Enum FrameInstPos 

pub enum FrameInstPos {
    Post,
    Pre,
}

An instruction position for a program point.

We attach debug metadata to a position on an offset in the text (code) section, either “post” or “pre”. The “post” position logically comes first, and is associated with the instruction that ends at this offset (i.e., the previous instruction). The “pre” position comes next, and is associated with the instruction that begins at this offset (i.e., the next instruction).

We make this distinction because metadata lookups sometimes occur with a PC that is after the instruction (e.g., the return address after a call instruction), and sometimes at the instruction (e.g., a trapping PC address). The lookup context will know which one to use – e.g., when walking the stack, “pre” for a trapping PC and “post” for every frame after that – so we simply encode it as part of the position and allow searching on it.

The need for this distinction can be understood by way of an example; say we have:

call ...
trapping_store ...

where both instructions have debug metadata. We might look up the PC of trapping_store once as we walk the stack from within the call (we will get this PC because it is the return address) and once when trapping_store itself traps; and we want different metadata in each case.

An alternative is to universally attach tags to the end offset of an instruction, which allows us to handle return addresses naturally but requires traps to adjust their PC. However, this requires trap handlers to know the length of the trapping instruction, which is not always easy – in the most general case, on variable-length instruction sets, it requires a full instruction decoder.

Variants

Post

The “post” position at an offset attaches to the instruction that ends at this offset, i.e., came previously.

Pre

The “pre” position at an offset attaches to the instruction that begins at this offset, i.e., comes next.

Trait Implementations

impl Clone for FrameInstPos

fn clone(&self) -> FrameInstPos

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for FrameInstPos

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Hash for FrameInstPos

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl PartialEq for FrameInstPos

fn eq(&self, other: &FrameInstPos) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl Copy for FrameInstPos

impl Eq for FrameInstPos

impl StructuralPartialEq for FrameInstPos