スタックフレームのアライメント要件

x86(i386)アーキテクチャ向けにGCCが出力するコードで、スタックフレームが16byteアライメントされる理由を求めて。¹

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経緯

1. SysV ABI当初ではワード(4byte)アライメントのみが要請されていた。
2. MMX, SSEなどのSIMD命令登場により、スタック変数でも16byteアライメントが要請されるケースが発生。またアライメント必須でない命令であってもパフォーマンス的には16byteアライメントが好ましい。
3. Mac OS X i386 ABIでは16byteアライメントを明確に要請。
4. GCCでサポートするLinux/BSD/OSX間の一貫性の観点から16byteアライメントを採用。
5. 新しいSysV psABIでは16byteアライメントに変更。

GCC Bugzilla

• #27537 XMM alignment fault when compiling for i386 with -Os
• #38496 Gcc misaligns arrays when stack is forced follow the x8632 ABI
• #40838 gcc shouldn't assume that the stack is aligned

ABI仕様

x86-64

System V Application Binary Interface, AMD64 Architecture Processor Supplement, Draft Version 0.99.6：

3.2.2 The Stack Frame
In addition to registers, each function has a frame on the run-time stack. This stack grows downwards from high addresses. Figure 3.3 shows the stack organization. The end of the input argument area shall be aligned on a 16 (32, if __m256 is passed on stack) byte boundary. In other words, the value (%rsp + 8) is always a multiple of 16 (32) when control is transferred to the function entry point. The stack pointer, %rsp, always points to the end of the latest allocated stack frame.

i386

SYSTEM V APPLICATION BINARY INTERFACE, Intel386 Architecture Processor Supplement：

Registers and the Stack Frame
Some registers have assigned roles in the standard calling sequence:
%esp: The stack pointer holds the limit of the current stack frame, which is the address of the stack’s bottom-most, valid word. At all times, the stack pointer should point to a word-aligned area.

OS X ABI Function Call Guide, IA-32 Function Calling Conventions：

The function calling conventions used in the IA-32 environment are the same as those used in the System V IA-32 ABI, with the following exceptions:

• The stack is 16-byte aligned at the point of function calls

System V Application Binary Interface, Intel386 Architecture Processor Supplement, Version 1.1：

2.2.2 The Stack Frame
In addition to registers, each function has a frame on the run-time stack. This stack grows downwards from high addresses. Table 2.2 shows the stack organization.
The end of the input argument area shall be aligned on a 16 (32 or 64, if __m256 or __m512 is passed on stack) byte boundary. In other words, the value (%esp + 4) is always a multiple of 16 (32 or 64) when control is transferred to the function entry point. The stack pointer, %esp, always points to the end of the latest allocated stack frame.

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