Add capability to flatdata-cpp for analysing data access patterns (#239)

Add an option to analyse data access patterns:
* To provide analysis of data accessed from each resource
* bytes / pages accessed
* analysis of inefficient struct layout (e.g. members that are never accessed)
* Performance is not an objective, this will cause significant slowdowns

All of this will be a compile-time opt-in.

Example of output (based on flatdata benchmark)

```
===== flatdata debug statistics =====
Total pages: 185635
Usage statistics:
    benchmark.flatdata/edge_data(63.1573% of pages): 
        bytes [count]: 160073112 out of 480219352
        bytes [accessed]: 33.3333%
        pages [count]: 117242 out of 117241
        pages [data access]: 33.3331%
        unused struct members:
            byte   0 redundant: 100%
            byte   1 redundant: 100%
            byte   2 redundant: 100%
            byte   3 redundant: 100%
            byte   4 redundant: 100%
            byte   5 redundant: 100%
            byte   6 redundant: 100%
            byte   7 redundant: 100%
            byte  16 redundant: 100%
            byte  17 redundant: 100%
            byte  18 redundant: 100%
            byte  19 redundant: 100%
            byte  20 redundant: 100%
            byte  21 redundant: 100%
            byte  22 redundant: 100%
            byte  23 redundant: 100%
    benchmark.flatdata/adjacent_edges(21.0526% of pages): 
        bytes [count]: 160073112 out of 160073120
        bytes [accessed]: 100%
        pages [count]: 39081 out of 39080
        pages [data access]: 99.9983%
        unused struct members:
    benchmark.flatdata/nodes(15.7896% of pages): 
        bytes [count]: 40018288 out of 120054856
        bytes [accessed]: 33.3333%
        pages [count]: 29311 out of 29310
        pages [data access]: 33.3325%
        unused struct members:
            byte   0 redundant: 100%
            byte   1 redundant: 100%
            byte   2 redundant: 100%
            byte   3 redundant: 100%
            byte   4 redundant: 100%
            byte   5 redundant: 100%
            byte   6 redundant: 100%
            byte   7 redundant: 100%
```

.github/workflows/cpp.yml

@@ -1,4 +1,4 @@
-name: flatdata-rs
+name: flatdata-cpp
 on:
   push:
     branches: [ master ]
@@ -12,6 +12,9 @@ env:
 jobs:
   GCC:
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        debug-stats: ["", "-DWITH_DEBUG_DATA_ACCESS_STATISTICS=ON"]
     steps:
       - uses: actions/checkout@v2
       - name: Dependencies
@@ -20,9 +23,12 @@ jobs:
         run: pip3 install ./flatdata-generator
       - name: Build and Test
         run: |
-           CC=gcc CXX=g++ flatdata-cpp/ci/build-and-test-cpp.sh
+           CC=gcc CXX=g++ EXTRA_CMAKE_ARGS=${{ matrix.debug-stats }} flatdata-cpp/ci/build-and-test-cpp.sh
   Clang:
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        debug-stats: ["", "-DWITH_DEBUG_DATA_ACCESS_STATISTICS=ON"]
     steps:
       - uses: actions/checkout@v2
       - name: Dependencies
@@ -31,4 +37,4 @@ jobs:
         run: pip3 install ./flatdata-generator
       - name: Build and Test
         run: |
-          CC=clang CXX=clang++ flatdata-cpp/ci/build-and-test-cpp.sh
+          CC=clang CXX=clang++ EXTRA_CMAKE_ARGS=${{ matrix.debug-stats }} flatdata-cpp/ci/build-and-test-cpp.sh

flatdata-cpp/CMakeLists.txt

@@ -18,6 +18,11 @@ add_library(flatdata STATIC ${FLATDATA_SOURCE})
 
 set_target_properties(flatdata PROPERTIES POSITION_INDEPENDENT_CODE ON)
 
+if(WITH_DEBUG_DATA_ACCESS_STATISTICS)
+  message(STATUS "WITH_DEBUG_DATA_ACCESS_STATISTICS enabled")
+  target_compile_definitions(flatdata PUBLIC DEBUG_DATA_ACCESS_STATISTICS)
+endif()
+
 target_include_directories(flatdata
     PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
     PUBLIC $<INSTALL_INTERFACE:include>

flatdata-cpp/ci/build-and-test-cpp.sh

@@ -2,6 +2,6 @@
 set -ex
 mkdir build
 cd build
-cmake ../flatdata-cpp -DCMAKE_CXX_FLAGS="-Wall -pedantic -Wextra"
+cmake ../flatdata-cpp -DCMAKE_CXX_FLAGS="-Wall -pedantic -Wextra" $EXTRA_CMAKE_ARGS
 make -j$(nproc)
-make test
+make test ARGS="--verbose"