User Guide for RISC-V Target

Introduction

The RISC-V target provides code generation for processors implementing supported variations of the RISC-V specification. It lives in the llvm/lib/Target/RISCV directory.

Specification Documents

There have been a number of revisions to the RISC-V specifications. LLVM aims to implement the most recent ratified version of the standard RISC-V base ISAs and ISA extensions with pragmatic variances. The most recent specification can be found at: https://github.com/riscv/riscv-isa-manual/releases/.

The official RISC-V International specification page. is also worth checking, but tends to significantly lag the specifications linked above. Make sure to check the wiki for not yet integrated extensions and note that in addition, we sometimes carry support for extensions that have not yet been ratified (these will be marked as experimental - see below) and support various vendor-specific extensions (see below).

The current known variances from the specification are:

• Unconditionally allowing instructions from zifencei, zicsr, zicntr, and zihpm without gating them on the extensions being enabled. Previous revisions of the specification included these instructions in the base ISA, and we preserve this behavior to avoid breaking existing code. If a future revision of the specification reuses these opcodes for other extensions, we may need to reevaluate this choice, and thus recommend users migrate build systems so as not to rely on this.

• Allowing CSRs to be named without gating on specific extensions. This applies to all CSR names, not just those in zicsr, zicntr, and zihpm.

• The ordering of z*, s*, and x* prefixed extension names is not enforced in user-specified ISA naming strings (e.g. -march).

We are actively deciding not to support multiple specification revisions at this time. We acknowledge a likely future need, but actively defer the decisions making around handling this until we have a concrete example of real hardware having shipped and an incompatible change to the specification made afterwards.

Base ISAs

The specification defines five base instruction sets: RV32I, RV32E, RV64I, RV64E, and RV128I. Currently, LLVM fully supports RV32I, and RV64I. RV32E and RV64E are supported by the assembly-based tools only. RV128I is not supported.

To specify the target triple:

Table 113 RISC-V Architectures

Architecture	Description
riscv32	RISC-V with XLEN=32 (i.e. RV32I or RV32E)
riscv64	RISC-V with XLEN=64 (i.e. RV64I or RV64E)

To select an E variant ISA (e.g. RV32E instead of RV32I), use the base architecture string (e.g. riscv32) with the extension e.

Profiles

Supported profile names can be passed using -march instead of a standard ISA naming string. Currently supported profiles:

• rvi20u32

• rvi20u64

• rva20u64

• rva20s64

• rva22u64

• rva22s64

• rva23u64

• rva23s64

• rvb23u64

• rvb23s64

Note that you can also append additional extension names to be enabled, e.g. rva20u64_zicond will enable the zicond extension in addition to those in the rva20u64 profile.

Profiles that are not yet ratified cannot be used unless -menable-experimental-extensions (or equivalent for other tools) is specified. This applies to the following profiles:

• rvm23u32

Extensions

The following table provides a status summary for extensions which have been ratified and thus have finalized specifications. When relevant, detailed notes on support follow.

Table 114 Ratified Extensions by Status

Extension	Status
A	Supported
B	Supported
C	Supported
D	Supported
F	Supported
E	Supported (See note)
H	Assembly Support
M	Supported
Sha	Supported
Shcounterenw	Assembly Support (See note)
Shgatpa	Assembly Support (See note)
Shtvala	Assembly Support (See note)
Shvsatpa	Assembly Support (See note)
Shvstvala	Assembly Support (See note)
Shvstvecd	Assembly Support (See note)
Smaia	Supported
Smcdeleg	Supported
Smcsrind	Supported
Smdbltrp	Supported
Smepmp	Supported
Smmpm	Supported
Smnpm	Supported
Smrnmi	Assembly Support
Smstateen	Assembly Support
Ssaia	Supported
Ssccfg	Supported
Ssccptr	Assembly Support (See note)
Sscofpmf	Assembly Support
Sscounterenw	Assembly Support (See note)
Sscsrind	Supported
Ssdbltrp	Supported
Ssnpm	Supported
Sspm	Supported
Ssqosid	Assembly Support
Ssstateen	Assembly Support (See note)
Ssstrict	Assembly Support (See note)
Sstc	Assembly Support
Sstvala	Assembly Support (See note)
Sstvecd	Assembly Support (See note)
Ssu64xl	Assembly Support (See note)
Supm	Supported
Svade	Assembly Support (See note)
Svadu	Assembly Support
Svbare	Assembly Support (See note)
Svinval	Assembly Support
Svnapot	Assembly Support
Svpbmt	Supported
Svvptc	Supported
V	Supported
Za128rs	Supported (See note)
Za64rs	Supported (See note)
Zaamo	Assembly Support
Zabha	Supported
Zacas	Supported (See note)
Zalrsc	Assembly Support
Zama16b	Supported (See note)
Zawrs	Assembly Support
Zba	Supported
Zbb	Supported
Zbc	Supported
Zbkb	Supported (See note)
Zbkc	Supported
Zbkx	Supported (See note)
Zbs	Supported
Zca	Supported
Zcb	Supported
Zcd	Supported
Zcf	Supported
Zcmop	Supported
Zcmp	Supported
Zcmt	Assembly Support
Zdinx	Supported
Zfa	Supported
Zfbfmin	Supported
Zfh	Supported
Zfhmin	Supported
Zfinx	Supported
Zhinx	Supported
Zhinxmin	Supported
Zic64b	Supported (See note)
Zicbom	Assembly Support
Zicbop	Supported
Zicboz	Assembly Support
Ziccamoa	Supported (See note)
Ziccif	Supported (See note)
Zicclsm	Supported (See note)
Ziccrse	Supported (See note)
Zicntr	(See Note)
Zicond	Supported
Zicsr	(See Note)
Zifencei	(See Note)
Zihintntl	Supported
Zihintpause	Assembly Support
Zihpm	(See Note)
Zimop	Supported
Zkn	Supported
Zknd	Supported (See note)
Zkne	Supported (See note)
Zknh	Supported (See note)
Zksed	Supported (See note)
Zksh	Supported (See note)
Zk	Supported
Zkr	Supported
Zks	Supported
Zkt	Supported
Zmmul	Supported
Ztso	Supported
Zvbb	Supported
Zvbc	Supported (See note)
Zve32x	(Partially) Supported
Zve32f	(Partially) Supported
Zve64x	Supported
Zve64f	Supported
Zve64d	Supported
Zvfbfmin	Supported
Zvfbfwma	Supported
Zvfh	Supported
Zvfhmin	Supported
Zvkb	Supported
Zvkg	Supported (See note)
Zvkn	Supported (See note)
Zvknc	Supported (See note)
Zvkned	Supported (See note)
Zvkng	Supported (See note)
Zvknha	Supported (See note)
Zvknhb	Supported (See note)
Zvks	Supported (See note)
Zvksc	Supported (See note)
Zvksed	Supported (See note)
Zvksg	Supported (See note)
Zvksh	Supported (See note)
Zvkt	Supported
Zvl32b	(Partially) Supported
Zvl64b	Supported
Zvl128b	Supported
Zvl256b	Supported
Zvl512b	Supported
Zvl1024b	Supported
Zvl2048b	Supported
Zvl4096b	Supported
Zvl8192b	Supported
Zvl16384b	Supported
Zvl32768b	Supported
Zvl65536b	Supported

Assembly Support

LLVM supports the associated instructions in assembly. All assembly related tools (e.g. assembler, disassembler, llvm-objdump, etc..) are supported. Compiler and linker will accept extension names, and linked binaries will contain appropriate ELF flags and attributes to reflect use of named extension.

Supported

Fully supported by the compiler. This includes everything in Assembly Support, along with - if relevant - C language intrinsics for the instructions and pattern matching by the compiler to recognize idiomatic patterns which can be lowered to the associated instructions.

E

Support of RV32E/RV64E and ilp32e/lp64e ABIs are experimental. To be compatible with the implementation of ilp32e in GCC, we don’t use aligned registers to pass variadic arguments. Furthermore, we set the stack alignment to 4 bytes for types with length of 2*XLEN.

Zbkb, Zbkx

Pattern matching support for these instructions is incomplete.

Zknd, Zkne, Zknh, Zksed, Zksh

No pattern matching exists. As a result, these instructions can only be used from assembler or via intrinsic calls.

Zvbc, Zvkg, Zvkn, Zvknc, Zvkned, Zvkng, Zvknha, Zvknhb, Zvks, Zvks, Zvks, Zvksc, Zvksed, Zvksg, Zvksh.

No pattern matching exists. As a result, these instructions can only be used from assembler or via intrinsic calls.

Zve32x, Zve32f, Zvl32b

LLVM currently assumes a minimum VLEN (vector register width) of 64 bits during compilation, and as a result Zve32x and Zve32f are supported only for VLEN>=64. Assembly support doesn’t have this restriction.

Zicntr, Zicsr, Zifencei, Zihpm

Between versions 2.0 and 2.1 of the base I specification, a backwards incompatible change was made to remove selected instructions and CSRs from the base ISA. These instructions were grouped into a set of new extensions, but were no longer required by the base ISA. This change is partially described in “Preface to Document Version 20190608-Base-Ratified” from the specification document (the zicntr and zihpm bits are not mentioned). LLVM currently implements version 2.1 of the base specification. To maintain compatibility, instructions from these extensions are accepted without being in the -march string. LLVM also allows the explicit specification of the extensions in an -march string.

Za128rs, Za64rs, Zama16b, Zic64b, Ziccamoa, Ziccif, Zicclsm, Ziccrse, Shcounterenvw, Shgatpa, Shtvala, Shvsatpa, Shvstvala, Shvstvecd, Ssccptr, Sscounterenw, Ssstateen, Ssstrict, Sstvala, Sstvecd, Ssu64xl, Svade, Svbare

These extensions are defined as part of the RISC-V Profiles specification. They do not introduce any new features themselves, but instead describe existing hardware features.

Zacas

The compiler will not generate amocas.d on RV32 or amocas.q on RV64 due to ABI compatibilty. These can only be used in the assembler.

Atomics ABIs

At the time of writing there are three atomics mappings (ABIs) defined for RISC-V. As of LLVM 19, LLVM defaults to “A6S”, which is compatible with both the original “A6” and the future “A7” ABI. See the psABI atomics document for more information on these mappings.

Note that although the “A6S” mapping is used, the ELF attribute recording the mapping isn’t currently emitted by default due to a bug causing a crash in older versions of binutils when processing files containing this attribute.

Experimental Extensions

LLVM supports (to various degrees) a number of experimental extensions. All experimental extensions have experimental- as a prefix. There is explicitly no compatibility promised between versions of the toolchain, and regular users are strongly advised not to make use of experimental extensions before they reach ratification.

The primary goal of experimental support is to assist in the process of ratification by providing an existence proof of an implementation, and simplifying efforts to validate the value of a proposed extension against large code bases. Experimental extensions are expected to either transition to ratified status, or be eventually removed. The decision on whether to accept an experimental extension is currently done on an entirely case by case basis; if you want to propose one, attending the bi-weekly RISC-V sync-up call is strongly advised.

experimental-zalasr

LLVM implements the 0.0.5 draft specification.

experimental-zicfilp, experimental-zicfiss

LLVM implements the 1.0 release specification.

experimental-zvbc32e, experimental-zvkgs

LLVM implements the 0.7 release specification.

experimental-smctr, experimental-ssctr

LLVM implements the 1.0-rc3 specification.

experimental-svukte

LLVM implements the 0.3 draft specification.

To use an experimental extension from clang, you must add -menable-experimental-extensions to the command line, and specify the exact version of the experimental extension you are using. To use an experimental extension with LLVM’s internal developer tools (e.g. llc, llvm-objdump, llvm-mc), you must prefix the extension name with experimental-. Note that you don’t need to specify the version with internal tools, and shouldn’t include the experimental- prefix with clang.

Vendor Extensions

Vendor extensions are extensions which are not standardized by RISC-V International, and are instead defined by a hardware vendor. The term vendor extension roughly parallels the definition of a non-standard extension from Section 1.3 of the Volume I: RISC-V Unprivileged ISA specification. In particular, we expect to eventually accept both custom extensions and non-conforming extensions.

Inclusion of a vendor extension will be considered on a case by case basis. All proposals should be brought to the bi-weekly RISCV sync calls for discussion. For a general idea of the factors likely to be considered, please see the Clang documentation.

It is our intention to follow the naming conventions described in riscv-non-isa/riscv-toolchain-conventions. Exceptions to this naming will need to be strongly motivated.

The current vendor extensions supported are:

XTHeadBa

LLVM implements the THeadBa (address-generation) vendor-defined instructions specified in by T-HEAD of Alibaba. Instructions are prefixed with th. as described in the specification.

XTHeadBb

LLVM implements the THeadBb (basic bit-manipulation) vendor-defined instructions specified in by T-HEAD of Alibaba. Instructions are prefixed with th. as described in the specification.

XTHeadBs

LLVM implements the THeadBs (single-bit operations) vendor-defined instructions specified in by T-HEAD of Alibaba. Instructions are prefixed with th. as described in the specification.

XTHeadCondMov

LLVM implements the THeadCondMov (conditional move) vendor-defined instructions specified in by T-HEAD of Alibaba. Instructions are prefixed with th. as described in the specification.

XTHeadCmo

LLVM implements the THeadCmo (cache management operations) vendor-defined instructions specified in by T-HEAD of Alibaba. Instructions are prefixed with th. as described in the specification.

XTHeadFMemIdx

LLVM implements the THeadFMemIdx (indexed memory operations for floating point) vendor-defined instructions specified in by T-HEAD of Alibaba. Instructions are prefixed with th. as described in the specification.

XTheadMac

LLVM implements the XTheadMac (multiply-accumulate instructions) vendor-defined instructions specified in by T-HEAD of Alibaba. Instructions are prefixed with th. as described in the specification.

XTHeadMemIdx

LLVM implements the THeadMemIdx (indexed memory operations) vendor-defined instructions specified in by T-HEAD of Alibaba. Instructions are prefixed with th. as described in the specification.

XTHeadMemPair

LLVM implements the THeadMemPair (two-GPR memory operations) vendor-defined instructions specified in by T-HEAD of Alibaba. Instructions are prefixed with th. as described in the specification.

XTHeadSync

LLVM implements the THeadSync (multi-core synchronization instructions) vendor-defined instructions specified in by T-HEAD of Alibaba. Instructions are prefixed with th. as described in the specification.

XTHeadVdot

LLVM implements version 1.0.0 of the THeadV-family custom instructions specification by T-HEAD of Alibaba. All instructions are prefixed with th. as described in the specification, and the riscv-toolchain-convention document linked above.

XVentanaCondOps

LLVM implements version 1.0.0 of the VTx-family custom instructions specification by Ventana Micro Systems. All instructions are prefixed with vt. as described in the specification, and the riscv-toolchain-convention document linked above. These instructions are only available for riscv64 at this time.

XSfvcp

LLVM implements version 1.1.0 of the SiFive Vector Coprocessor Interface (VCIX) Software Specification by SiFive. All instructions are prefixed with sf.vc. as described in the specification, and the riscv-toolchain-convention document linked above.

XSfvqmaccdod, XSfvqmaccqoq

LLVM implements version 1.1.0 of the SiFive Int8 Matrix Multiplication Extensions Specification by SiFive. All instructions are prefixed with sf. as described in the specification linked above.

Xsfvfnrclipxfqf

LLVM implements version 1.0.0 of the FP32-to-int8 Ranged Clip Instructions Extension Specification by SiFive. All instructions are prefixed with sf. as described in the specification linked above.

Xsfvfwmaccqqq

LLVM implements version 1.0.0 of the Matrix Multiply Accumulate Instruction Extension Specification by SiFive. All instructions are prefixed with sf. as described in the specification linked above.

XCVbitmanip

LLVM implements version 1.0.0 of the CORE-V Bit Manipulation custom instructions specification by OpenHW Group. All instructions are prefixed with cv. as described in the specification.

XCVelw

LLVM implements version 1.0.0 of the CORE-V Event load custom instructions specification by OpenHW Group. All instructions are prefixed with cv. as described in the specification. These instructions are only available for riscv32 at this time.

XCVmac

LLVM implements version 1.0.0 of the CORE-V Multiply-Accumulate (MAC) custom instructions specification by OpenHW Group. All instructions are prefixed with cv.mac as described in the specification. These instructions are only available for riscv32 at this time.

XCVmem

LLVM implements version 1.0.0 of the CORE-V Post-Increment load and stores custom instructions specification by OpenHW Group. All instructions are prefixed with cv. as described in the specification. These instructions are only available for riscv32 at this time.

XCValu

LLVM implements version 1.0.0 of the Core-V ALU custom instructions specification by Core-V. All instructions are prefixed with cv. as described in the specification. These instructions are only available for riscv32 at this time.

XCVsimd

LLVM implements version 1.0.0 of the CORE-V SIMD custom instructions specification by OpenHW Group. All instructions are prefixed with cv. as described in the specification.

XCVbi

LLVM implements version 1.0.0 of the CORE-V immediate branching custom instructions specification by OpenHW Group. All instructions are prefixed with cv. as described in the specification. These instructions are only available for riscv32 at this time.

XSiFivecdiscarddlone

LLVM implements the SiFive sf.cdiscard.d.l1 instruction specified in by SiFive.

XSiFivecflushdlone

LLVM implements the SiFive sf.cflush.d.l1 instruction specified in by SiFive.

XSfcease

LLVM implements the SiFive sf.cease instruction specified in by SiFive.

Xwchc

LLVM implements the custom compressed opcodes present in some QingKe cores by WCH / Nanjing Qinheng Microelectronics. The vendor refers to these opcodes by the name “XW”.

experimental-Xqcia

LLVM implements version 0.2 of the Qualcomm uC Arithmetic extension specification by Qualcomm. All instructions are prefixed with qc. as described in the specification. These instructions are only available for riscv32.

experimental-Xqcics

LLVM implements version 0.2 of the Qualcomm uC Conditional Select extension specification by Qualcomm. All instructions are prefixed with qc. as described in the specification. These instructions are only available for riscv32.

experimental-Xqcicsr

LLVM implements version 0.2 of the Qualcomm uC CSR extension specification by Qualcomm. All instructions are prefixed with qc. as described in the specification. These instructions are only available for riscv32.

experimental-Xqcilsm

LLVM implements version 0.2 of the Qualcomm uC Load Store Multiple extension specification by Qualcomm. All instructions are prefixed with qc. as described in the specification. These instructions are only available for riscv32.

experimental-Xqcisls

LLVM implements version 0.2 of the Qualcomm uC Scaled Load Store extension specification by Qualcomm. All instructions are prefixed with qc. as described in the specification. These instructions are only available for riscv32.

Experimental C Intrinsics

In some cases an extension is non-experimental but the C intrinsics for that extension are still experimental. To use C intrinsics for such an extension from clang, you must add -menable-experimental-extensions to the command line. This currently applies to the following extensions:

No extensions have experimental intrinsics.

Long (>32-bit) Instruction Support

RISC-V is a variable-length ISA, but the standard currently only defines 16- and 32-bit instructions. The specification describes longer instruction encodings, but these are not ratified.

The LLVM disassembler, llvm-objdump, does use the longer instruction encodings described in the specification to guess the instruction length (up to 176 bits) and will group the disassembly view of encoding bytes correspondingly.

The LLVM integrated assembler for RISC-V supports two different kinds of .insn directive, for assembling instructions that LLVM does not yet support:

• .insn type, args* which takes a known instruction type, and a list of fields. You are strongly recommended to use this variant of the directive if your instruction fits an existing instruction type.

• .insn [ length , ] encoding which takes an (optional) explicit length (in bytes) and a raw encoding for the instruction. When given an explicit length, this variant can encode instructions up to 64 bits long. The encoding part of the directive must be given all bits for the instruction, none are filled in for the user. When used without the optional length, this variant of the directive will use the LSBs of the raw encoding to work out if an instruction is 16 or 32 bits long. LLVM does not infer that an instruction might be longer than 32 bits - in this case, the user must give the length explicitly.

It is strongly recommended to use the .insn directive for assembling unsupported instructions instead of .word or .hword, because it will produce the correct mapping symbols to mark the word as an instruction, not data.

Global Pointer (GP) Relaxation and the Small Data Limit

Some of the RISC-V psABI variants reserve gp (x3) for use as a “Global Pointer”, to make generating data addresses more efficient.

To use this functionality, you need to be doing all of the following:

• Use the medlow (aka small) code model;

• Not use the gp register for any other uses (some platforms use it for the shadow stack and others as a temporary – as denoted by the Tag_RISCV_x3_reg_usage build attribute);

• Compile your objects with Clang’s -mrelax option, to enable relaxation annotations on relocatable objects (this is the default, but -mno-relax disables these relaxation annotations);

• Compile for a position-dependent static executable (not a shared library, and -fno-PIC / -fno-pic / -fno-pie); and

• Use LLD’s --relax-gp option.

LLD will relax (rewrite) any code sequences that materialize an address within 2048 bytes of __global_pointer$ (which will be defined if it is used and does not already exist) to instead generate the address using gp and the correct (signed) 12-bit immediate. This usually saves at least one instruction compared to materialising a full 32-bit address value.

There can only be one gp value in a process (as gp is not changed when calling into a function in a shared library), so the symbol is is only defined and this relaxation is only done for executables, and not for shared libraries. The linker expects executable startup code to put the value of __global_pointer$ (from the executable) into gp before any user code is run.

Arguably, the most efficient use for this addressing mode is for smaller global variables, as larger global variables likely need many more loads or stores when they are being accessed anyway, so the cost of materializing the upper bits can be shared.

Therefore the compiler can place smaller global variables into sections with names starting with .sdata or .sbss (matching sections with names starting with .data and .bss respectively). LLD knows to define the global_pointer$ symbol close to these sections, and to lay these sections out adjacent to the .data section.

Clang’s -msmall-data-limit= option controls what the threshold size is (in bytes) for a global variable to be considered small. -msmall-data-limit=0 disables the use of sections starting .sdata and .sbss. The -msmall-data-limit= option will not move global variables that have an explicit data section, and will keep globals in separate sections if you are using -fdata-sections.

The small data limit threshold is also used to separate small constants into sections with names starting with .srodata. LLD does not place these with the .sdata and .sbss sections as .srodata sections are read only and the other two are writable. Instead the .srodata sections are placed adjacent to .rodata.

Data suggests that these options can produce significant improvements across a range of benchmarks.