Document target_feature and cfg_target_feature.

src/attributes.md

@@ -216,6 +216,7 @@ The following is an index of all built-in attributes.
   - [`inline`] — Hint to inline code.
   - [`cold`] — Hint that a function is unlikely to be called.
   - [`no_builtins`] — Disables use of certain built-in functions.
+  - [`target_feature`] — Configure platform-specific code generation.
 - Documentation
   - `doc` — Specifies documentation. See [The Rustdoc Book] for more
     information. [Doc comments] are transformed into `doc` attributes.
@@ -282,6 +283,7 @@ The following is an index of all built-in attributes.
 [`recursion_limit`]: attributes/limits.html#the-recursion_limit-attribute
 [`repr`]: type-layout.html#representations
 [`should_panic`]: attributes/testing.html#the-should_panic-attribute
+[`target_feature`]: attributes/codegen.html#the-target_feature-attribute
 [`test`]: attributes/testing.html#the-test-attribute
 [`used`]: abi.html#the-used-attribute
 [`warn`]: attributes/diagnostics.html#lint-check-attributes

src/attributes/codegen.md

@@ -42,7 +42,112 @@ The *`no_builtins` [attribute]* may be applied at the crate level to disable
 optimizing certain code patterns to invocations of library functions that are
 assumed to exist.
 
+## The `target_feature` attribute
+
+The *`target_feature` [attribute]* may be applied to an [unsafe function] to
+enable code generation of that function for specific platform architecture
+features. It uses the [_MetaListNameValueStr_] syntax with a single key of
+`enable` whose value is a string of comma-separated feature names to enable.
+
+```rust,ignore
+#[target_feature(enable = "avx2")]
+unsafe fn foo_avx2() {}
+```
+
+Each [target architecture] has a set of features that may be enabled. It is an
+error to specify a feature for a target architecture that the crate is not
+being compiled for.
+
+It is [undefined behavior] to call a function that is compiled with a feature
+that is not supported on the current platform the code is running on.
+
+Functions marked with `target_feature` are not inlined into a context that
+does not support the given features. The `#[inline(always)]` attribute may not
+be used with a `target_feature` attribute.
+
+### Available features
+
+The following is a list of the available feature names.
+
+#### `x86` or `x86_64`
+
+Feature     | Implicitly Enables | Description
+------------|--------------------|-------------------
+`aes`       | `sse2`   | [AES] — Advanced Encryption Standard
+`avx`       | `sse4.2` | [AVX] — Advanced Vector Extensions
+`avx2`      | `avx`    | [AVX2] — Advanced Vector Extensions 2
+`bmi1`      |          | [BMI1] — Bit Manipulation Instruction Sets
+`bmi2`      |          | [BMI2] — Bit Manipulation Instruction Sets 2
+`fma`       | `avx`    | [FMA3] — Three-operand fused multiply-add
+`fxsr`      |          | [`fxsave`] and [`fxrstor`] — Save and restore x87 FPU, MMX Technology, and SSE State
+`lzcnt`     |          | [`lzcnt`] — Leading zeros count
+`pclmulqdq` | `sse2`   | [`pclmulqdq`] — Packed carry-less multiplication quadword
+`popcnt`    |          | [`popcnt`] — Count of bits set to 1
+`rdrand`    |          | [`rdrand`] — Read random number
+`rdseed`    |          | [`rdseed`] — Read random seed
+`sha`       | `sse2`   | [SHA] — Secure Hash Algorithm
+`sse`       |          | [SSE] — Streaming <abbr title="Single Instruction Multiple Data">SIMD</abbr> Extensions
+`sse2`      | `sse`    | [SSE2] — Streaming SIMD Extensions 2
+`sse3`      | `sse2`   | [SSE3] — Streaming SIMD Extensions 3
+`sse4.1`    | `sse3`   | [SSE4.1] — Streaming SIMD Extensions 4.1
+`sse4.2`    | `sse4.1` | [SSE4.2] — Streaming SIMD Extensions 4.2
+`ssse3`     | `sse3`   | [SSSE3] — Supplemental Streaming SIMD Extensions 3
+`xsave`     |          | [`xsave`] — Save processor extended states
+`xsavec`    |          | [`xsavec`] — Save processor extended states with compaction
+`xsaveopt`  |          | [`xsaveopt`] — Save processor extended states optimized
+`xsaves`    |          | [`xsaves`] — Save processor extended states supervisor
+
+<!-- Keep links near each table to make it easier to move and update. -->
+
+[AES]: https://en.wikipedia.org/wiki/AES_instruction_set
+[AVX]: https://en.wikipedia.org/wiki/Advanced_Vector_Extensions
+[AVX2]: https://en.wikipedia.org/wiki/Advanced_Vector_Extensions#AVX2
+[BMI1]: https://en.wikipedia.org/wiki/Bit_Manipulation_Instruction_Sets
+[BMI2]: https://en.wikipedia.org/wiki/Bit_Manipulation_Instruction_Sets#BMI2
+[FMA3]: https://en.wikipedia.org/wiki/FMA_instruction_set
+[`fxsave`]: https://www.felixcloutier.com/x86/fxsave
+[`fxrstor`]: https://www.felixcloutier.com/x86/fxrstor
+[`lzcnt`]: https://www.felixcloutier.com/x86/lzcnt
+[`pclmulqdq`]: https://www.felixcloutier.com/x86/pclmulqdq
+[`popcnt`]: https://www.felixcloutier.com/x86/popcnt
+[`rdrand`]: https://en.wikipedia.org/wiki/RdRand
+[`rdseed`]: https://en.wikipedia.org/wiki/RdRand
+[SHA]: https://en.wikipedia.org/wiki/Intel_SHA_extensions
+[SSE]: https://en.wikipedia.org/wiki/Streaming_SIMD_Extensions
+[SSE2]: https://en.wikipedia.org/wiki/SSE2
+[SSE3]: https://en.wikipedia.org/wiki/SSE3
+[SSE4.1]: https://en.wikipedia.org/wiki/SSE4#SSE4.1
+[SSE4.2]: https://en.wikipedia.org/wiki/SSE4#SSE4.2
+[SSSE3]: https://en.wikipedia.org/wiki/SSSE3
+[`xsave`]: https://www.felixcloutier.com/x86/xsave
+[`xsavec`]: https://www.felixcloutier.com/x86/xsavec
+[`xsaveopt`]: https://www.felixcloutier.com/x86/xsaveopt
+[`xsaves`]: https://www.felixcloutier.com/x86/xsaves
+
+### Additional information
+
+See the [`target_feature` conditional compilation option] for selectively
+enabling or disabling compilation of code based on compile-time settings. Note
+that this option is not affected by the `target_feature` attribute, and is
+only driven by the features enabled for the entire crate.
+
+See the [`is_x86_feature_detected`] macro in the standard library for runtime
+feature detection on the x86 platforms.
+
+> Note: `rustc` has a default set of features enabled for each target and CPU.
+> The CPU may be chosen with the [`-C target-cpu`] flag. Individual features
+> may be enabled or disabled for an entire crate with the
+> [`-C target-feature`] flag.
+
+[_MetaListNameValueStr_]: attributes.html#meta-item-attribute-syntax
+[`-C target-cpu`]: ../rustc/codegen-options/index.html#target-cpu
+[`-C target-feature`]: ../rustc/codegen-options/index.html#target-feature
+[`is_x86_feature_detected`]: ../std/macro.is_x86_feature_detected.html
+[`target_feature` conditional compilation option]: conditional-compilation.html#target_feature
 [attribute]: attributes.html
 [attributes]: attributes.html
 [functions]: items/functions.html
+[target architecture]: conditional-compilation.html#target_arch
 [trait]: items/traits.html
+[undefined behavior]: behavior-considered-undefined.html
+[unsafe function]: unsafe-functions.html

src/behavior-considered-undefined.md

@@ -38,6 +38,8 @@ code.
   * A discriminant in an `enum` not included in the type definition.
   * A value in a `char` which is a surrogate or above `char::MAX`.
   * Non-UTF-8 byte sequences in a `str`.
+* Executing code compiled with platform features that the current platform
+  does not support (see [`target_feature`]).
 
 > **Note**: Undefined behavior affects the entire program. For example, calling
 > a function in C that exhibits undefined behavior of C means your entire
@@ -48,9 +50,10 @@ code.
 [noalias]: http://llvm.org/docs/LangRef.html#noalias
 [pointer aliasing rules]: http://llvm.org/docs/LangRef.html#pointer-aliasing-rules
 [undef]: http://llvm.org/docs/LangRef.html#undefined-values
-[`offset`]: https://doc.rust-lang.org/std/primitive.pointer.html#method.offset
-[`std::ptr::copy_nonoverlapping_memory`]: https://doc.rust-lang.org/std/ptr/fn.copy_nonoverlapping.html
-[`UnsafeCell<U>`]: https://doc.rust-lang.org/std/cell/struct.UnsafeCell.html
-[`read_unaligned`]: https://doc.rust-lang.org/std/ptr/fn.read_unaligned.html
-[`write_unaligned`]: https://doc.rust-lang.org/std/ptr/fn.write_unaligned.html
+[`offset`]: ../std/primitive.pointer.html#method.offset
+[`std::ptr::copy_nonoverlapping_memory`]: ../std/ptr/fn.copy_nonoverlapping.html
+[`target_feature`]: attributes/codegen.html#the-target_feature-attribute
+[`UnsafeCell<U>`]: ../std/cell/struct.UnsafeCell.html
+[`read_unaligned`]: ../std/ptr/fn.read_unaligned.html
+[`write_unaligned`]: ../std/ptr/fn.write_unaligned.html
 [Rustonomicon]: ../nomicon/index.html

src/conditional-compilation.md

@@ -90,6 +90,25 @@ Example values:
 * `"arm"`
 * `"aarch64"`
 
+### `target_feature`
+
+Key-value option set for each platform feature available for the current
+compilation target.
+
+Example values:
+
+* `"avx"`
+* `"avx2"`
+* `"crt-static"`
+* `"rdrand"`
+* `"sse"`
+* `"sse2"`
+* `"sse4.1"`
+
+See the [`target_feature` attribute] for more details on the available
+features. An additional feature of `crt-static` is available to the
+`target_feature` option to indicate that a [static C runtime] is available.
+
 ### `target_os`
 
 Key-value option set once with the target's operating system. This value is
@@ -305,6 +324,8 @@ println!("I'm running on a {} machine!", machine_kind);
 [`cfg` macro]: #the-cfg-macro
 [`cfg_attr`]: #the-cfg_attr-attribute
 [`debug_assert!`]: ../std/macro.debug_assert.html
+[`target_feature` attribute]: attributes/codegen.html#the-target_feature-attribute
 [attribute]: attributes.html
 [attributes]: attributes.html
 [crate type]: linkage.html
+[static C runtime]: linkage.html#static-and-dynamic-c-runtimes

src/linkage.md

@@ -168,7 +168,7 @@ that it's linked as you would expect after the compiler succeeds.
 Crates may also learn about how the C runtime is being linked. Code on MSVC, for
 example, needs to be compiled differently (e.g. with `/MT` or `/MD`) depending
 on the runtime being linked. This is exported currently through the
-`target_feature` attribute (note this is a nightly feature):
+[`cfg` attribute `target_feature` option]:
 
 ```rust,ignore
 #[cfg(target_feature = "crt-static")]
@@ -210,5 +210,6 @@ a statically linked binary on MSVC you would execute:
 RUSTFLAGS='-C target-feature=+crt-static' cargo build --target x86_64-pc-windows-msvc
 ```
 
+[`cfg` attribute `target_feature` option]: conditional-compilation.html#target_feature
 [configuration option]: conditional-compilation.html
-[procedural macros]: procedural-macros.html
+[procedural macros]: procedural-macros.html