|
For Unix targeting CoreRT, this will provide platform specific unwind
info which is a dwarf format.
Unlike window’s UNWIND_INFO, we won’t encode the format within RyuJit
since it is not only complex/error-prone but also needs to be adjusted
depending on platforms.
Instead, CFI (call frame information) pseudo instructions are encoded,
which will be passed to CoreRT/ObjectWriter which will translate
them to directly emit CFI directives to LLVM MC stream to establish
frames and layout eh_frame section.
A jit flag is used to dynamically dispatch either Windows’s unwind
blob or this CFI instruction table. No JIT/EE interface change is needed
since the API already expects an opaque blob.
Initially when I looked at what Clang does, the prologue and the
sequence of CFI emissions are a bit different than Windows.
Since we will emit the same sequence of code with the same runtime that
we define, I assume this is unnecessary – I’ve verified the CFI sequence
here can work correctly with libunwind.
Basically we need only 3 operations – push reg is a combination of 1 and
2 below.
1. Allocation – increase stack frame. Normally subtract esp in x64.
2. Store – Copy a (callee save) register to stack (memory)
3. SaveFP – Set frame pointer register
Since Windows operation is based on the relative value (all offsets,
etc are computed from the current point), I also use the similar form of
CFI instructions.
So, mostly one window’s code corresponds to two CFIs – we might optimize
this by aggregating allocation, but I’d like to keep the current
syntax/semantic same as Windows.
This results in a very simple transformation on par with Windows.
|
