About

Enablement of auto-vectorization in LLVM for RISC-V, targeting the V extension version 1.0.   While the long term goal is to focus on vector length agnostic (VLA) approaches to vectorization, some of LLVM's vectorizer may still be biased towards fixed vector sizes.  Thus we expect to find cases that are not well handled using VLA approaches and we expect to support VLS approaches to vectorization a stop-gap alternatives.

LLVM's support for auto-vectorization on RISC-V appears to be improving regularly, but it is sensitive to having reasonable micro-architectural data available.  Thus it may be necessary to stub-out values for these key parameters when enabling auto-vectorization on a new micro-architecture, or to disable the costing model.

Stakeholders/Partners

RISE:

Ventana: 1 FTE focused on getting necessary uarch data ready

Ventana: 1 FTE Reference/target implementation of key x264 loops, breakdown of tasks that need to be solved to achieve desired code generation

SiFive: Craig Topper, Alexey Bataev, Kolya Panchenko

Rivos:

External:

Alex Bradbury

Dependencies

The most pressing upstream dependencies are:

1. PSABI specification for vector argument passing and return values
2. Kernel support to enable discovery of the V extension
3. glibc support for libmvec to enable vector API for key math library functions such as sin, cos, sqrt, etc (does LLVM support libmvec calls?)

Status

Updates

11 Jul 2024 

• Moved to 2H2024
• Patch for using VP intrinsics for unary and binary operators https://github.com/llvm/llvm-project/pull/93854

11 Apr 2024 

• First VP intrinsic vectorizer patch merged https://github.com/llvm/llvm-project/pull/76172. First step to vectorizing using strip mined loops like examples in the vector spec.

30 Aug 2023 

• Improving vectorization split off as distinct project