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use core::num::NonZeroU64;
/// An absolute code address for a stack frame. Can either be taken directly from the
/// instruction pointer ("program counter"), or from a return address.
///
/// These addresses are "AVMAs", i.e. Actual Virtual Memory Addresses, i.e. addresses
/// in the virtual memory of the profiled process.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum FrameAddress {
/// This address is the instruction pointer / program counter. This is what unwinding
/// starts with.
InstructionPointer(u64),
/// This is a return address, i.e. the address to which the CPU will jump to when
/// returning from a function. This is the address of the instruction *after* the
/// call instruction.
///
/// Unwinding produces a list of return addresses.
ReturnAddress(NonZeroU64),
}
impl FrameAddress {
/// Create a [`FrameAddress::InstructionPointer`].
pub fn from_instruction_pointer(ip: u64) -> Self {
FrameAddress::InstructionPointer(ip)
}
/// Create a [`FrameAddress::ReturnAddress`]. This returns `None` if the given
/// address is zero.
pub fn from_return_address(return_address: u64) -> Option<Self> {
Some(FrameAddress::ReturnAddress(NonZeroU64::new(
return_address,
)?))
}
/// The raw address (AVMA).
pub fn address(self) -> u64 {
match self {
FrameAddress::InstructionPointer(address) => address,
FrameAddress::ReturnAddress(address) => address.into(),
}
}
/// The address (AVMA) that should be used for lookup.
///
/// If this address is taken directly from the instruction pointer, then the lookup
/// address is just the raw address.
///
/// If this address is a return address, then the lookup address is that address **minus
/// one byte**. This adjusted address will point inside the call instruction. This
/// subtraction of one byte is needed if you want to look up unwind information or
/// debug information, because you usually want the information for the call, not for
/// the next instruction after the call.
///
/// Furthermore, this distinction matters if a function calls a noreturn function as
/// the last thing it does: If the call is the final instruction of the function, then
/// the return address will point *after* the function, into the *next* function.
/// If, during unwinding, you look up unwind information for that next function, you'd
/// get incorrect unwinding.
/// This has been observed in practice with `+[NSThread exit]`.
pub fn address_for_lookup(self) -> u64 {
match self {
FrameAddress::InstructionPointer(address) => address,
FrameAddress::ReturnAddress(address) => u64::from(address) - 1,
}
}
/// Returns whether this address is a return address.
pub fn is_return_address(self) -> bool {
match self {
FrameAddress::InstructionPointer(_) => false,
FrameAddress::ReturnAddress(_) => true,
}
}
}