's x86 (32bit) architecture, supporting it natively. 64-bit code can be slower with pointer-heavy data structures, because larger pointers means bigger cache footprint. When a function is called, an 8 byte return address is automatically pushed onto the stack and the function then saves any non-volatile registers that it will use. This is dramatically different from the typical convention where you push arguments unto the stack in reverse order prior to calling or a method. Recall that the first 4 parameters are passed in . You know that the caller has (according to the calling convention) taken care of its stack alignment and . Space is allocated on the call stack as a shadow store for callees to save those registers. Note 3: Stack is 16 byte aligned at time of call. The Windows x64 ABI (Application Binary Interface) presents some new challenges for assembly programming that don't exist for x86. Most of the time, 8 bytes works fine; this happens pretty often when working with ROP chains. [Bug target/54412] minimal 32-byte stack alignment with -mavx on 64-bit Windows. The precise ABI of aggregates and vectors depends on the precise ABIs of their fields. So, for consistency's sake, it makes sense to adopt that calling convention for your own 32-bit DLL's as well. There are two primary versions of thiscall used depending on the compiler and whether or not the function uses variable arguments.. For the GCC compiler, thiscall is almost identical to cdecl: The caller cleans the stack, and the parameters are passed in right-to-left order. On x64 it must also allocate space for parameters of the called functions. . When writing code for 64-bit Linux that integrates with a C library, you must follow the calling conventions explained in the AMD64 ABI Reference. At least 4 * 8 = 32 bytes must be allocated for this purpose. " The stack pointer must be aligned to 16 bytes, except for leaf functions, in any region of code that isn't part of an epilog or prolog. . Stack alignment. They're not variables at all—they're arguments. There is a strict one-to-one correspondence between a function call's arguments and the registers used for those arguments. On the other hand C.ADDI16SP instruction assumes the stack pointer is kept 16-byte aligned and is useless if the stack pointer is not 16-byte aligned. The only guarantee you have for local variables is that 32-bit Windows aligns them on a 4-byte boundary and 64-bit Windows aligns them on an 8-byte boundary. endian-ness, and alignment of data types and structures •Calling Conventions -how functions are called, parameters passed, etc. This makes pure asm programming (without macros) quite difficult and requies new coding style. In the Microsoft x64 calling convention, it is the caller's responsibility to allocate 32 bytes of "shadow space" on the stack right before calling the . Unfortunately, there is actually no such thing as "The MIPS Calling Convention". When compiling for the x64 architecture in a Windows context (whether using Microsoft or non-Microsoft tools), stdcall, thiscall, cdecl, and fastcall all resolve to using this convention. " The stack pointer must be aligned to 16 bytes, except for leaf functions, in any region of code that isn't part of an epilog or prolog. RSP is restored by subtracting 0x48 bytes from the stack. In the generic procedure call standard, all function arguments passed on the stack consume slots in multiples of 8 bytes. x86-64 calling conventions are quite confusing for me, to say the least. In Assembly Language, Seventh Edition for x86 Processors by Kip Irvine, on page 211, it says under 5.53 The x86 Calling Convention which addresses the Microsoft x64 Calling Convention, When calling a subroutine, the stack pointer ( RSP) must be aligned on a 16-byte boundary (a multiple of 16). For 64-bit mode, the stack must be aligned by 16 bytes minus 8 before calling an API so that SSE2 . From the MSDN blog 'The Old New Thing', The history of calling conventions, part 5: amd64: Fast calling convention that specifies that arguments are passed in registers rather than on the stack. There's a strict one-to-one correspondence between a function call's arguments and the registers used for those arguments. On x86 "sub esp, n" is used to allocated stack space for local variables. The current RISC-V ISA 2.1 requires 16-byte alignment on hardware floating point implementations. 调用约定描述了被调用代码的接口：. stack frame Address Value (64 bit) 0000 7FFF FFFF E870 0000 0000 0000 0000 The lack of stack data alignment facilities has not become really critical until the appearance of the SSE instruction set. . But I'd really suggest just not defining them at all. 参数是如何被传递的（放置在堆栈上，或是寄存器中，亦或两者混合）. itself satisfy calling convention ABI. This space can be left uninitialized, it doesn't matter. (So are data alignment issues, no clue what you're talking about with alignment being a bigger issue for x86-64). In x64 fastcalls, the calling function is responsible for cleaning up the stack. You can find . Many x64 stack defaults to 64-bit width so it is easy to keep the stack 8-byte aligned. The called procedure assumes that the stack has room to store four 64-bit registers and that the address contained in RSP (stack pointer) is 16 byte aligned. The CALL instruction subtracts 8 from the RSP (stack pointer) register, since addresses are 64-bits long. Binghamton University CS-220 . Calling convention in x64 always uses the RCX, RDX, R8, R9 registers for passing the first 4 arguments (anything up to 64 bit values or pointers), while additionally to those 4 registers, RAX, R10 and R11 are considered volatile. Additional arguments are pushed onto the stack (right to left). It is sometimes referred to as x86-64, AMD64, or, cloned by Intel, EM64T or Intel64. A couple of the changes that must be taken into account can can be seen as very positive. . To understand this better, we will dissect the information in junks starting with arguments/parameters. __vectorcall on targets that support this calling convention (X86/X64). For soft-float it is not required. Most recent C/C++ compilers have directives to align stack data, but we are dealing with MASM. A CALL puts an 8 byte return address on the stack, so to maintain stack pointer alignment, 40 bytes of "shadow space" is reserved on the stack. How did we know the argument to puts was supposed to go in RDI? Stack and Structure alignment. If a frame function allocates a fixed amount of stack space, it must maintain 16-byte alignment of the stack pointer in the body of the function (outside the prolog and . the C type long long is a 64-bit integer, float is a 32-bit IEEE 754-2008 ﬂoating-point . On the other hand C.ADDI16SP instruction assumes the stack pointer is kept 16-byte aligned and is useless if the stack pointer is not 16-byte aligned. The 64-bit calling convention does, in general, seem to increase the stack consumption of the program. So the underlying issue is that gcc uses "aligned" vector-instructions for "unaligned" temporary stack variables. There are two primary versions of thiscall used depending on the compiler and whether or not the function uses a variable number of arguments.. For the GCC compiler, thiscall is almost identical to cdecl: The caller cleans the stack, and the parameters are passed in right-to . THE 64 BIT X86 C CALLING CONVENTION 1.2 The C Calling Convention The C calling convention is based heavily on the use of the hardware-supported stack. Wikipedia's list of x86 calling conventions also mentions 16-byte alignment for Windows x64 and (apparently due to a . that will be of use due to the 2 byte alignment MBE - 05/08/2015 x64, ARM, Windows 53 . So for instance, I believe these two types have different ABIs on x64 linux even though they have identical size, alignment, and type-kind: # [repr(C)] struct Homo(u64, u64); # [repr(C . System V; . Calling Conventions Calling Conventions •where to put function arguments •where to put return value •who saves and restores registers, and how •stack discipline Why? The stack must be kept 16-byte aligned. . but GCC on Linux uses a 16 byte stack alignment. The "Stdcall" convention is used throughout by the 32-bit Windows API (which consists of DLL's). We certainly could have devised our own calling conv. fld / fld / faddp. times you need to use this calling convention is when you either call a function in the Windows API, or you call a function written in C or C++. For soft-float it is not required. Function calling conventions. This shadow space is 32 bytes long (since we are in 64-bit assembly). 1.2.4. The problem lies in fact that at time of performing CALL instruction the stack has to be 16-byte aligned. You merely allocate the space in your prologue and it's good for the whole function. The x64 calling convention is also referred to as the x64 ABI (Application Binary Interface). This document summarizes the calling conventions that we expect you to use in ECE 314 for homework problems and assembly language programming projects. Typically they save register parameters in their shadow locations (if . Given the expanded register set, x64 uses the __fastcall calling convention and a RISC-based exception-handling model. It gives an option to preserve it by providing the -fno-omit-frame-pointer flag. sprinkled all over to get the right calling convention. I would follow Viorel_'s suggestion of seeing what the compiler does, since VC is what sets the platform's calling conventions. But once you know the important things, like EAX, EDX and . . •Object File and Debug Formats . This provides a way to be sure that when entering a function (that is, after a call instruction), the value of the stack pointer is always 8 modulo 16. I came here from riscv/riscv-isa-manual#44. Calling convention. And the calling "conventions" of the Microsoft compiler has almost become a de factostandard for the Windows operating system. Presumably, these are intended to encapsulate the Windows x64 calling convention for argument passing. To correct your code, insert add rsp, 8 before and spl, -16, for example. A stack machine is like reverse-polish notation, like x87 registers. Either caller or callee clean-up thiscall. Space is allocated on the call stack as a shadow store for callees to save those registers. 2 CHAPTER 1. lecture != book != gcc != spim != web On this processor, a word is deﬁned to be 16 bits in size, a dword 32 bits and a qword 64 bits. - Callee cleans up the stack - Standard calling convention for the Win32 API • fastcall - First two arguments are put into ECX, and EDX, the . x64 calling conventions Two important differences between x86 and x64 are: 64-bit addressing capability Sixteen 64-bit registers for general use. All other part of MS' ABI might, or might not be compatible to that one of gcc (and llvm?). Argument Passing in General. . Stack Alignment in 64-bit Calling Conventions - discusses stack alignment for x86-64 calling conventions; The Lost Art of Structure Packing by Eric S. Raymond This page was last edited on 5 May 2022, at 14:40 (UTC).