This PR uses the `measureme` crate to profile the call stack of the
program being interpreted by Miri. This is accomplished by starting a
measureme 'event' when we enter a function call, and ending the event
when we exit the call. The `measureme` tooling can be used to produce a
call stack from the generated profile data.
Limitations:
* We currently record every single entry/exit. This might generate very
large profile outputs for programs with a large number of function
calls. In follow-up work, we might want to explore sampling (e.g. only
recording every N function calls).
* This does not integrate very well with Miri's concurrency support.
Each event we record starts when we push a frame, and ends when we pop
a frame. As a result, switching between virtual threads will cause
events from different threads to be interleaved. Additionally, the
recorded for a particular frame will include all of the work Miri does
before that frame completes, including executing another thread.
The `measureme` integration is off by default, and must be enabled via
`-Zmiri-measureme=<output_name>`
1. The double quotes around <untagged> are changed to backspaces, so <untagged>
will render correctly in markdown.
2. Clarify that -Zmiri-track-raw-pointers will never accept code that Miri
would not have accepted without -Zmiri-track-raw-pointers.
Add an `fn_ptr` field to `MiriFrame`
The `backtrace-rs` crate can use this to implement
`Frame::symbol_address`, which is used to skip frames
above the call to `Backtrace::capture` on the stack.
The function pointer will not be useable for comparison purposes if the
function is generic, as CTFE creates a new function pointer for each
cast of a (monomorphized) generic function. However, this already
affects code running under Miri, and isn't a problem for `backtrace-rs`
(which only casts a non-generic function).
I've added logic to allow `MiriFrame` to have either 4 or 5 fields - if
a 5th field is present, we write the function pointer to it.
The `backtrace-rs` crate can use this to implement
`Frame::symbol_address`, which is used to skip frames
above the call to `Backtrace::capture` on the stack.
The function pointer will not be useable for comparison purposes if the
function is generic, as CTFE creates a new function pointer for each
cast of a (monomorphized) generic function. However, this already
affects code running under Miri, and isn't a problem for `backtrace-rs`
(which only casts a non-generic function).
I've added logic to allow `MiriFrame` to have either 4 or 5 fields - if
a 5th field is present, we write the function pointer to it.
