in favor of CPU features. Also rearrange the `std.Target`
data structure.
* note: `@import("builtin")` was already deprecated in favor of
`@import("std").builtin`.
* `std.builtin.arch` is now deprecated in favor of
`std.builtin.cpu.arch`.
* `std.Target.CpuFeatures.Cpu` is now `std.Target.Cpu.Model`.
* `std.Target.CpuFeatures` is now `std.Target.Cpu`.
* `std.Target` no longer has an `arch` field. Instead it has a
`cpu` field, which has `arch`, `model`, and `features`.
* `std.Target` no longer has a `cpu_features` field.
* `std.Target.Arch` is moved to `std.Target.Cpu.Arch` and
it is an enum instead of a tagged union.
* `std.Target.parseOs` is moved to `std.Target.Os.parse`.
* `std.Target.parseAbi` is moved to `std.Target.Abi.parse`.
* `std.Target.parseArchSub` is only for arch now and moved
to `std.Target.Cpu.Arch.parse`.
* `std.Target.parse` is improved to accept CPU name and features.
* `std.Target.Arch.getBaselineCpuFeatures` is moved to
`std.Target.Cpu.baseline`.
* `std.Target.allCpus` is renamed to `std.Target.allCpuModels`.
* `std.Target.defaultAbi` is moved to `std.Target.Abi.default`.
* Significant cleanup of aarch64 and arm CPU features, resulting in
the needed bit count for cpu feature set going from 174 to 138.
* Add `std.Target.Cpu.Feature.Set.addFeatureSet` for merging
feature sets together.
`-target-feature` and `-target-cpu` are removed in favor of
`-mcpu`, to conform to established conventions, and it gains
additional power to support cpu features. The syntax is:
-mcpu=name+on1+on2-off1-off2
closes#4261
* libc_installation.cpp is deleted.
src-self-hosted/libc_installation.zig is now used for both stage1 and
stage2 compilers.
* (breaking) move `std.fs.File.access` to `std.fs.Dir.access`. The API
now encourages use with an open directory handle.
* Add `std.os.faccessat` and related functions.
* Deprecate the "C" suffix naming convention for null-terminated
parameters. "C" should be used when it is related to libc. However
null-terminated parameters often have to do with the native system
ABI rather than libc. "Z" suffix is the new convention. For example,
`std.os.openC` is deprecated in favor of `std.os.openZ`.
* Add `std.mem.dupeZ` for using an allocator to copy memory and add a
null terminator.
* Remove dead struct field `std.ChildProcess.llnode`.
* Introduce `std.event.Batch`. This API allows expressing concurrency
without forcing code to be async. It requires no Allocator and does
not introduce any failure conditions. However it is not thread-safe.
* There is now an ongoing experiment to transition away from
`std.event.Group` in favor of `std.event.Batch`.
* `std.os.execvpeC` calls `getenvZ` rather than `getenv`. This is
slightly more efficient on most systems, and works around a
limitation of `getenv` lack of integration with libc.
* (breaking) `std.os.AccessError` gains `FileBusy`, `SymLinkLoop`, and
`ReadOnlyFileSystem`. Previously these error codes were all reported
as `PermissionDenied`.
* Add `std.Target.isDragonFlyBSD`.
* stage2: access to the windows_sdk functions is done with a manually
maintained .zig binding file instead of `@cImport`.
* Update src-self-hosted/libc_installation.zig with all the
improvements that stage1 has seen to src/libc_installation.cpp until
now. In addition, it now takes advantage of Batch so that evented I/O
mode takes advantage of concurrency, but it still works in blocking
I/O mode, which is how it is used in stage1.
Clean up the code a bit and introduce a few checks meant to avoid
overshooting the end of the frame chain.
The code is now stable enough not to cause panics during the call frame
walking.
Previously it was a tagged union which was one of:
* baseline
* a specific CPU
* a set of features
Now, it's possible to have a CPU but also modify the CPU's feature set
on top of that. This is closer to what LLVM does.
This is more correct because Zig's notion of CPUs (and LLVM's) is not
exact CPU models. For example "skylake" is not one very specific model;
there are several different pieces of hardware that match "skylake" that
have different feature sets enabled.
comment from this commit reproduced here:
I have observed the CPU name reported by LLVM being incorrect. On
the SourceHut build services, LLVM 9.0 reports the CPU as "athlon-xp",
which is a 32-bit CPU, even though the system is 64-bit and the reported
CPU features include, among other things, +64bit.
So the strategy taken here is that we observe both reported CPU, and the
reported CPU features. The features are trusted more; but if the features
match exactly the features of the reported CPU, then we trust the reported CPU.
The new plan to support hobby operating systems is #3784.
And what kind of name is "Zen" anyway? There's already a
[Zen programming language](http://zenlang.sourceforge.net/)
and that's just confusing.
