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Rollup merge of rust-lang#130877 - taiki-e:riscv-atomic, r=Amanieu 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](llvm/llvm-project@8be079c)) - `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](llvm/llvm-project@6b74449)) - `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](llvm/llvm-project@8be079c)) (Zacas Extension is not included here because it is still marked as experimental in LLVM 19 llvm/llvm-project@70e7d26 and will become non-experimental in LLVM 20 llvm/llvm-project@614aeda) `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" (riscv/riscv-isa-manual@e87412e), and [`a` implies `zaamo` and `zalrsc` in GCC](https://github.com/gcc-mirror/gcc/blob/08693e29ec186fd7941d0b73d4d466388971fe2f/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](https://github.com/gcc-mirror/gcc/blob/08693e29ec186fd7941d0b73d4d466388971fe2f/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
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