固定在 IL 中是如何表示的

Question

我想知道 .Net 的 IL 语言如何表达字段固定，所以我看了一下示例代码：

struct S
{
    public fixed int buf[8];
}
S s = default(S);
public void MyMethod() {              
    fixed (int* ptr = s.buf){            
        *ptr = 2;        
    }
}

这会生成 IL：

.method private hidebysig instance void MyMethod () cil managed 
{
    // Method begins at RVA 0x2050
    // Code size 25 (0x19)
    .maxstack 2
    .locals init (
        [0] int32& pinned
    )

    IL_0000: ldarg.0              // Load argument 0 onto the stack
    IL_0001: ldflda valuetype C/S C::s // Push the address of field of object obj on the stack
    IL_0006: ldflda valuetype C/S/'<buf>e__FixedBuffer' C/S::buf // Push the address of field of object obj on the stack
    IL_000b: ldflda int32 C/S/'<buf>e__FixedBuffer'::FixedElementField // Push the address of field of object obj on the stack
    IL_0010: stloc.0              // Pop a value from stack into local variable 0
    IL_0011: ldloc.0              // Load local variable 0 onto stack
    IL_0012: conv.i               // Convert to native int, pushing native int on stack
    IL_0013: ldc.i4.2             // Push 2 onto the stack as int32
    IL_0014: stind.i4             // Store value of type int32 into memory at address
    IL_0015: ldc.i4.0             // Push 0 onto the stack as int32
    IL_0016: conv.u               // Convert to native unsigned int, pushing native int on stack
    IL_0017: stloc.0              // Pop a value from stack into local variable 0
    IL_0018: ret                  // Return from method, possibly with a value
} // end of method C::MyMethod

我在这里没有看到任何明确告诉 GC 固定数组的内容，哪条指令实际上负责固定？另外，是否还有其他涉及固定的基本操作"under the hood"？

Answer 1

.locals init (
    [0] int32& pinned
)

pinned 的使用负责固定。本文对此进行了解释：How does the 'fixed' keyword work? The article points to the following excerpt from Standard ECMA-335 Common Language Infrastructure (CLI):

II.7.1.2 pinned

The signature encoding for pinned shall appear only in signatures that describe local variables (§II.15.4.1.3). While a method with a pinned local variable is executing, the VES shall not relocate the object to which the local refers. That is, if the implementation of the CLI uses a garbage collector that moves objects, the collector shall not move objects that are referenced by an active pinned local variable.

[Rationale: If unmanaged pointers are used to dereference managed objects, these objects shall be pinned. This happens, for example, when a managed object is passed to a method designed to operate with unmanaged data. end rationale]

VES = 虚拟执行系统，CLI = 公共语言基础设施

固定在 IL 中是如何表示的

How is pinning represented in IL

c#

pinvoke

cil

II.7.1.2 pinned