SIC/XE LLVM backend

This project contains the SIC/XE (Simplified Instructional Computer) backend for the LLVM compiler infrastructure. The version of LLVM used is 3.9.

The modified clang compiler (with support for 24-bit integers that are used in SIC/XE) is available in the sic-clang repository.

Building

git clone --recurse-submodules https://github.com/jakoberzar/sic-llvm.git
cd sic-llvm
mkdir build
cd build
cmake \
    -DCMAKE_BUILD_TYPE=Release \
    -DLLVM_TARGETS_TO_BUILD="SIC" \
    -DCMAKE_C_COMPILER=clang \
    -DCMAKE_CXX_COMPILER=clang++ \
    -DLLVM_USE_LINKER=gold \
    -DLLVM_PARALLEL_LINK_JOBS=2 \
    ./../
make -j4

The built binaries are then available in the build/bin folder.

Usage

• Compiling with clang:

  ./build/bin/clang -target sic-unknown-linux-gnu -S ./examples/SIC/fib-recursive.c -O3 -o fib-recursive.sic.asm

• Compiling from C to LLVM IR with clang:

  ./build/bin/clang -target sic-unknown-linux-gnu -S -emit-llvm ./examples/SIC/fib-recursive.c -O3 -o fib-recursive.ll

• Compiling from LLVM IR to SIC/XE assembly with llc:

  ./build/bin/llc ./fib-recursive.ll -O3 -o fib-recursive.sic.asm

Relevant files are shown in the EXAMPLE.md file.

Features

• Working compilation from C to SIC/XE assembly using the sic-clang modified version of clang
• Use of 24-bit integers for the int data type
• It uses a set of conventions:
  • Register X is used as a stack pointer, as it can be used with indexed addressing
  • Arguments are passed via stack, as there aren't many registers available
  • Function results are returned in register S
  • A special symbol, MEMREG is used for loading and storing to arbitrary addresses in memory

Current limitations

• No support for the standard library
• No support for the SIC/XE 48-bit floating point register
• No possible SIC object file program output
• Incomplete support for some LLVM IR instructions, which appear rarely or are a result of optimizations
• No support for inline SIC/XE assembly in C

Location of modifications

The relevant modifications are spread accross the LLVM project, but can be mostly found in the lib/Target/SIC folder.

More information

This work was done as a part of my bachelor's thesis, LLVM backend for SIC/XE, which includes setup instructions, a guide through the work done and some examples with results.