[gdal-dev] Different results when reading a GeoTIFF with GDAL 2.1.2 vs 3.x

[Even Rouault]

>      Is it possible that this is a compiler issue? Anyone else that can
> test this on an AMD CPU? Mine are old Phenom II X4 965s.

Perhaps rather a runtime issue. I suspect this is related to a SSSE3 
optimization that has been added in GDAL 2.2 that is used in this use case 
(unpacking of byte buffer with a 2 byte stride to packed buffer)

>From what I found on the net, I think this CPU doesn't support the SSSE3 
instruction set (SSSE3 with three S, not to be confused with SSE3...)
Can you check the following ?
cat /proc/cpuinfo | grep ssse3

What is strange is that GDAL does have a runtime check to detect if SSSE3 is 
available, so I'm not sure why you would get that issue. Unless that Docker 
would wrongly expose SSSE3, but Docker is not a VM technology so it shouldn't 
mess up with CPU capability discoveries. This is weird.

Can you try the following ?

docker run --rm osgeo/gdal su -c "apt install -y cpuid; cpuid" | grep SSSE3

In a DEBUG build of GDAL, you could disable at runtime the SSSE3 optimization 
by defining the environment variable/configuration option GDAL_USE_SSSE3 to 
NO, but this will not work on the release builds available on Docker.

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