rustc_target: RISC-V: feature addition batch 2 #139440
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This commit adds unprivileged ratified extensions that are either
dicoverable from the `riscv_hwprobe` syscall of the Linux kernel (as of
version 6.14) plus 1 minus 2 extensions.
Plus 1:
* "B"
This is a combination of "Zba", "Zbb" and "Zbs".
Note:
Although not required by the RISC-V specification, it is convenient to
imply "B" from its three members (will be implemented in LLVM 21/22) but
this is not yet implemented in Rust due to current implication handling.
It still implies three members *from* "B".
Minus 2:
* "Zcf" (target_arch = "riscv32" only)
This is the compression instruction subset corresponding "F".
This is implied from RV32 + "C" + "F" but this complex handling is
not yet supported by Rust's feature handling.
* "Zcd"
This is the compression instruction subset corresponding "D".
This is implied from "C" + "D" but this complex handling is
not yet supported by Rust's feature handling.
Notes:
* Because this commit adds the "Zca" extension (an integer subset of the
"C" extension), the "C" extension is modified to imply "Zca".
* The "Supm" extension requires LLVM 20 yet it should not affect codegen
on LLVM 19 because this extension does not specify any mechanism to
control pointer masking (just tells the availability from the user mode
and *some* way to control pointer masking like a system call).
The "B" extension is ratified as a combination of three extensions: "Zba", "Zbb" and "Zbs". To maximize discoverability of the RISC-V target features, this commit makes use of the "B" extension instead of its three members. This way, `#[cfg(target_feature = "b")]` can also be used instead of: `#[cfg(all(target_feature = "zba", target_feature = "zbb", target_feature = "zbs"))]`
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Of ratified RISC-V extensions, this commit adds ones satisfying following criteria:
riscv_hwprobesystem call on Linux 6.14or should be very helpful even on basic needs (the
Bextension) andDue to 2., the author excluded
Zcf(RV32 only) andZcdfrom the list despite that they are discoverable from Linux 6.14.This is based on those two specifications:
Linux Definition: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/riscv/include/uapi/asm/hwprobe.h
LLVM Definitions:
BZcaZcbZcmopZfaZicbozZicondZihintntlZimopZtsoSupm(LLVM 19 (experimental) / LLVM 20 (non-experimental and enabled on Rust side through this PR))The author also adds required implication:
CimpliesZca.Android RISC-V target is also updated to include the
Bextension (this is just a shorthand combination ofZba,ZbbandZbsextensions but possibly simplifiestarget_featurehandling).Note that absence of non-experimental
Supmextension on LLVM 19 should not affect the codegen because of this extension's position: unlike most of the RISC-V extensions, theSupmextension defines some mechanism to control pointer masking behavior but provides none of additional instructions, CSRs nor some architectural behaviors/constraints (i.e. it tells the existence of an external mechanism such like a system call of an operating system which changes the behavior of the pointer masking using features defined bySsnpmand/orSmnpmextensions, notSupm).Related:
#[target_feature]#44839(
riscv_target_feature)I-related important extensions #138823(my previous batch)
-Ctarget-features must be restrained on RISCV #132618(stabilization of the
Zfaextension is blocked by this)@rustbot r? @Amanieu
@rustbot label +T-compiler +O-riscv +A-target-feature