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aarch64-linux: Default to FramePointer::NonLeaf
For aarch64-apple and aarch64-windows, platform docs state that code must use frame pointers correctly. This is because the AAPCS64 mandates that a platform specify its frame pointer conformance requirements:
- Apple: https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms#Respect-the-purpose-of-specific-CPU-registers
- Windows: https://learn.microsoft.com/en-us/cpp/build/arm64-windows-abi-conventions?view=msvc-170#integer-registers
- AAPCS64: 4492d1570e/aapcs64/aapcs64.rst (the-frame-pointer)
Unwinding code either requires unwind tables or frame pointers, and on aarch64 the expectation is that one can use frame pointers for this. Most Linux targets represent a motley variety of possible distributions, so it is unclear who to defer to on conformance, other than perhaps Arm. In the absence of a specific edict for a given aarch64-linux target, Rust will assume aarch64-linux targets also use non-leaf frame pointers. This reflects what compilers like clang do.
rustc_target contains some very low-level details that are
specific to different compilation targets and so forth.
For more information about how rustc works, see the rustc dev guide.