Re: [gil] Broken bit-aligned pixel locator on MSVC++ 64-bit optimized builds only

From: Mateusz Loskot (mateusz_at_[hidden])
Date: 2020-03-14 23:30:50

On Wed, 11 Mar 2020 at 01:02, Mateusz Loskot <mateusz_at_[hidden]> wrote:
>
> Over the last few months, intermittently I've been chasing a peculiar
> bug that has been revealing itself in failing checksum tests in GIL's
> test/legacy/image.cpp [1]
> since we started testing GIL with MSVC++ (VS2017, VS2019) with
>
> b2 toolset=msvc variant=release address-model=64
> [...]
> auto v = view(img);
> auto loc = v.xy_at(0, v.height() - 1);
> for (std::ptrdiff_t y = 0; y < v.height(); ++y)
> {
>     *loc = bgr121_blue; // set blue pixel value
>     ++loc.x();          // INCREMENT X FIRST
>     --loc.y();
> }

Ha!
I've just noticed failures between VS 2017 and 2019 are a bit different,
still for bgr121_image_t though, still for 64-bit optimised builds only:

VS2017 build fails at bgr121_views_90cw test case:

====== BEGIN OUTPUT =====
...
checking checksum for bgr121_basic_red_x (crc=7f6e24e7)
Checking checksum for bgr121_basic_white_x (crc=e4aaa1d3)
Checking checksum for bgr121_views_original (crc=7e5bb87d)
Checking checksum for bgr121_views_cropped (crc=43305e15)
Checking checksum for bgr121_views_gray8 (crc=bbabe219)
Checking checksum for bgr121_views_my_gray8 (crc=3086d22f)
Checking checksum for bgr121_views_transpose (crc=da2ff80)
Checking checksum for bgr121_views_rot180 (crc=68f37202)
Checksum error in bgr121_views_90cw (crc=8ffd852b != 8565efd8)
Unknown exception
 ====== END OUTPUT =

VS2019 build fails much sooner:

====== BEGIN OUTPUT =====
...
Checksum error in bgr121_basic_red_x (crc=85c86c53 != 7f6e24e7)
Unknown exception
====== END OUTPUT =

Clearly, there is something fishy going on near the MSVC++ optimisation.

Looking at the C++ code, I'm getting very suspicious about what is going
on in the decrement operator, or boost::iterator_facade implementor.

I have not compared assembly for the minimal_test.cpp cases.

Best regards,
-- 
Mateusz Loskot, http://mateusz.loskot.net