rustc_target: RISC-V: feature addition batch 3 #140507
Open
+21
−1
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
rustbot
added
S-waiting-on-review
The Za64rs extension (reservation set -- a primitive memory unit of LR/SC atomic operations -- is naturally aligned and *at most* 64 bytes) is a superset of the Za128rs extension (*at most* 128 bytes; note that smaller the reservation set is, more fine grained control over atomics). This commit handles this as a feature implication.
This commit adds a part of RISC-V extensions that are mandatory part of the RVA23U64 profile (application-class processor profile) and related to memory/atomic constraints. The Zic64b extension constrains the cache line to naturally-aligned 64 bytes that would make certain memory operations (like zeroing the memory using the Zicboz extension) easier. The Zicbom and Zicbop extensions enable managing cache block-based operations (the Zicbop contains hints that will work as a NOP when this extension is absent and the Zicbom contains control instructions). Of which, the Zicbom extension is going to be discoverable from the Linux kernel (as of the version 6.15-rc4) and this commit prepares for corresponding stdarch changes. The Zicc* extensions add certain constraints to "the main memory" (usually true on the user mode application on the application-class processor but those extensions make sure such constraints exist).
This commit adds three ratified unprivileged RISC-V extensions related to BFloat16 (BF16) handling. Although that they are far from stabilization due to ABI issues, they are optional extensions of the RVA23U64 profile (application-class processor profile) and going to be discoverable from the Linux kernel (as of version 6.15-rc4). This commit mainly prepares runtime detection of those extensions.
a4lg
force-pushed
the
riscv-feature-addition-batch-3
branch
from
69d1db7 to
501a539
Compare
|
@bors r+ rollup |
bors
added
S-waiting-on-bors
VlaDexa
added a commit
to VlaDexa/rust
that referenced
this pull request
May 2, 2025
…3, r=Amanieu
rustc_target: RISC-V: feature addition batch 3
This is the last batch (3 of 3) for RISC-V feature enhancements intended for the version 1.88 cycle.
The author's primary criteria are:
1. The extension is ratified and unprivileged one.
2. The extension is in the RVA23U64 profile (to be a baseline of the application-class RISC-V software ecosystem in the near future), either mandatory or optional.
3. Either:
1. To be discoverable through a `riscv_hwprobe` system call on (currently unreleased) Linux 6.15 (as of rc4) or
2. Helps memory/atomics-related operations more efficient and/or more robust.
This is based on the specifications:
* [The latest ratified ISA Manuals (version 20240411)](https://lf-riscv.atlassian.net/wiki/spaces/HOME/pages/16154769/RISC-V+Technical+Specifications)
* [RVA23/RVB23 profiles](https://github.com/riscv/riscv-profiles/releases/tag/rva23-rvb23-ratified)
* [RISC-V BF16 extensions](https://github.com/riscv/riscv-bfloat16/releases/tag/v183a3dac863d7c18187a739eb52b0c8f0d16854d)
LLVM Definitions:
* [`Zicbop`](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/RISCV/RISCVFeatures.td#L82-87)
* [`Zicbom`](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/RISCV/RISCVFeatures.td#L75-L80)
* [`Zic64b`](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/RISCV/RISCVFeatures.td#L71-L73)
* [`Ziccamoa`](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/RISCV/RISCVFeatures.td#L97-L99)
* [`Ziccif`](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/RISCV/RISCVFeatures.td#L101-L103)
* [`Zicclsm`](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/RISCV/RISCVFeatures.td#L105-L107)
* [`Ziccrse`](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/RISCV/RISCVFeatures.td#L109-L111)
* [`Zfbfmin`](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/RISCV/RISCVFeatures.td#L320-L325)
* [`Zvfbfmin`](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/RISCV/RISCVFeatures.td#L697-L702)
* [`Zvfbfwma`](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/RISCV/RISCVFeatures.td#L704-L710)
The `Zicbop` extension (mandatory in RVA23U64) adds prefetch hints to prepare for subsequent memory operations (will be executed as no-op if the hardware does not support this extension).
The `Zicbom` extension (mandatory in RVA23U64) adds cache block-management instructions. The author did not include this in the batch 2 (because of limited use cases compared to the `Zicboz` extension) but added because it will be discoverable from Linux (as of version 6.15-rc4). Along with `Zicbop`, Rust now supports all CMO extensions.
The `Zic64b` extension (mandatory in RVA23U64) constraints the cache block to be naturally-aligned and exactly 64 bytes. Along with CMO instructions, it can improve efficiency handling with memory (e.g. efficient memory zeroing using `Zicboz` + `Zic64b`).
The `Zicc*` extensions (mandatory in RVA23U64) add constraints to the main memory properties. They are normally satisfied in the application environment with regular OSes but profiles like RVA23U64 ensures such properties are satisfied (through those *constraint* extensions).
The `Zfbf*` and `Zvfbf*` extensions (optional in RVA23U64) add instructions to handle BF16 (BFloat16) data. Although stabilization of FP-related extensions are relatively far due to ABI-related issues, they are included in this batch because they will be discoverable from Linux (as of version 6.15-rc4).
The author also adds the extension implication: `Za64rs` → `Za128rs` (superset) which the author missed to include in rust-lang#140139.
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
This is the last batch (3 of 3) for RISC-V feature enhancements intended for the version 1.88 cycle.
The author's primary criteria are:
riscv_hwprobesystem call on (currently unreleased) Linux 6.15 (as of rc4) orThis is based on the specifications:
LLVM Definitions:
ZicbopZicbomZic64bZiccamoaZiccifZicclsmZiccrseZfbfminZvfbfminZvfbfwmaThe
Zicbopextension (mandatory in RVA23U64) adds prefetch hints to prepare for subsequent memory operations (will be executed as no-op if the hardware does not support this extension).The
Zicbomextension (mandatory in RVA23U64) adds cache block-management instructions. The author did not include this in the batch 2 (because of limited use cases compared to theZicbozextension) but added because it will be discoverable from Linux (as of version 6.15-rc4). Along withZicbop, Rust now supports all CMO extensions.The
Zic64bextension (mandatory in RVA23U64) constraints the cache block to be naturally-aligned and exactly 64 bytes. Along with CMO instructions, it can improve efficiency handling with memory (e.g. efficient memory zeroing usingZicboz+Zic64b).The
Zicc*extensions (mandatory in RVA23U64) add constraints to the main memory properties. They are normally satisfied in the application environment with regular OSes but profiles like RVA23U64 ensures such properties are satisfied (through those constraint extensions).The
Zfbf*andZvfbf*extensions (optional in RVA23U64) add instructions to handle BF16 (BFloat16) data. Although stabilization of FP-related extensions are relatively far due to ABI-related issues, they are included in this batch because they will be discoverable from Linux (as of version 6.15-rc4).The author also adds the extension implication:
Za64rs→Za128rs(superset) which the author missed to include in #140139.Notes
Za64rstoZa128rsseem counterintuitive but they denote the maximum reservation set size as used in the LR/SC-based atomic operations. That means, smaller the reservation set is, more fine-grained control of atomics.Related
#[target_feature]#44839(
riscv_target_feature)I-related important extensions #138823-Ctarget-features must be restrained on RISCV #132618(stabilization of the
Zfbf*andZvfbf*extensions is blocked by this)History
Version 1 (2025-04-30)
The initial proposal.
Version 2 (2025-05-01)
Clarified that the feature implication of the
Zfbfminextension is not fully described in the source code (it also depends on load/store/move-(from/to)-GPR instructions from theZfhextension or its subset: theZfhminextension) and needs extra care.@rustbot r? @Amanieu
@rustbot label +T-compiler +O-riscv +A-target-feature