acaa6cee07
rustc_target: Add RISC-V atomic-related features
This adds the following three target features to unstable riscv_target_feature.
- `zaamo` (Zaamo Extension 1.0.0): Atomic Memory Operations (`amo*.{w,d}{,.aq,.rl,.aqrl}`)
([definition in LLVM](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/RISCV/RISCVFeatures.td#L229-L231), [available since LLVM 19](8be079cddd))
- `zabha` (Zabha Extension 1.0.0): Byte and Halfword Atomic Memory Operations (`amo*.{b,h}{,.aq,.rl,.aqrl}`)
([definition in LLVM](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/RISCV/RISCVFeatures.td#L238-L240), [available since LLVM 19](6b7444964a))
- `zalrsc` (Zalrsc Extension 1.0.0): Load-Reserved/Store-Conditional Instructions (`lr.{w,d}{,.aq,.rl,.aqrl}` and `sc.{w,d}{,.aq,.rl,.aqrl}`)
([definition in LLVM](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/RISCV/RISCVFeatures.td#L261-L263), [available since LLVM 19](8be079cddd))
(Zacas Extension is not included here because it is still marked as experimental in LLVM 19 70e7d26e56 and will become non-experimental in LLVM 20 614aeda93b)
`a` implies `zaamo` and `zalrsc`, and `zabha` implies `zaamo`:
- After Zaamo and Zalrsc Extensions are frozen, riscv-isa-manual says "The A extension comprises instructions provided by the Zaamo and Zalrsc extensions" (e87412e621), and [`a` implies `zaamo` and `zalrsc` in GCC](08693e29ec/gcc/config/riscv/arch-canonicalize (L44)). However, in LLVM, [`a` does not define them as implying `zaamo` and `zalrsc`](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/RISCV/RISCVFeatures.td#L206).
- Zabha and Zaamo are in a similar situation, [riscv-isa-manual](https://github.com/riscv/riscv-isa-manual/blob/main/src/zabha.adoc) says "The Zabha extension depends upon the Zaamo standard extension", and [`zabha` implies `zaamo` in GCC](08693e29ec/gcc/config/riscv/arch-canonicalize (L45-L46)), but [does not in LLVM (but enabling `zabha` without `zaamo` or `a` is not allowed)](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/TargetParser/RISCVISAInfo.cpp#L776-L778).
r? `@Amanieu`
`@rustbot` label +O-riscv +A-target-feature
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc dev guide.