e39b4aba1c
Previously, every call was lowered on AArch64 to a `call` instruction, which takes a signed 26-bit PC-relative offset. Including the 2-bit left shift, this gives a range of +/- 128 MB. Longer-distance offsets would cause an impossible relocation record to be emitted (or rather, a record that a more sophisticated linker would fix up by inserting a shim/veneer). This commit adds a notion of "relocation distance" in the MachInst backends, and provides this information for every call target and symbol reference. The intent is that backends on architectures like AArch64, where there are different offset sizes / addressing strategies to choose from, can either emit a regular call or a load-64-bit-constant / call-indirect sequence, as necessary. This avoids the need to implement complex linking behavior. The MachInst driver code provides this information based on the "colocated" bit in the CLIF symbol references, which appears to have been designed for this purpose, or at least a similar one. Combined with the `use_colocated_libcalls` setting, this allows client code to ensure that library calls can link to library code at any location in the address space. Separately, the `simplejit` example did not handle `Arm64Call`; rather than doing so, it appears all that is necessary to get its tests to pass is to set the `use_colocated_libcalls` flag to false, to make use of the above change. This fixes the `libcall_function` unit-test in this crate.
This crate provides a simple JIT library that uses Cranelift.
This crate is extremely experimental.
See the example program for a brief overview of how to use this.