xelert / llvm-pass

Implementation of my DCE LLVM Pass

Sber LLVM Pass

About

This is the third task at Sber Compilers additional semester class: Implementation of dead code elimination pass. Zero propagation and memory to register transformations are made before DCE pass.

Requirements

• Cmake version 3.13.4
• LLVM

Install & Build

    $ git clone https://github.com/XelerT/llvm-pass.git
    $ cd llvm-pass/
    $ cmake -S . -B build -DCMAKE_BUILD_TYPE=Release && cmake --build build

Run

You need to generate llvm IR file and then feed it to stasyan:

    $ cd build/
    $ ./stasyan input_file.ll

Example of input file

    define dso_local i32 @sub(i32 noundef %0, i32 noundef %1) { 
            %3 = mul i32 %0, 0
            ret i32 %3
    }

Example of work

Firstly, you need to create llvm IR file (from build/):

    $ clang -S -emit-llvm -O0 ../test/data/sources/ex3.cpp
    $ ./stasyan  ../test/data/ex3.ll

Output of the program:

    ; Function Attrs: mustprogress noinline nounwind optnone sspstrong uwtable
    define dso_local noundef i32 @_Z5code3i(i32 noundef %0) #0 {
            ret i32 %0
    }

Three passes worked on the IR (see code after every transformation):

1. Zero propagation from apselon

    define dso_local noundef i32 @_Z5code3i(i32 noundef %0) #0 {
            %2 = alloca i32, align 4
            %3 = alloca i32, align 4
            %4 = alloca i32, align 4
            %5 = alloca i32, align 4
            %6 = alloca i32, align 4
            store i32 %0, ptr %2, align 4
            store i32 5, ptr %3, align 4
            store i32 10, ptr %4, align 4
            store i32 15, ptr %5, align 4
            store i32 20, ptr %6, align 4
            %7 = load i32, ptr %2, align 4
            %8 = load i32, ptr %4, align 4
            %9 = add nsw i32 %7, %8
            %10 = load i32, ptr %3, align 4
            %11 = add nsw i32 %9, %10
            %12 = load i32, ptr %5, align 4
            %13 = add nsw i32 %11, %12
            %14 = load i32, ptr %6, align 4
            %15 = add nsw i32 %13, %14
            store i32 %15, ptr %6, align 4
            %16 = load i32, ptr %2, align 4
            ret i32 %16
    }
   

2. Memory to registers from llvm:

    define dso_local noundef i32 @_Z5code3i(i32 noundef %0) #0 {
            %2 = add nsw i32 %0, 10
            %3 = add nsw i32 %2, 5
            %4 = add nsw i32 %3, 15
            %5 = add nsw i32 %4, 20
            ret i32 %0
    }
   

3. Dead code elimination from me:

    define dso_local noundef i32 @_Z5code3i(i32 noundef %0) #0 {
            ret i32 %0
    }