pub struct StackMap { /* private fields */ }
Expand description

Stack maps record which words in a stack frame contain live GC references at a given instruction pointer.

Logically, a set of stack maps for a function record a table of the form:

+---------------------+-------------------------------------------+
| Instruction Pointer | SP-Relative Offsets of Live GC References |
+---------------------+-------------------------------------------+
| 0x12345678          | 2, 6, 12                                  |
| 0x1234abcd          | 2, 6                                      |
| ...                 | ...                                       |
+---------------------+-------------------------------------------+

Where “instruction pointer” is an instruction pointer within the function, and “offsets of live GC references” contains the offsets (in units of words) from the frame’s stack pointer where live GC references are stored on the stack. Instruction pointers within the function that do not have an entry in this table are not GC safepoints.

Because

  • offsets of live GC references are relative from the stack pointer, and
  • stack frames grow down from higher addresses to lower addresses,

to get a pointer to a live reference at offset x within a stack frame, you add x from the frame’s stack pointer.

For example, to calculate the pointer to the live GC reference inside “frame 1” below, you would do frame_1_sp + x:

          Stack
        +-------------------+
        | Frame 0           |
        |                   |
   |    |                   |
   |    +-------------------+ <--- Frame 0's SP
   |    | Frame 1           |
 Grows  |                   |
 down   |                   |
   |    | Live GC reference | --+--
   |    |                   |   |
   |    |                   |   |
   V    |                   |   x = offset of live GC reference
        |                   |   |
        |                   |   |
        +-------------------+ --+--  <--- Frame 1's SP
        | Frame 2           |
        | ...               |

An individual StackMap is associated with just one instruction pointer within the function, contains the size of the stack frame, and represents the stack frame as a bitmap. There is one bit per word in the stack frame, and if the bit is set, then the word contains a live GC reference.

Note that a caller’s OutgoingArg stack slots and callee’s IncomingArg stack slots overlap, so we must choose which function’s stack maps record live GC references in these slots. We record the IncomingArgs in the callee’s stack map.

Implementations

Create a vec of Bitsets from a slice of bools.

Returns a specified bit.

Returns the raw bitmap that represents this stack map.

Returns the number of words represented by this stack map.

Trait Implementations

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

Formats the value using the given formatter. Read more

This method tests for self and other values to be equal, and is used by ==. Read more

This method tests for !=.

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

Compare self to key and return true if they are equal.

Returns the argument unchanged.

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

The resulting type after obtaining ownership.

Creates owned data from borrowed data, usually by cloning. Read more

🔬 This is a nightly-only experimental API. (toowned_clone_into)

Uses borrowed data to replace owned data, usually by cloning. Read more

The type returned in the event of a conversion error.

Performs the conversion.

The type returned in the event of a conversion error.

Performs the conversion.