gcov对Clang和GCC的研究结果不同。

Question

我正在尝试理解如何使用C++、googletest和gcov来正确地构造一个CMake项目。我想构建一个通用的CMakeLists.txt，它适用于任何平台/编译器。

这是我第一次尝试。但是，如果我试图构建这个项目，然后运行lcov (生成报告)，我会发现如果我使用CLang (正确的结果)或GCC (错误的结果)，我会得到不同的结果。注意，我在MacOs上，我通过brew (brew install gcc)安装了gcc。

此外，我在我的主CMakeLists.txt中使用了以下标志

if(CODE_COVERAGE)
    SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fprofile-arcs -ftest-coverage" )
endif()

注意:如果您在我的CMakeLists.txt文件或lcov使用中发现错误/奇怪的地方，我可以接受任何形式的反馈！

我的图书馆

#include "library.h"

#include <iostream>

void foo(){
    std::cout << "Foo!" << std::endl;
}

void bar(int n){
    if (n > 0){
        std::cout << "n is grater than 0!" << std::endl;
    }
    else if (n < 0){
        std::cout << "n is less than 0!" << std::endl;
    }
    else{
        std::cout << "n is exactly 0!" << std::endl;
    }
}

void baz(){  // LCOV_EXCL_START
    std::cout << "Baz!" << std::endl;
}
// LCOV_EXCL_STOP

我的测试

#ifndef GCOV_TUTORIAL_TEST_LIBRARY_H
#define GCOV_TUTORIAL_TEST_LIBRARY_H

#include "../src/library.h"

#include <gtest/gtest.h>


namespace gcov_tutorial::tests {
    TEST(TestFooSuite,TestFoo){
        foo();
    }
    TEST(TestBarSuite,TestBarGreaterThanZero){
        bar(100);
    }
    TEST(TestBarSuite,TestBarEqualToZero){
        //bar(0);
    }
    TEST(TestBarSuite,TestBarLessThanZero){
        bar(-100);
    }
}

#endif //GCOV_TUTORIAL_TEST_LIBRARY_H

CLang编译

#!/bin/bash

# Rationale: https://vaneyckt.io/posts/safer_bash_scripts_with_set_euxo_pipefail/
set -euxo pipefail

# BASE_DIR is the project's directory, containing the src/ and tests/ folders.
BASE_DIR=$PWD
COVERAGE_FILE=coverage.info

GCOV_PATH=/usr/bin/gcov
CLANG_PATH=/usr/bin/clang
CLANGPP_PATH=/usr/bin/clang++

rm -rf build
mkdir build && cd build

# Configure
cmake -DCMAKE_C_COMPILER=$CLANG_PATH -DCMAKE_CXX_COMPILER=$CLANGPP_PATH -DCODE_COVERAGE=ON -DCMAKE_BUILD_TYPE=Release ..

# Build (for Make on Unix equivalent to `make -j $(nproc)`)
cmake --build . --config Release

# Clean-up for any previous run.
rm -f $COVERAGE_FILE
lcov --zerocounters --directory .
# Run tests
./tests/RunTests
# Create coverage report by taking into account only the files contained in src/
lcov --capture --directory tests/ -o $COVERAGE_FILE --include "$BASE_DIR/src/*" --gcov-tool $GCOV_PATH
# Create HTML report in the out/ directory
genhtml $COVERAGE_FILE --output-directory out
# Show coverage report to the terminal
lcov --list $COVERAGE_FILE
# Open HTML
open out/index.html

​

​

GCC编撰

#!/bin/bash

# Rationale: https://vaneyckt.io/posts/safer_bash_scripts_with_set_euxo_pipefail/
set -euxo pipefail

# BASE_DIR is the project's directory, containing the src/ and tests/ folders.
BASE_DIR=$PWD
COVERAGE_FILE=coverage.info

GCOV_PATH=/usr/local/bin/gcov-11
GCC_PATH=/usr/local/bin/gcc-11
GPP_PATH=/usr/local/bin/g++-11

rm -rf build
mkdir build && cd build

# Configure
cmake -DCMAKE_C_COMPILER=$GCC_PATH -DCMAKE_CXX_COMPILER=$GPP_PATH -DCODE_COVERAGE=ON -DCMAKE_BUILD_TYPE=Release ..

# Build (for Make on Unix equivalent to `make -j $(nproc)`)
cmake --build . --config Release

# Clean-up for any previous run.
rm -f $COVERAGE_FILE
lcov --zerocounters --directory .
# Run tests
./tests/RunTests
# Create coverage report by taking into account only the files contained in src/
lcov --capture --directory tests/ -o $COVERAGE_FILE --include "$BASE_DIR/src/*" --gcov-tool $GCOV_PATH
# Create HTML report in the out/ directory
genhtml $COVERAGE_FILE --output-directory out
# Show coverage report to the terminal
lcov --list $COVERAGE_FILE
# Open HTML
open out/index.html

​

​

Answer 1

你实际上是在问两个问题，这里。

1. 为什么这两个编译器的覆盖率结果不同？
2. 如何构造用于代码覆盖的CMake项目？

回答1:覆盖面差异

这里的简单答案是，您正在构建的是Release模式，而不是RelWithDebInfo模式。在默认情况下，GCC并没有像Clang那样将更多的调试信息放入其中。在我的系统中，将-DCMAKE_CXX_FLAGS="-g"添加到build-and-run-cov-gcc.sh脚本中会产生与Clang相同的结果，就像在RelWithDebInfo中构建一样。

无论出于何种原因，Clang似乎都会在默认情况下或在启用覆盖率时跟踪更多的调试信息。GCC没有同样的护栏。要吸取的教训是:收集覆盖率信息是一种调试形式；如果要获得准确的结果，必须为编译器使用可调试的配置。

答2:建立系统结构

在构建中设置CMAKE_CXX_FLAGS通常是个糟糕的主意。该变量的作用是让构建的用户插入他们自己的标志。正如我在这个网站上的另一个答案中详细介绍的那样，存储这些设置的现代方法是在https://cmake.org/cmake/help/latest/manual/cmake-presets.7.html中。

我将去掉顶级CMakeLists.txt的CMakeLists.txt部分，然后创建以下CMakePresets.json文件：

{
  "version": 4,
  "cmakeMinimumRequired": {
    "major": 3,
    "minor": 23,
    "patch": 0
  },
  "configurePresets": [
    {
      "name": "gcc-coverage",
      "displayName": "Code coverage (GCC)",
      "description": "Enable code coverage on GCC-compatible compilers",
      "binaryDir": "${sourceDir}/build",
      "cacheVariables": {
        "CMAKE_BUILD_TYPE": "RelWithDebInfo",
        "CMAKE_CXX_FLAGS": "-fprofile-arcs -ftest-coverage"
      }
    }
  ],
  "buildPresets": [
    {
      "name": "gcc-coverage",
      "configurePreset": "gcc-coverage",
      "configuration": "RelWithDebInfo"
    }
  ]
}

然后，您的构建脚本可以大大简化。

#!/bin/bash

# Rationale: https://vaneyckt.io/posts/safer_bash_scripts_with_set_euxo_pipefail/
set -euxo pipefail

# Set up defaults for CC, CXX, GCOV_PATH
export CC="${CC:-gcc-11}"
export CXX="${CXX:-g++-11}"
: "${GCOV_PATH:=gcov-11}"

# Record the base directory
BASE_DIR=$PWD

# Clean up old build
rm -rf build

# Configure
cmake --preset gcc-coverage

# Build
cmake --build --preset gcc-coverage

# Enter build directory
cd build

# Clean-up counters for any previous run.
lcov --zerocounters --directory .

# Run tests
./tests/RunTests

# Create coverage report by taking into account only the files contained in src/
lcov --capture --directory tests/ -o coverage.info --include "$BASE_DIR/src/*" --gcov-tool $GCOV_PATH

# Create HTML report in the out/ directory
genhtml coverage.info --output-directory out

# Show coverage report to the terminal
lcov --list coverage.info

# Open HTML
open out/index.html

这里的关键是以下几行：

# Configure
cmake --preset gcc-coverage
# Build
cmake --build --preset gcc-coverage

这个脚本现在允许您通过环境变量更改编译器和覆盖率工具，并且CMakeLists.txt不需要对使用什么编译器做任何假设。

在我的(Linux)系统上，我可以成功地运行以下命令：

$ CC=gcc-12 CXX=g++-12 GCOV=gcov-12 ./build-and-run-cov.sh

​

​

$ CC=clang-13 CXX=clang++-13 GCOV=$PWD/llvm-cov-13.sh ./build-and-run-cov.sh

其中，llvm-cov-13.sh是llvm-cov-13的包装器，用于与--gcov-tool标志兼容。有关更多细节，请参见这个答案。

#!/bin/bash
exec llvm-cov-13 gcov "$@"

​

​

正如您所看到的，由于使用了正确的标志，结果是无法区分的。