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Update codegen.md

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Lokathor 2023-06-27 13:10:11 -06:00 committed by GitHub
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@ -361,6 +361,10 @@ It uses the [_MetaListPath_] syntax, and a path comprised of the architecture fa
[_MetaListPath_]: ../attributes.md#meta-item-attribute-syntax
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.
@ -375,9 +379,7 @@ fn foo_arm_code() {}
fn bar_thumb_code() {}
```
If your function has neither the `instruction_set` attribute nor inline assembly, then the code you write within that function should not presume any particular instruction set.
This ends up creating a limitation to how often code is inlined:
Using the `instruction_set` attribute has the following effects:
* If a function has an `instruction_set` attribute it won't inline into a function of another instruction set.
* If a function does not have an `instruction_set` attribute but *does* contain inline assembly, then the inline assembly is assumed to require the default instruction set of the build target, and so inlining between different instruction sets won't happen.
* Otherwise, inlining happens normally.
* 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.