Direct users towards using Rust target feature names in CLI
This PR consists of a couple of changes on how we handle target features.
In particular there is a bug-fix wherein we avoid passing through features that aren't prefixed by `+` or `-` to LLVM. These appear to be causing LLVM to assert, which is pretty poor a behaviour (and also makes it pretty clear we expect feature names to be prefixed).
The other commit, I anticipate to be somewhat more controversial is outputting a warning when users specify a LLVM-specific, or otherwise unknown, feature name on the CLI. In those situations we request users to either replace it with a known Rust feature name (e.g. `bmi` -> `bmi1`) or file a feature request. I've a couple motivations for this: first of all, if users are specifying these features on the command line, I'm pretty confident there is also a need for these features to be usable via `#[cfg(target_feature)]` machinery. And second, we're growing a fair number of backends recently and having ability to provide some sort of unified-ish interface in this place seems pretty useful to me.
Sponsored by: standard.ai
Partially move cg_ssa towards using a single builder
Not all codegen backends can handle hopping between blocks well. For example Cranelift requires blocks to be terminated before switching to building a new block. Rust-gpu requires a `RefCell` to allow hopping between blocks and cg_gcc currently has a buggy implementation of hopping between blocks. This PR reduces the amount of cases where cg_ssa switches between blocks before they are finished and mostly fixes the block hopping in cg_gcc. (~~only `scalar_to_backend` doesn't handle it correctly yet in cg_gcc~~ fixed that one.)
`@antoyo` please review the cg_gcc changes.
Ensure that queries only return Copy types.
This should pervent the perf footgun of returning a result with an expensive `Clone` impl (like a `Vec` of a hash map).
I went for the stupid solution of allocating on an arena everything that was not `Copy`. Some query results could be made Copy easily, but I did not really investigate.
This puts every function and data object in their own section. This
allows the linker to omit unused functions and data objects with
--gc-sections.
On linux this shrinks a hello world binary without optimizations
(neither sysroot nor binary) from 17MB to 13MB. It shrinks a hello world
binary with only sysroot optimizations from 14MB to 13MB. For comparison
cg_llvm produces a 3.5MB debug mode hello world binary with an optimized
sysroot. Cg_clif produces a 10MB debug mode hello world binary without
an optimized sysroot.
Previously foreign statics would actually cause a local static to be
defined and exported. This issue was found because std::env::vars() was
found to return no env vars despite many being defined. This was caused
by libstd importing environ as foreign static. The accidental definition
of environ caused libstd to read a null pointer which was interpreted as
there being no environment variables at all.
Also fix tests. STDOUT is not defined by libc. The correct name is stdout.
This previously worked as STDOUT was incorrectly defined as null pointer
during codegen.
