Machine independent passes to optimise LLVM intermediate representation.

In order to run the optimization passes, it is necessary having LLVM 17.0.6 source code installed.
Official repository here.

The following steps assume that LLVM 17.0.6 has been installed.
Clone the current repository:

git clone https://github.com/RaffaeleTranfaglia/LLVM-Middle-End-Optimizations.git

src/LocalOpts.cpp file must be moved to the following directory:

$SRC/llvm/lib/Transforms/Utils

Add LocalOpts.cpp in $SRC/llvm/lib/Transforms/Utils/CMakeLists.txt.
src/LocalOpts.h file must be moved to the following directory:

$SRC/llvm/include/llvm/Transforms/Utils

Replace SRC/llvm/lib/Passes/PassRegistry.def with the provided src/PassRegistry.def.
Add the follwing line to SRC/llvm/lib/Passes/PassBuilder.cpp:

#include "llvm/Transforms/Utils/LocalOpts.h"

Where $SRC is the source folder of the project.
To compile the source code:

cd $ROOT/BUILD
make opt
make install

Otherwise, to install the source code already containig the optimized passes' files: here.

Constant Folding execute at compile time operations involving contant values.
Examples:

• x = 4 + 9 → x = 13 + 0
• x = 6 * 2 → x = 12 + 0

Observation: The assignment of the computed constant is carryed out using an addition of the given constant with zero.
Afterwards the introduced add will furtherly be optimized by the Algebric Identity optimization.

Algebraic Identity aims to optimise operation containing neutral values.
Examples:

• y = x + 0 → every use of y is replaced with x
• a = b * 1 → every use of a is replaced with b

A strength reduction pass replace mul instructions with shift instruction to reduce computational complexity. Example:

• x * 15 → (x << 4) - x
• x * 17 → (x << 5) - 15x
• x / 16 → x >> 4

Observations:
In case the element "subtracted" can be optimized, the algorithm will optimize further more the operation. In case it is used on divisions, the divisor must be an exact multiple of 2.

Multi instruction optimization operates in cases where, given a SSA register, the same fixed amount is addend and subtracted from it. Examples:

• y = x + 2; z = y - 2 → every use of z is replaced with x
• y = x + 2; z = y / 2 → every use of z is replaced with x

DataFlowAnalysis folder contains global optimizations algorithms.
Optimization tasks addressed:

• Very Busy Expressions
• Dominator Analysis
• Constant Propagation

Instructions that meet the following requirements are candidate for code motion:

• are invariant
• dominate their uses
• dominate the exits of the loop or are dead outside the loop

Candidated instruction may be moved outside the loop (just before the loop header, in the so called preheader block) in order to be executed only one time.

LoopOpts.cpp and LoopOpts.h files contain the Loop Invariant Code Motion pass.
In order to make the pass work, src/LoopOpts.cpp file must be moved to the following directory:

$SRC/llvm/lib/Transforms/Utils

Add LoopOpts.cpp in $SRC/llvm/lib/Transforms/Utils/CMakeLists.txt.
src/LoopOpts.h file must be moved to the following directory:

$SRC/llvm/include/llvm/Transforms/Utils

Replace SRC/llvm/lib/Passes/PassRegistry.def with the provided src/PassRegistry.def.
Add the follwing line to SRC/llvm/lib/Passes/PassBuilder.cpp:

#include "llvm/Transforms/Utils/LoopOpts.h"

Where $SRC is the source folder of the project.
To compile the source code:

cd $ROOT/BUILD
make opt
make install

Tests folder contains different LLVM files to test the optimizations.

Compile the source code:

cd $ROOT/BUILD
make opt
make install

To run tests:

opt -p <optimization_pass_name> <file_name>.ll -o <file_name_optimized>.bc
llvm-dis <file_name_optimized>.bc -o <file_name_optimized>.ll

Note: Implemented passes names are localopts and loopopts.

• Raffaele Tranfaglia
• Matteo Venturi
• Mirco Piccinini