我很想知道我的 64 位应用程序是否存在对齐错误。

来自 Windows Data Alignment on IPF, x86, and x64 ^archive :

In Windows, an application program that generates an alignment fault will raise an exception, EXCEPTION_DATATYPE_MISALIGNMENT.

• On the x64 architecture, the alignment exceptions are disabled by default, and the fix-ups are done by the hardware. The application can enable alignment exceptions by setting a couple of register bits, in which case the exceptions will be raised unless the user has the operating system mask the exceptions with SEM_NOALIGNMENTFAULTEXCEPT. (For details, see the AMD Architecture Programmer's Manual Volume 2: System Programming.)

[Ed. emphasis mine]

• On the x86 architecture, the operating system does not make the alignment fault visible to the application. On these two platforms, you will also suffer performance degradation on the alignment fault, but it will be significantly less severe than on the Itanium, because the hardware will make the multiple accesses of memory to retrieve the unaligned data.

• On the Itanium, by default, the operating system (OS) will make this exception visible to the application, and a termination handler might be useful in these cases. If you do not set up a handler, then your program will hang or crash. In Listing 3, we provide an example that shows how to catch the EXCEPTION_DATATYPE_MISALIGNMENT exception.

忽略引用AMD 架构程序员手册 的方向，我将改为引用Intel 64 and IA-32 Architectures Software Developer’s Manual

5.10.5 Checking Alignment

When the CPL is 3, alignment of memory references can be checked by setting the AM flag in the CR0 register and the AC flag in the EFLAGS register. Unaligned memory references generate alignment exceptions (#AC). The processor does not generate alignment exceptions when operating at privilege level 0, 1, or 2. See Table 6-7 for a description of the alignment requirements when alignment checking is enabled.

非常好。我不确定那是什么意思，但非常好。

然后还有:

2.5 CONTROL REGISTERS

Control registers (CR0, CR1, CR2, CR3, and CR4; see Figure 2-6) determine operating mode of the processor and the characteristics of the currently executing task. These registers are 32 bits in all 32-bit modes and compatibility mode.

In 64-bit mode, control registers are expanded to 64 bits. The MOV CRn instructions are used to manipulate the register bits. Operand-size prefixes for these instructions are ignored.

The control registers are summarized below, and each architecturally defined control field in these control registers are described individually. In Figure 2-6, the width of the register in 64-bit mode is indicated in parenthesis (except for CR0).

• CR0 — Contains system control flags that control operating mode and states of the processor

AM
Alignment Mask (bit 18 of CR0) — Enables automatic alignment checking when set; disables alignment checking when clear. Alignment checking is performed only when the AM flag is set, the AC flag in the EFLAGS register is set, CPL is 3, and the processor is operating in either protected or virtual- 8086 mode.

我试过了

我实际使用的语言是 Delphi，但假装它是与语言无关的伪代码:

void UnmaskAlignmentExceptions()
{
   asm
      mov rax, cr0; //copy CR0 flags into RAX
      or  rax, 0x20000; //set bit 18 (AM)
      mov cr0, rax; //copy flags back
}

第一条指令

mov rax, cr0;

因特权指令异常而失败。

如何在 x64 上为我的进程启用对齐异常？

推送

我发现x86有指令:

• PUSHF、POPF:将 EFLAGS 的前 16 位插入/弹出堆栈
• PUSHFD、POPFD:将 EFLAGS 的所有 32 位插入/弹出堆栈

这让我想到了 x64 版本:

• PUSHFQ、POPFQ:将 RFLAGS quad 插入/弹出堆栈

(在 64 位世界中，EFLAGS 被重命名为 RFLAGS)。

所以我写道:

void EnableAlignmentExceptions;
{
    asm
       PUSHFQ;                //Push RFLAGS quadword onto the stack
       POP       RAX;         //Pop them flags into RAX
       OR        RAX, $20000; //set bit 18 (AC=Alignment Check) of the flags
       PUSH      RAX;         //Push the modified flags back onto the stack
       POPFQ;                 //Pop the stack back into RFLAGS;
}

而且它没有崩溃或触发保护异常。我不知道它是否按照我的意愿行事。

红利阅读

• How to catch data-alignment faults on x86 (aka SIGBUS on Sparc) (不相关的问题；x86 不是 x64，Ubuntu 不是 Windows，gcc vs 不是)

最佳答案

在 x64 上运行的应用程序可以访问标志寄存器(有时称为 EFLAGS )。该寄存器中的第 18 位允许应用程序在发生对齐错误时获得异常。因此理论上，要启用对齐错误异常，程序所要做的就是修改标志寄存器。

然而

为了让它真正起作用，操作系统内核必须设置cr0允许它的第 18 位。 Windows 操作系统不会那样做。为什么不？谁知道呢？

应用程序不能在控制寄存器中设置值。只有内核可以做到这一点。设备驱动程序在内核中运行，因此他们也可以设置它。

可以通过创 build 备驱动程序来搞砸并尝试让它工作，请参阅:

Old New Thing - Disabling the program crash dialog ^archive

以及随后的评论。请注意，这篇文章已有十多年历史，因此一些链接已失效。

您可能还会发现此评论(以及此问题中的其他一些答案)很有用:

Larry Osterman - 07-28-2004 2:22 AM

We actually built a version of NT with alignment exceptions turned on for x86 (you can do that as Skywing mentioned).

We quickly turned it off, because of the number of apps that broke :)

关于windows - 如何在 x64 上为我的进程启用对齐异常？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/26919269/