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Merge pull request #1307 from Lokathor/patch-1 

Clearly specify the `instruction_set` effects
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Eric Huss 2023-08-01 16:07:09 +00:00 committed by GitHub
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src/attributes/codegen.md
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@ -361,15 +361,20 @@ trait object whose methods are attributed.
## The `instruction_set` attribute
The *`instruction_set` attribute* may be applied to a function to enable code generation for a specific
instruction set supported by the target architecture. It uses the [_MetaListPath_] syntax and a path
comprised of the architecture and instruction set to specify how to generate the code for
architectures where a single program may utilize multiple instruction sets.
The *`instruction_set` [attribute]* may be applied to a function to control which instruction set the function will be generated for.
This allows mixing more than one instruction set in a single program on CPU architectures that support it.
It uses the [_MetaListPath_] syntax, and a path comprised of the architecture family name and instruction set name.
The following values are available on targets for the `ARMv4` and `ARMv5te` architectures:
[_MetaListPath_]: ../attributes.md#meta-item-attribute-syntax
* `arm::a32` - Uses ARM code.
* `arm::t32` - Uses Thumb code.
It is a compilation error to use the `instruction_set` attribute on a target that does not support it.
### 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
@ -380,4 +385,7 @@ fn foo_arm_code() {}
fn bar_thumb_code() {}
```
[_MetaListPath_]: ../attributes.md#meta-item-attribute-syntax
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.