program-markers inserts markers (as function calls) in C or C++ source code to enable differential testing for missed optimizations:

1. Instrument a program.
2. Compile it with two or more compilers (or compiler versions, or optimization levels, etc).
3. For each output: the markers whose corresponding calls are still present in the generated assembly are alive, the remaining are dead.
4. Differential testing: compare the sets of alive and dead markers across outputs.

There are two kinds of markers supported:

• DCE (Dead Code Elimination) Markers
• VR (Value Range) Markers

A DCEMarker tests if a compiler dead code eliminates a piece of code (basic block). For example,

if (Cond){
  DCEMarker0_();
  STMT0;
  STMT1;
  //...
}

If call DCEMarker0_(); is not present in the generated assembly code then the compiler determined that Cond must always be false and therefore the body of this if statement is dead.

A VRMarker tests if a compiler can determine subsets of the value ranges of variables, for example:

if (a <= C)
    VRMarker0_();

If call VRMarker0_(); is not present in the generated assembly code then the compiler determined that a's value is always a > C. Currently only integer variables are instrumented.

To build just the clang tool

Prerequisites: cmake, make, clang/llvm 13/14.

mkdir build
cd build
cmake .. 
cmake --build . [--parallel]
cmake --install . --prefix=/where/to/install/

Usage

cat test.c
int foo(int a) {
    if (a == 0)
        return 1;
    else {
        a = 5;
    }

    return a;
}


dead-instrument test.c --


cat test.c | clang-format
#if defined DisableDCEMarker0_
#define DCEMARKERMACRO0_ ;
#elif defined UnreachableDCEMarker0_
#define DCEMARKERMACRO0_ __builtin_unreachable();
#else
#define DCEMARKERMACRO0_ DCEMarker0_();
void DCEMarker0_(void);
#endif
#if defined DisableDCEMarker1_
#define DCEMARKERMACRO1_ ;
#elif defined UnreachableDCEMarker1_
#define DCEMARKERMACRO1_ __builtin_unreachable();
#else
#define DCEMARKERMACRO1_ DCEMarker1_();
void DCEMarker1_(void);
#endif
int foo(int a) {
  if (a == 0)
  {
    DCEMARKERMACRO1_
    return 1;
  }
  else {
    DCEMARKERMACRO0_
    a = 5;
  }

  return a;
}

Individual markers can be disabled or turned into unreachables (useful for helping the compiler optimize parts known to be dead):

gcc -E -P -DDisableDCEMarker0_ -DUnreachableDCEMarker1_ test.c | clang-format
int foo(int a) {
  if (a == 0) {
    __builtin_unreachable();
    return 1;
  } else {
    ;
    a = 5;
  }
  return a;
}

Passing --ignore-functions-with-macros to program-markers will cause it to ignore any functions that contain macro expansions.

Value range markers can be emitted instead by using --mode=vr:

cat test.c
int foo(int a) {
  if (a == 0)
    return 1;
  return 0;
}

program-markers --mode=vr test.c --

cat test.c | clang-format 
#if defined DisableVRMarkerLE0_
#define VRMARKERMACROLE0_(VAR)
#elif defined UnreachableVRMarkerLE0_
#define VRMARKERMACROLE0_(VAR)         \
  if ((VAR) <= VRMarkerConstantLE0_)   \
    __builtin_unreachable();
#else
#define VRMARKERMACROLE0_(VAR)         \
  if ((VAR) <= VRMarkerConstantLE0_)   \
    VRMarkerLE0_();
void VRMarkerLE0_(void);
#endif
#ifndef VRMarkerConstantLE0_
#define VRMarkerConstantLE0_ 0
#endif
#if defined DisableVRMarkerGE1_
#define VRMARKERMACROGE1_(VAR)
#elif defined UnreachableVRMarkerGE1_
#define VRMARKERMACROGE1_(VAR)         \
  if ((VAR) >= VRMarkerConstantGE1_)   \
    __builtin_unreachable();
#else
#define VRMARKERMACROGE1_(VAR)         \
  if ((VAR) >= VRMarkerConstantGE1_)   \
    VRMarkerGE1_();
void VRMarkerGE1_(void);
#endif
#ifndef VRMarkerConstantGE1_
#define VRMarkerConstantGE1_ 0
#endif
int foo(int a) {
  VRMARKERMACROLE0_(a)
  VRMARKERMACROGE1_(a)
  if (a == 0)
    return 1;
  return 0;
}

The ranges that each marker test can be adjucted via macros and individual markers can be disabled or turned into unreachables:

gcc -E -P -DDisableVRMarkerLE0_ -DVRMarkerConstantGE1_=8 test.c | clang-format
void VRMarkerGE1_(void);
int foo(int a) {

  if ((a) >= 8)
    VRMarkerGE1_();
  if (a == 0)
    return 1;
  return 0;
}

Python wrapper

pip install program-markers

To use the instrumenter in python import from program_markers.instrumenter import instrument_program: instrument_program(program: diopter.SourceProgram, ignore_functions_with_macros: bool) -> InstrumentedProgram.

Building the python wrapper

Local build

./build_python_wheel_local.sh #this will build the current branch
pip install .

Docker based build

docker run --rm -e REVISION=REV -v `pwd`:/io theodort/manylinux-with-llvm:latest /io/build_python_wheel_docker.sh

This will build multiple wheels for REV with multiple python versions. The output is stored in the wheelhouse directory. The docker image is based on https://github.com/thetheodor/manylinux-with-llvm.