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a18fd41064
Our usage of `ucontext_t` in the standard library was kind of problematic. We unnecessarily mimiced libc-specific structures, and our `getcontext` implementation was overkill for our use case of stack tracing. This commit introduces a new namespace, `std.debug.cpu_context`, which contains "context" types for various architectures (currently x86, x86_64, ARM, and AARCH64) containing the general-purpose CPU registers; the ones needed in practice for stack unwinding. Each implementation has a function `current` which populates the structure using inline assembly. The structure is user-overrideable, though that should only be necessary if the standard library does not have an implementation for the *architecture*: that is to say, none of this is OS-dependent. Of course, in POSIX signal handlers, we get a `ucontext_t` from the kernel. The function `std.debug.cpu_context.fromPosixSignalContext` converts this to a `std.debug.cpu_context.Native` with a big ol' target switch. This functionality is not exposed from `std.c` or `std.posix`, and neither are `ucontext_t`, `mcontext_t`, or `getcontext`. The rationale is that these types and functions do not conform to a specific ABI, and in fact tend to get updated over time based on CPU features and extensions; in addition, different libcs use different structures which are "partially compatible" with the kernel structure. Overall, it's a mess, but all we need is the kernel context, so we can just define a kernel-compatible structure as long as we don't claim C compatibility by putting it in `std.c` or `std.posix`. This change resulted in a few nice `std.debug` simplifications, but nothing too noteworthy. However, the main benefit of this change is that DWARF unwinding---sometimes necessary for collecting stack traces reliably---now requires far less target-specific integration. Also fix a bug I noticed in `PageAllocator` (I found this due to a bug in my distro's QEMU distribution; thanks, broken QEMU patch!) and I think a couple of minor bugs in `std.debug`. Resolves: #23801 Resolves: #23802