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+# Using RUSTFLAGS
+
+One of the easiest ways to benefit from SIMD is to allow the compiler
+to generate code using certain vector instruction extensions.
+
+The environment variable `RUSTFLAGS` can be used to pass options for code
+generation to the Rust compiler. These flags will affect **all** compiled crates.
+
+There are two flags which can be used to enable specific vector extensions:
+
+## target-feature
+
+- Syntax: `-C target-feature=<features>`
+
+- Provides the compiler with a comma-separated set of instruction extensions
+  to enable.
+
+  **Example**: Use `-C target-feature=+sse3,+avx` to enable generating instructions
+  for [Streaming SIMD Extensions 3](https://en.wikipedia.org/wiki/SSE3) and
+  [Advanced Vector Extensions](https://en.wikipedia.org/wiki/Advanced_Vector_Extensions).
+
+- To list target triples for all targets supported by Rust, use:
+
+  ```sh
+  rustc --print target-list
+  ```
+
+- To list all support target features for a certain target triple, use:
+
+  ```sh
+  rustc --target=${TRIPLE} --print target-features
+  ```
+
+- Note that all CPU features are independent, and will have to be enabled individually.
+
+  **Example**: Setting `-C target-feature=+avx2` will _not_ enable `fma`, even though
+  all CPUs which support AVX2 also support FMA. To enable both, one has to use
+  `-C target-feature=+avx2,+fma`
+
+- Some features also depend on other features, which need to be enabled for the
+  target instructions to be generated.
+
+  **Example**: Unless `v7` is specified as the target CPU (see below), to enable
+  NEON on ARM it is necessary to use `-C target-feature=+v7,+neon`.
+
+## target-cpu
+
+- Syntax: `-C target-cpu=<cpu>`
+
+- Sets the identifier of a CPU family / model for which to build and optimize the code.
+
+  **Example**: `RUSTFLAGS='-C target-cpu=cortex-a75'`
+
+- To list all supported target CPUs for a certain target triple, use:
+
+  ```sh
+  rustc --target=${TRIPLE} --print target-cpus
+  ```
+
+  **Example**:
+
+  ```sh
+  rustc --target=i686-pc-windows-msvc --print target-cpus
+  ```
+
+- The compiler will translate this into a list of target features. Therefore,
+  individual feature checks (`#[cfg(target_feature = "...")]`) will still
+  work properly.
+
+- It will cause the code generator to optimize the generated code for that
+  specific CPU model.
+
+- Using `native` as the CPU model will cause Rust to generate and optimize code
+  for the CPU running the compiler. It is useful when building programs which you
+  plan to only use locally. This should never be used when the generated programs
+  are meant to be run on other computers, such as when packaging for distribution
+  or cross-compiling.