RISC-V Optimization Guide

This document may be of use to any software developers who need to operate at an instruction set level. For example, this would include:

This guide is intended to be vendor-neutral and implementation-neutral. It is meant to present optimizations that are applicable to many different implementations of RISC-V application processors, using standard RISC-V instructions.

However, to reach peak performance, this document will also highlight some implementation-specific optimizations. These will be clearly outlined like such:

The use of non-RISC-V-standard ISA extensions is out of scope of this guide, and shall not be documented here.

In some cases it can be desirable to create an executable with multiple instances of the same algorithm compiled with different target options. A version of the algorithm compiled with base options, e.g., RV64G, might be supplied so that the binary can be run on a wide range of devices, while a more performant version compiled with some additional extensions, e.g., Zba or Vector, could also be provided. With current toolchains, to support multiple versions of code compiled for different architectural features, separate compilation of translation units in addition to a manual runtime check for extensions (using riscv_hwprobe for example) must be used. In the future, features to support multi-versioning within a single translation unit are planned.

GCC and Clang both support a function attribute called target_clones that can be used to compile multiple versions of a given function with different compiler flags. An ifunc resolver function is automatically created that ensures that the function symbol is resolved to the most suitable version of the function at load time. At the time of writing, the compilers have yet to add support for the target_clones attribute for RISC-V.

Use the assembler MV mnemonic, which translates to ADDI rd, rs1, 0, to copy values from one register to another. For example use,

in preference to any of the following instructions.

The Zicond extension adds the two conditional operations czero.eqz and czero.nez. Where available, these operations can reduce pressure on the branch predictor, at the cost of a longer critical path. We believe this to generally be a good tradeoff, but examine particular hot instances carefully. Experience with other architectures shows this is frequently a complicated tradeoff.

As an example, assume the following code segment relies on an unpredictable branch to determine which constant to load into a0. If the original value of a0 is non-zero we set a0 to constant1 otherwise it is set to constant2, i.e., (a0 = a0 ? constant1 : constant2;).

The branch can be eliminated using the Zicond instructions CZERO.EQZ and CZERO.NEZ followed by an OR.

After the czero.nez instruction has executed, t3 will contain (constant2 - constant1) if a0 is zero or zero if a0 is non-zero. Adding t2, which contains constant1, to t3 yields either constant2 if a0 is zero, or constant 1 if it is not. An additional optimization is possible if constant1 fits in 12 bits. In this case the initial li t2, constant1 instruction can be eliminated and the final add instruction can be replaced by an addi, e.g.,

The above code sequence can also be written using only the base integer ISA albeit with the additional cost of three extra instructions.

The combination of the instructions

results in t0 having no bits set if a0 is zero, or all the bits set if a0 is non-zero. The resulting value in t0 is then a mask which can be ANDed with the constants in its original or inverted form to generate the operands for the OR instruction.

When the 'M' extension is available a shorter sequence using the MUL instruction is possible

In the following sequence of instructions

SEQZ sets t0 to either 1 or 0 depending on whether a0 contains 0. The MUL instructions multiplies t0, which is either 1 or 0, by the result of the earlier XOR instruction, storing either 0 or the xor of the two constants in t1.

When constant1 and constant2 are 0 and 1 respectively, the above code sequence can be written using a single instruction from the base integer ISA.

Use canonical NOPs, NOP ( ADDI X0, X0, 0 ) and C.NOP ( C.ADDI X0, 0 ), to add padding within a function. Use the canonical illegal instruction ( either 2 or 4 bytes of zeros depending on whether the C extension is supported ) to add padding between functions.

For a cpu that doesn’t support fast misaligned memory accesses, increasing alignment might enable wider load/store usage for memory copy. For example, align to 4 for rv32 and align to 8 for rv64 might enable using LW/SW or LD/SD. For a cpu that supports fast misaligned memory accesses, wider load/store would be more profitable regardless of the data alignment.

Use shifts if the immediate can’t fit in signed 12-bit of ANDI.

For example

rather than

The above shift example is for a 32-bit system. The shift amount for a 64-bit target will be 52 in this case.

Prefer instructions with static round modes where possible. For many common scalar floating point operations, rounding can be controlled by the instruction opcode, and this is strictly better than using FRM. For example, it’s better to write

than

Use immediates for FRM writes (as shown above) and avoid redundant FRM writes where possible. Save/restore idioms for FRM may be expensive. Try to save/restore over the largest region feasible.

The v0 register defined by the RISC-V vector extension is special in that it can be used both as a general purpose vector register and also as a mask register. As a preference, use registers other than v0 for non-mask values. Otherwise data will have to be moved out of v0 when a mask is required in an operation. v0 may be used when all other registers are in use, and using v0 would avoid spilling register state to memory.

Vector length agnostic (“VLA”) code is designed to run on any implementation of the vector specification. Vector length specific (“VLS”) code will run on exactly one VLEN. There’s also a middle ground where code may run on a family of VLEN values, but not all possible VLENs.

For benchmarking purposes, using vector length specific code is strongly recommended.

One common technique to write fully generic and performant code is to dispatch to multiple implementations based on the dynamic value of the vlenb CSR, at the expense of additional code size.

Use only immediate VTYPE encodings, vsetvli and vsetivli. The vsetvl instruction should be reserved for context-restoring type operations.

Choose the value used for LMUL/EMUL carefully.

Fold immediate scalar values into integer vector operations where possible using the .vi instruction variant.

Prefer .vi, .vx and .vf variants over .vv variants when vector register pressure is high.

For example, to add 1.0 to each element of an array of 32 bit floats whose length is stored in a1, we might write

Use vmv.v.x or vmv.v.i to splat a value across all body lanes. For example, to broadcast 3 across all elements of the register group starting at v8, use

Use v(f)merge.vxm to perform a masked splat. The following code splats alternating values of 0xaaaaaaaa and 0xbbbbbbbb into v2.

If only the first lane is active, use vmv.s.x. Prefer vmv.v.i vd, 0 to zero a vector register. Using these two pieces of advice together in an example, we can set the first element of a vector register to 2 and the remaining elements to 0 as follows.

Use vmv1r.v (and variants) to perform whole register copies, ignoring predication. Use vmv.v.v to perform register moves while respecting inactive lanes.

See the corresponding scalar section. The discussion of redundant FRM writes applies for vector operations as well. Unfortunately, most vector operations do not have static round mode overrides, so vector code is more sensitive to FRM pressure.

The RISC-V vector specification provides a wide range of different load and store operations. These operations may not all share the same performance characteristics. Where possible, employ these instructions in the following order of descending preference.