codegen.md

r[attributes.codegen]
# Code generation attributes


The following [attributes] are used for controlling code generation.

r[attributes.codegen.hint]
## Optimization hints

r[attributes.codegen.hint.cold-inline]
The `cold` and `inline` [attributes] give suggestions to generate code in a
way that may be faster than what it would do without the hint. The attributes
are only hints, and may be ignored.

r[attributes.codegen.hint.usage]
Both attributes can be used on [functions]. When applied to a function in a
[trait], they apply only to that function when used as a default function for
a trait implementation and not to all trait implementations. The attributes
have no effect on a trait function without a body.

r[attributes.codegen.inline]
### The `inline` attribute

r[attributes.codegen.inline.intro]
The *`inline` [attribute]* suggests that a copy of the attributed function
should be placed in the caller, rather than generating code to call the
function where it is defined.

> ***Note***: The `rustc` compiler automatically inlines functions based on
> internal heuristics. Incorrectly inlining functions can make the program
> slower, so this attribute should be used with care.

r[attributes.codegen.inline.modes]
There are three ways to use the inline attribute:

* `#[inline]` *suggests* performing an inline expansion.
* `#[inline(always)]` *suggests* that an inline expansion should always be
  performed.
* `#[inline(never)]` *suggests* that an inline expansion should never be
  performed.

> ***Note***: `#[inline]` in every form is a hint, with no *requirements*
> on the language to place a copy of the attributed function in the caller.

r[attributes.codegen.cold]
### The `cold` attribute


The *`cold` [attribute]* suggests that the attributed function is unlikely to
be called.

r[attributes.codegen.no_builtins]
## The `no_builtins` attribute


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.

r[attributes.codegen.target_feature]
## The `target_feature` attribute

r[attributes.codegen.target_feature.intro]
The *`target_feature` [attribute]* may be applied to a 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
# #[cfg(target_feature = "avx2")]
#[target_feature(enable = "avx2")]
unsafe fn foo_avx2() {}
```

r[attributes.codegen.target_feature.arch]
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.

r[attributes.codegen.target_feature.target-ub]
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, *except*
if the platform explicitly documents this to be safe.

r[attributes.codegen.target_feature.inline]
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.

r[attributes.codegen.target_feature.availability]
### Available features


The following is a list of the available feature names.

r[attributes.codegen.target_feature.x86]
#### `x86` or `x86_64`


Executing code with unsupported features is undefined behavior on this platform.
Hence this platform requires that `#[target_feature]` is only applied to [`unsafe`
functions][unsafe function].

Feature     | Implicitly Enables | Description
------------|--------------------|-------------------
`adx`       |          | [ADX] --- Multi-Precision Add-Carry Instruction Extensions
`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
`cmpxchg16b`|          | [`cmpxchg16b`] --- Compares and exchange 16 bytes (128 bits) of data atomically
`f16c`      | `avx`    | [F16C] --- 16-bit floating point conversion instructions
`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
`movbe`     |          | [`movbe`] --- Move data after swapping bytes
`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`    | `ssse3`  | [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. -->

[ADX]: https://en.wikipedia.org/wiki/Intel_ADX
[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
[`cmpxchg16b`]: https://www.felixcloutier.com/x86/cmpxchg8b:cmpxchg16b
[F16C]: https://en.wikipedia.org/wiki/F16C
[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
[`movbe`]: https://www.felixcloutier.com/x86/movbe
[`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

r[attributes.codegen.target_feature.aarch64]
#### `aarch64`


This platform requires that `#[target_feature]` is only applied to [`unsafe`
functions][unsafe function].

Further documentation on these features can be found in the [ARM Architecture
Reference Manual], or elsewhere on [developer.arm.com].

[ARM Architecture Reference Manual]: https://developer.arm.com/documentation/ddi0487/latest
[developer.arm.com]: https://developer.arm.com

> ***Note***: The following pairs of features should both be marked as enabled
> or disabled together if used:
> - `paca` and `pacg`, which LLVM currently implements as one feature.


Feature        | Implicitly Enables | Feature Name
---------------|--------------------|-------------------
`aes`          | `neon`         | FEAT_AES & FEAT_PMULL --- Advanced <abbr title="Single Instruction Multiple Data">SIMD</abbr> AES & PMULL instructions
`bf16`         |                | FEAT_BF16 --- BFloat16 instructions
`bti`          |                | FEAT_BTI --- Branch Target Identification
`crc`          |                | FEAT_CRC --- CRC32 checksum instructions
`dit`          |                | FEAT_DIT --- Data Independent Timing instructions
`dotprod`      |                | FEAT_DotProd --- Advanced SIMD Int8 dot product instructions
`dpb`          |                | FEAT_DPB --- Data cache clean to point of persistence
`dpb2`         |                | FEAT_DPB2 --- Data cache clean to point of deep persistence
`f32mm`        | `sve`          | FEAT_F32MM --- SVE single-precision FP matrix multiply instruction
`f64mm`        | `sve`          | FEAT_F64MM --- SVE double-precision FP matrix multiply instruction
`fcma`         | `neon`         | FEAT_FCMA --- Floating point complex number support
`fhm`          | `fp16`         | FEAT_FHM --- Half-precision FP FMLAL instructions
`flagm`        |                | FEAT_FlagM --- Conditional flag manipulation
`fp16`         | `neon`         | FEAT_FP16 --- Half-precision FP data processing
`frintts`      |                | FEAT_FRINTTS --- Floating-point to int helper instructions
`i8mm`         |                | FEAT_I8MM --- Int8 Matrix Multiplication
`jsconv`       | `neon`         | FEAT_JSCVT --- JavaScript conversion instruction
`lse`          |                | FEAT_LSE --- Large System Extension
`lor`          |                | FEAT_LOR --- Limited Ordering Regions extension
`mte`          |                | FEAT_MTE & FEAT_MTE2 --- Memory Tagging Extension
`neon`         |                | FEAT_FP & FEAT_AdvSIMD --- Floating Point and Advanced SIMD extension
`pan`          |                | FEAT_PAN --- Privileged Access-Never extension
`paca`         |                | FEAT_PAuth --- Pointer Authentication (address authentication)
`pacg`         |                | FEAT_PAuth --- Pointer Authentication (generic authentication)
`pmuv3`        |                | FEAT_PMUv3 --- Performance Monitors extension (v3)
`rand`         |                | FEAT_RNG --- Random Number Generator
`ras`          |                | FEAT_RAS & FEAT_RASv1p1 --- Reliability, Availability and Serviceability extension
`rcpc`         |                | FEAT_LRCPC --- Release consistent Processor Consistent
`rcpc2`        | `rcpc`         | FEAT_LRCPC2 --- RcPc with immediate offsets
`rdm`          |                | FEAT_RDM --- Rounding Double Multiply accumulate
`sb`           |                | FEAT_SB --- Speculation Barrier
`sha2`         | `neon`         | FEAT_SHA1 & FEAT_SHA256 --- Advanced SIMD SHA instructions
`sha3`         | `sha2`         | FEAT_SHA512 & FEAT_SHA3 --- Advanced SIMD SHA instructions
`sm4`          | `neon`         | FEAT_SM3 & FEAT_SM4 --- Advanced SIMD SM3/4 instructions
`spe`          |                | FEAT_SPE --- Statistical Profiling Extension
`ssbs`         |                | FEAT_SSBS & FEAT_SSBS2 --- Speculative Store Bypass Safe
`sve`          | `fp16`         | FEAT_SVE --- Scalable Vector Extension
`sve2`         | `sve`          | FEAT_SVE2 --- Scalable Vector Extension 2
`sve2-aes`     | `sve2`, `aes`  | FEAT_SVE_AES --- SVE AES instructions
`sve2-sm4`     | `sve2`, `sm4`  | FEAT_SVE_SM4 --- SVE SM4 instructions
`sve2-sha3`    | `sve2`, `sha3` | FEAT_SVE_SHA3 --- SVE SHA3 instructions
`sve2-bitperm` | `sve2`         | FEAT_SVE_BitPerm --- SVE Bit Permute
`tme`          |                | FEAT_TME --- Transactional Memory Extension
`vh`           |                | FEAT_VHE --- Virtualization Host Extensions

r[attributes.codegen.target_feature.riscv]
#### `riscv32` or `riscv64`


This platform requires that `#[target_feature]` is only applied to [`unsafe`
functions][unsafe function].

Further documentation on these features can be found in their respective
specification. Many specifications are described in the [RISC-V ISA Manual] or
in another manual hosted on the [RISC-V GitHub Account].

[RISC-V ISA Manual]: https://github.com/riscv/riscv-isa-manual
[RISC-V GitHub Account]: https://github.com/riscv

Feature     | Implicitly Enables  | Description
------------|---------------------|-------------------
`a`         |                     | [A][rv-a] --- Atomic instructions
`c`         |                     | [C][rv-c] --- Compressed instructions
`m`         |                     | [M][rv-m] --- Integer Multiplication and Division instructions
`zb`        | `zba`, `zbc`, `zbs` | [Zb][rv-zb] --- Bit Manipulation instructions
`zba`       |                     | [Zba][rv-zb-zba] --- Address Generation instructions
`zbb`       |                     | [Zbb][rv-zb-zbb] --- Basic bit-manipulation
`zbc`       |                     | [Zbc][rv-zb-zbc] --- Carry-less multiplication
`zbkb`      |                     | [Zbkb][rv-zb-zbkb] --- Bit Manipulation Instructions for Cryptography
`zbkc`      |                     | [Zbkc][rv-zb-zbc] --- Carry-less multiplication for Cryptography
`zbkx`      |                     | [Zbkx][rv-zb-zbkx] --- Crossbar permutations
`zbs`       |                     | [Zbs][rv-zb-zbs] --- Single-bit instructions
`zk`        | `zkn`, `zkr`, `zks`, `zkt`, `zbkb`, `zbkc`, `zkbx` | [Zk][rv-zk] --- Scalar Cryptography
`zkn`       | `zknd`, `zkne`, `zknh`, `zbkb`, `zbkc`, `zkbx`     | [Zkn][rv-zkn] --- NIST Algorithm suite extension
`zknd`      |                                                    | [Zknd][rv-zknd] --- NIST Suite: AES Decryption
`zkne`      |                                                    | [Zkne][rv-zkne] --- NIST Suite: AES Encryption
`zknh`      |                                                    | [Zknh][rv-zknh] --- NIST Suite: Hash Function Instructions
`zkr`       |                                                    | [Zkr][rv-zkr] --- Entropy Source Extension
`zks`       | `zksed`, `zksh`, `zbkb`, `zbkc`, `zkbx`            | [Zks][rv-zks] --- ShangMi Algorithm Suite
`zksed`     |                                                    | [Zksed][rv-zksed] --- ShangMi Suite: SM4 Block Cipher Instructions
`zksh`      |                                                    | [Zksh][rv-zksh] --- ShangMi Suite: SM3 Hash Function Instructions
`zkt`       |                                                    | [Zkt][rv-zkt] --- Data Independent Execution Latency Subset

<!-- Keep links near each table to make it easier to move and update. -->

[rv-a]: https://github.com/riscv/riscv-isa-manual/blob/de46343a245c6ee1f7b1a40c92fe1a86bd4f4978/src/a-st-ext.adoc
[rv-c]: https://github.com/riscv/riscv-isa-manual/blob/de46343a245c6ee1f7b1a40c92fe1a86bd4f4978/src/c-st-ext.adoc
[rv-m]: https://github.com/riscv/riscv-isa-manual/blob/de46343a245c6ee1f7b1a40c92fe1a86bd4f4978/src/m-st-ext.adoc
[rv-zb]: https://github.com/riscv/riscv-bitmanip
[rv-zb-zba]: https://github.com/riscv/riscv-bitmanip/blob/main/bitmanip/zba.adoc
[rv-zb-zbb]: https://github.com/riscv/riscv-bitmanip/blob/main/bitmanip/zbb.adoc
[rv-zb-zbc]: https://github.com/riscv/riscv-bitmanip/blob/main/bitmanip/zbc.adoc
[rv-zb-zbkb]: https://github.com/riscv/riscv-bitmanip/blob/main/bitmanip/zbkb.adoc
[rv-zb-zbkc]: https://github.com/riscv/riscv-bitmanip/blob/main/bitmanip/zbkc.adoc
[rv-zb-zbkx]: https://github.com/riscv/riscv-bitmanip/blob/main/bitmanip/zbkx.adoc
[rv-zb-zbs]: https://github.com/riscv/riscv-bitmanip/blob/main/bitmanip/zbs.adoc
[rv-zk]: https://github.com/riscv/riscv-crypto/blob/e2dd7d98b7f34d477e38cb5fd7a3af4379525189/doc/scalar/riscv-crypto-scalar-zk.adoc
[rv-zkn]: https://github.com/riscv/riscv-crypto/blob/e2dd7d98b7f34d477e38cb5fd7a3af4379525189/doc/scalar/riscv-crypto-scalar-zkn.adoc
[rv-zkne]: https://github.com/riscv/riscv-crypto/blob/e2dd7d98b7f34d477e38cb5fd7a3af4379525189/doc/scalar/riscv-crypto-scalar-zkne.adoc
[rv-zknd]: https://github.com/riscv/riscv-crypto/blob/e2dd7d98b7f34d477e38cb5fd7a3af4379525189/doc/scalar/riscv-crypto-scalar-zknd.adoc
[rv-zknh]: https://github.com/riscv/riscv-crypto/blob/e2dd7d98b7f34d477e38cb5fd7a3af4379525189/doc/scalar/riscv-crypto-scalar-zknh.adoc
[rv-zkr]: https://github.com/riscv/riscv-crypto/blob/e2dd7d98b7f34d477e38cb5fd7a3af4379525189/doc/scalar/riscv-crypto-scalar-zkr.adoc
[rv-zks]: https://github.com/riscv/riscv-crypto/blob/e2dd7d98b7f34d477e38cb5fd7a3af4379525189/doc/scalar/riscv-crypto-scalar-zks.adoc
[rv-zksed]: https://github.com/riscv/riscv-crypto/blob/e2dd7d98b7f34d477e38cb5fd7a3af4379525189/doc/scalar/riscv-crypto-scalar-zksed.adoc
[rv-zksh]: https://github.com/riscv/riscv-crypto/blob/e2dd7d98b7f34d477e38cb5fd7a3af4379525189/doc/scalar/riscv-crypto-scalar-zksh.adoc
[rv-zkt]: https://github.com/riscv/riscv-crypto/blob/e2dd7d98b7f34d477e38cb5fd7a3af4379525189/doc/scalar/riscv-crypto-scalar-zkt.adoc

r[attributes.codegen.target_feature.wasm]
#### `wasm32` or `wasm64`


`#[target_feature]` may be used with both safe and
[`unsafe` functions][unsafe function] on Wasm platforms. It is impossible to
cause undefined behavior via the `#[target_feature]` attribute because
attempting to use instructions unsupported by the Wasm engine will fail at load
time without the risk of being interpreted in a way different from what the
compiler expected.

Feature               | Implicitly Enables  | Description
----------------------|---------------------|-------------------
`bulk-memory`         |                     | [WebAssembly bulk memory operations proposal][bulk-memory]
`extended-const`      |                     | [WebAssembly extended const expressions proposal][extended-const]
`mutable-globals`     |                     | [WebAssembly mutable global proposal][mutable-globals]
`nontrapping-fptoint` |                     | [WebAssembly non-trapping float-to-int conversion proposal][nontrapping-fptoint]
`relaxed-simd`        | `simd128`           | [WebAssembly relaxed simd proposal][relaxed-simd]
`sign-ext`            |                     | [WebAssembly sign extension operators Proposal][sign-ext]
`simd128`             |                     | [WebAssembly simd proposal][simd128]
`multivalue`          |                     | [WebAssembly multivalue proposal][multivalue]
`reference-types`     |                     | [WebAssembly reference-types proposal][reference-types]
`tail-call`           |                     | [WebAssembly tail-call proposal][tail-call]

[bulk-memory]: https://github.com/WebAssembly/bulk-memory-operations
[extended-const]: https://github.com/WebAssembly/extended-const
[mutable-globals]: https://github.com/WebAssembly/mutable-global
[nontrapping-fptoint]: https://github.com/WebAssembly/nontrapping-float-to-int-conversions
[relaxed-simd]: https://github.com/WebAssembly/relaxed-simd
[sign-ext]: https://github.com/WebAssembly/sign-extension-ops
[simd128]: https://github.com/webassembly/simd
[reference-types]: https://github.com/webassembly/reference-types
[tail-call]: https://github.com/webassembly/tail-call
[multivalue]: https://github.com/webassembly/multi-value

r[attributes.codegen.target_feature.info]
### Additional information

r[attributes.codegen.target_feature.remark-cfg]
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.

r[attributes.codegen.target_feature.remark-rt]
See the [`is_x86_feature_detected`] or [`is_aarch64_feature_detected`] macros
in the standard library for runtime feature detection on these 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.

r[attributes.codegen.track_caller]
## The `track_caller` attribute

r[attributes.codegen.track_caller.allowed-positions]
The `track_caller` attribute may be applied to any function with [`"Rust"` ABI][rust-abi]
with the exception of the entry point `fn main`.

r[attributes.codegen.track_caller.traits]
When applied to functions and methods in trait declarations, the attribute applies to all implementations. If the trait provides a
default implementation with the attribute, then the attribute also applies to override implementations.

r[attributes.codegen.track_caller.extern]
When applied to a function in an `extern` block the attribute must also be applied to any linked
implementations, otherwise undefined behavior results. When applied to a function which is made
available to an `extern` block, the declaration in the `extern` block must also have the attribute,
otherwise undefined behavior results.

r[attributes.codegen.track_caller.behavior]
### Behavior

Applying the attribute to a function `f` allows code within `f` to get a hint of the [`Location`] of
the "topmost" tracked call that led to `f`'s invocation. At the point of observation, an
implementation behaves as if it walks up the stack from `f`'s frame to find the nearest frame of an
*unattributed* function `outer`, and it returns the [`Location`] of the tracked call in `outer`.

```rust
#[track_caller]
fn f() {
    println!("{}", std::panic::Location::caller());
}
```

> Note: `core` provides [`core::panic::Location::caller`] for observing caller locations. It wraps
> the [`core::intrinsics::caller_location`] intrinsic implemented by `rustc`.

> Note: because the resulting `Location` is a hint, an implementation may halt its walk up the stack
> early. See [Limitations](#limitations) for important caveats.

#### Examples

When `f` is called directly by `calls_f`, code in `f` observes its callsite within `calls_f`:

```rust
# #[track_caller]
# fn f() {
#     println!("{}", std::panic::Location::caller());
# }
fn calls_f() {
    f(); // <-- f() prints this location
}
```

When `f` is called by another attributed function `g` which is in turn called by `calls_g`, code in
both `f` and `g` observes `g`'s callsite within `calls_g`:

```rust
# #[track_caller]
# fn f() {
#     println!("{}", std::panic::Location::caller());
# }
#[track_caller]
fn g() {
    println!("{}", std::panic::Location::caller());
    f();
}

fn calls_g() {
    g(); // <-- g() prints this location twice, once itself and once from f()
}
```

When `g` is called by another attributed function `h` which is in turn called by `calls_h`, all code
in `f`, `g`, and `h` observes `h`'s callsite within `calls_h`:

```rust
# #[track_caller]
# fn f() {
#     println!("{}", std::panic::Location::caller());
# }
# #[track_caller]
# fn g() {
#     println!("{}", std::panic::Location::caller());
#     f();
# }
#[track_caller]
fn h() {
    println!("{}", std::panic::Location::caller());
    g();
}

fn calls_h() {
    h(); // <-- prints this location three times, once itself, once from g(), once from f()
}
```

And so on.

r[attributes.codegen.track_caller.limits]
### Limitations

r[attributes.codegen.track_caller.hint]
This information is a hint and implementations are not required to preserve it.

r[attributes.codegen.track_caller.decay]
In particular, coercing a function with `#[track_caller]` to a function pointer creates a shim which
appears to observers to have been called at the attributed function's definition site, losing actual
caller information across virtual calls. A common example of this coercion is the creation of a
trait object whose methods are attributed.

> Note: The aforementioned shim for function pointers is necessary because `rustc` implements
> `track_caller` in a codegen context by appending an implicit parameter to the function ABI, but
> this would be unsound for an indirect call because the parameter is not a part of the function's
> type and a given function pointer type may or may not refer to a function with the attribute. The
> creation of a shim hides the implicit parameter from callers of the function pointer, preserving
> soundness.

[_MetaListNameValueStr_]: ../attributes.md#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/arch/macro.is_x86_feature_detected.html
[`is_aarch64_feature_detected`]: ../../std/arch/macro.is_aarch64_feature_detected.html
[`target_feature` conditional compilation option]: ../conditional-compilation.md#target_feature
[attribute]: ../attributes.md
[attributes]: ../attributes.md
[functions]: ../items/functions.md
[target architecture]: ../conditional-compilation.md#target_arch
[trait]: ../items/traits.md
[undefined behavior]: ../behavior-considered-undefined.md
[unsafe function]: ../unsafe-keyword.md
[rust-abi]: ../items/external-blocks.md#abi
[`Location`]: core::panic::Location

r[attributes.codegen.instruction_set]
## The `instruction_set` attribute

r[attributes.codegen.instruction_set.allowed-positions]
The *`instruction_set` [attribute]* may be applied to a function to control which instruction set the function will be generated for.

r[attributes.codegen.instruction_set.behavior]
This allows mixing more than one instruction set in a single program on CPU architectures that support it.

r[attributes.codegen.instruction_set.syntax]
It uses the [_MetaListPath_] syntax, and a path comprised of the architecture family name and instruction set name.

[_MetaListPath_]: ../attributes.md#meta-item-attribute-syntax

r[attributes.codegen.instruction_set.target-limits]
It is a compilation error to use the `instruction_set` attribute on a target that does not support it.

r[attributes.codegen.instruction_set.arm]
### On ARM


For the `ARMv4T` and `ARMv5te` architectures, the following are supported:
* `arm::a32` --- Generate the function as A32 "ARM" code.
* `arm::t32` --- Generate the function as T32 "Thumb" code.

<!-- ignore: arm-only -->
```rust,ignore
#[instruction_set(arm::a32)]
fn foo_arm_code() {}

#[instruction_set(arm::t32)]
fn bar_thumb_code() {}
```

Using the `instruction_set` attribute has the following effects:

* If the address of the function is taken as a function pointer, the low bit of the address will be set to 0 (arm) or 1 (thumb) depending on the instruction set.
* Any inline assembly in the function must use the specified instruction set instead of the target default.