boost-gil-checksum-bug

TL;TR: Why Boost.GIL checksum tests for bit-aligned image fail on MSVC 64-bit optimized builds?

Boost.GIL

The bug was reproduced with GIL from:

• Boost.GIL develop branch
• Boost 1.67 through 1.72 (current release)

Test

The minimal_test.cpp performs the following test:

1. Create 3x3 image based on bit-aligned pixel (bgr121_image_t)
2. Fill image with red
3. Draw blue diagonal

Observation

Pixels are accessed using xy_locator which gives access x_iterator and y_iterator.

Pixels are iterated using for loop:

• if x_iterator is incremented before the y_iterator, then the blue diagonal is garbled.
• if x_iterator is incremented after the y_iterator, then the blue diagonal is drawn correctly - Where is the BUG?!

Pixels are iterated hand-rolled iteration without for loop:

• regardless if ++loc.x() is before or after --loc.y(), the blue diagonal is drawn correctly.

Mystery

The bug described above leaks only and only with 64-bit optimised build using MSVC++ 14.x

b2 toolset=msvc variant=release address-model=64 cxxstd=11,14,17,2a

It is not observed using all these numerous tested compilers:

• GCC: 4.8, 4.9, 5, 6, 7, 8, 9
• Clang: 3.9, 4.0, 5.0, 6.0, 7, 8, 9, 10
• XCode: 8.3, 9.0, 9.1, 9.2, 9.3, 9.4, 10

Run with Boost.Build

b2.exe -sBOOST_ROOT=D:\boost.win toolset=msvc cxxstd=11 variant=release,debug address-model=32,64 minimal

Run with CMake

cmake -G "Visual Studio 16 2019" -A Win32 -B build32 -DBOOST_ROOT=D:/boost.win
cmake -G "Visual Studio 16 2019" -A x64   -B build64 -DBOOST_ROOT=D:/boost.win

set CTEST_OUTPUT_ON_FAILURE=1
for %B in (32,64) do ( for %C in (Debug,Release) do (
    cmake --build build%B --config %C && cmake --build build%B --config %C --target RUN_TESTS
))