SG2042 Thead LLVM documentation

This document is about the installation and use thead llvm in SG2042.

1. Current status

Thead vendor extensions are extensions which are not standardized by RISC-V International, and are instead defined by a thead hardware. Check spec Here

Instructions are prefixed with th. as described in the specification. There are 13 vendor extensions in spec, but llvm supports 11 vendor extensions，they are:

1. XTHeadBa, address-generation.
2. XTHeadBb, basic bit-manipulation.
3. XTHeadBs, single-bit operations
4. XTHeadCondMov, conditional move
5. XTHeadCmo, cache management operations
6. XTHeadFMemIdx, indexed memory operations for floating point
7. XTheadMac, multiply-accumulate instructions
8. XTHeadMemIdx, indexed memory operations
9. XTHeadMemPair，two-GPR memory operations
10. XTHeadSync，multi-core synchronization instructions
11. XTHeadVdot, version 1.0.0 of the THeadV-family custom instructions

2. How to build

2.1 Install prerequisites

Several standard packages are needed to build the toolchain, so we need to prepare the build environment before build. Using ubuntu as an example：

$ apt-get -y install flex bison git-core build-essential ninja-build libssh-dev libc6-dev python3-pip clang  cmake

2.2 Build steps

$ git clone [email protected]:llvm/llvm-project.git
$ cd llvm-project
$ mkdir build && cd build
$ cmake -DLLVM_TARGETS_TO_BUILD="RISCV" -DLLVM_ENABLE_PROJECTS="clang" -DCMAKE_BUILD_TYPE="RELEASE" -G Ninja ../llvm
$ ninja

3. How to use

$.example, test.c

unsigned int clz_32(unsigned int a) {
  return __builtin_riscv_clz_32(a);
}

$llvm-project/build/bin/clang -march=rv64gc_zfh_xtheadba_xtheadbb_xtheadbs_xtheadcmo_xtheadcondmov_xtheadfmemidx_xtheadmac_xtheadmemidx_xtheadmempair_xtheadsync test.c 

Notes: If we add the -S option，we can see the thead instruction.

      not     a0, a0
        slli    a0, a0, 32
        th.ff0  a0, a0 ----- the thead instruction.
        ret