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Add unstable book entries for parts of asm that are not being stabilized
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‎src/doc/unstable-book/src/language-features/asm-const.md

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# `asm_const`
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The tracking issue for this feature is: [#72016]
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[#72016]: https://github.com/rust-lang/rust/issues/72016
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------------------------
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This feature adds a `const <expr>` operand type to `asm!` and `global_asm!`.
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- `<expr>` must be an integer constant expression.
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- The value of the expression is formatted as a string and substituted directly into the asm template string.

‎src/doc/unstable-book/src/language-features/asm-experimental-arch.md

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# `asm_experimental_arch`
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The tracking issue for this feature is: [#72016]
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[#72016]: https://github.com/rust-lang/rust/issues/72016
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------------------------
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This feature tracks `asm!` and `global_asm!` support for the following architectures:
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- NVPTX
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- PowerPC
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- Hexagon
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- MIPS32r2 and MIPS64r2
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- wasm32
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- BPF
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- SPIR-V
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- AVR
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## Register classes
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| Architecture | Register class | Registers | LLVM constraint code |
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| ------------ | -------------- | ---------------------------------- | -------------------- |
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| MIPS | `reg` | `$[2-25]` | `r` |
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| MIPS | `freg` | `$f[0-31]` | `f` |
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| NVPTX | `reg16` | None\* | `h` |
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| NVPTX | `reg32` | None\* | `r` |
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| NVPTX | `reg64` | None\* | `l` |
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| Hexagon | `reg` | `r[0-28]` | `r` |
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| PowerPC | `reg` | `r[0-31]` | `r` |
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| PowerPC | `reg_nonzero` | `r[1-31]` | `b` |
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| PowerPC | `freg` | `f[0-31]` | `f` |
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| PowerPC | `cr` | `cr[0-7]`, `cr` | Only clobbers |
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| PowerPC | `xer` | `xer` | Only clobbers |
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| wasm32 | `local` | None\* | `r` |
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| BPF | `reg` | `r[0-10]` | `r` |
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| BPF | `wreg` | `w[0-10]` | `w` |
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| AVR | `reg` | `r[2-25]`, `XH`, `XL`, `ZH`, `ZL` | `r` |
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| AVR | `reg_upper` | `r[16-25]`, `XH`, `XL`, `ZH`, `ZL` | `d` |
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| AVR | `reg_pair` | `r3r2` .. `r25r24`, `X`, `Z` | `r` |
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| AVR | `reg_iw` | `r25r24`, `X`, `Z` | `w` |
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| AVR | `reg_ptr` | `X`, `Z` | `e` |
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> **Notes**:
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> - NVPTX doesn't have a fixed register set, so named registers are not supported.
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>
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> - WebAssembly doesn't have registers, so named registers are not supported.
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# Register class supported types
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| Architecture | Register class | Target feature | Allowed types |
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| ------------ | ------------------------------- | -------------- | --------------------------------------- |
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| MIPS32 | `reg` | None | `i8`, `i16`, `i32`, `f32` |
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| MIPS32 | `freg` | None | `f32`, `f64` |
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| MIPS64 | `reg` | None | `i8`, `i16`, `i32`, `i64`, `f32`, `f64` |
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| MIPS64 | `freg` | None | `f32`, `f64` |
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| NVPTX | `reg16` | None | `i8`, `i16` |
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| NVPTX | `reg32` | None | `i8`, `i16`, `i32`, `f32` |
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| NVPTX | `reg64` | None | `i8`, `i16`, `i32`, `f32`, `i64`, `f64` |
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| Hexagon | `reg` | None | `i8`, `i16`, `i32`, `f32` |
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| PowerPC | `reg` | None | `i8`, `i16`, `i32` |
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| PowerPC | `reg_nonzero` | None | `i8`, `i16`, `i32` |
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| PowerPC | `freg` | None | `f32`, `f64` |
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| PowerPC | `cr` | N/A | Only clobbers |
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| PowerPC | `xer` | N/A | Only clobbers |
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| wasm32 | `local` | None | `i8` `i16` `i32` `i64` `f32` `f64` |
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| BPF | `reg` | None | `i8` `i16` `i32` `i64` |
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| BPF | `wreg` | `alu32` | `i8` `i16` `i32` |
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| AVR | `reg`, `reg_upper` | None | `i8` |
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| AVR | `reg_pair`, `reg_iw`, `reg_ptr` | None | `i16` |
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## Register aliases
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| Architecture | Base register | Aliases |
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| ------------ | ------------- | --------- |
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| Hexagon | `r29` | `sp` |
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| Hexagon | `r30` | `fr` |
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| Hexagon | `r31` | `lr` |
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| BPF | `r[0-10]` | `w[0-10]` |
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| AVR | `XH` | `r27` |
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| AVR | `XL` | `r26` |
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| AVR | `ZH` | `r31` |
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| AVR | `ZL` | `r30` |
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## Unsupported registers
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| Architecture | Unsupported register | Reason |
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| ------------ | --------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
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| All | `sp` | The stack pointer must be restored to its original value at the end of an asm code block. |
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| All | `fr` (Hexagon), `$fp` (MIPS), `Y` (AVR) | The frame pointer cannot be used as an input or output. |
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| All | `r19` (Hexagon) | This is used internally by LLVM as a "base pointer" for functions with complex stack frames. |
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| MIPS | `$0` or `$zero` | This is a constant zero register which can't be modified. |
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| MIPS | `$1` or `$at` | Reserved for assembler. |
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| MIPS | `$26`/`$k0`, `$27`/`$k1` | OS-reserved registers. |
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| MIPS | `$28`/`$gp` | Global pointer cannot be used as inputs or outputs. |
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| MIPS | `$ra` | Return address cannot be used as inputs or outputs. |
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| Hexagon | `lr` | This is the link register which cannot be used as an input or output. |
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| AVR | `r0`, `r1`, `r1r0` | Due to an issue in LLVM, the `r0` and `r1` registers cannot be used as inputs or outputs. If modified, they must be restored to their original values before the end of the block. |
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## Template modifiers
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| Architecture | Register class | Modifier | Example output | LLVM modifier |
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| ------------ | -------------- | -------- | -------------- | ------------- |
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| MIPS | `reg` | None | `$2` | None |
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| MIPS | `freg` | None | `$f0` | None |
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| NVPTX | `reg16` | None | `rs0` | None |
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| NVPTX | `reg32` | None | `r0` | None |
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| NVPTX | `reg64` | None | `rd0` | None |
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| Hexagon | `reg` | None | `r0` | None |
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| PowerPC | `reg` | None | `0` | None |
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| PowerPC | `reg_nonzero` | None | `3` | `b` |
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| PowerPC | `freg` | None | `0` | None |
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# Flags covered by `preserves_flags`
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These flags registers must be restored upon exiting the asm block if the `preserves_flags` option is set:
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- AVR
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- The status register `SREG`.

‎src/doc/unstable-book/src/language-features/asm-sym.md

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# `asm_sym`
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The tracking issue for this feature is: [#72016]
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[#72016]: https://github.com/rust-lang/rust/issues/72016
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------------------------
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This feature adds a `sym <path>` operand type to `asm!` and `global_asm!`.
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- `<path>` must refer to a `fn` or `static`.
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- A mangled symbol name referring to the item is substituted into the asm template string.
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- The substituted string does not include any modifiers (e.g. GOT, PLT, relocations, etc).
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- `<path>` is allowed to point to a `#[thread_local]` static, in which case the asm code can combine the symbol with relocations (e.g. `@plt`, `@TPOFF`) to read from thread-local data.

‎src/doc/unstable-book/src/language-features/asm-unwind.md

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# `asm_unwind`
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The tracking issue for this feature is: [#72016]
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[#72016]: https://github.com/rust-lang/rust/issues/72016
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------------------------
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This feature adds a `may_unwind` option to `asm!` which allows an `asm` block to unwind stack and be part of the stack unwinding process. This option is only supported by the LLVM backend right now.

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