opt-chance

An LLVM-based tool used for comparing binaries compiled with different compilers in order to find optimizations oportunities. It relies on Debug Info (DWARF only at the moment). This is still a WIP.

Clone

If you want to use upstream llvm:

$ git clone https://github.com/djtodoro/opt-chance.git --recursive

Otherwise (please do note that if you want to use your version of LLVM, you need to use EXTERN_LLVM_SRC_DIR cmake variable):

$ git clone https://github.com/djtodoro/opt-chance.git

Build (in-tree-llvm)

1. Build LLVM

$ mkdir build_llvm && cd build_llvm
$ cmake -G "Ninja" ../opt-chance/llvm-project/llvm -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_ENABLE_PROJECTS="llvm"
$ ninja

2. Build opt-chance

$ cd ..
$ mkdir build_tool && cd build_tool
$ cmake -G "Ninja" ../opt-chance/ -DLLVM_DIR="$PWD/../build_llvm/llvm/lib/cmake/llvm" -DLLVM_ENABLE_ASSERTIONS=ON -DCMAKE_BUILD_TYPE=Release
$ ninja opt-chance

Build (with custom LLVM)

Then, build the plugin with your version of llvm, e.g. llvm-project-llvmorg-16.0.4 (please note -DEXTERN_LLVM_SRC_DIR that contains src of the llvm and proper -DLLVM_DIR that contatins build of the llvm):

$ cmake -G"Ninja" -DEXTERN_LLVM_SRC_DIR="${PATH_TO_LLVM}/llvm-project-llvmorg-16.0.4" -DCMAKE_BUILD_TYPE=Release -DLLVM_DIR=$PWD/../build_llvm/lib/cmake/llvm -DLLVM_ENABLE_ASSERTIONS=ON ../opt-chance/
$ ninja opt-chance

Usage

First of all, both a.out and a.out need to be comipled with debug info (-g), since our tool reads debug info in order to find canindates for optimizations.

$ ../build_tool/opt-chance a2.out a.out 
=== opt-chance Tool ===
Analysis...

Candidates to optimize:
for function 'main' the diff is 18

Analysis done.

It means that main function is larger in the a.out for 18 bytes.

There is a --verbose option as well.

$ ../build_tool/opt-chance a2.out a.out --verbose
=== opt-chance Tool ===
Analysis...
Analysis of file: a2.out
 function main: 25 bytes
 function fn: 11 bytes
 function fn4: 11 bytes
 function fn3: 11 bytes
 function fn2: 11 bytes

Analysis of file: a.out
 function main: 43 bytes
 function fn: 11 bytes
 function fn4: 11 bytes
 function fn3: 11 bytes
 function fn2: 11 bytes

Candidates to optimize:
for function 'main' the diff is 18

Analysis done.