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

[Even Rouault]

On dimanche 29 décembre 2019 23:53:14 CET Momtchil Momtchev wrote:
>      I recompiled with the modified CPU detection, the SSE3 syntax error
> and with the default configure options (after a distclean) and I get the
> _incorrect_ result.

Hum, what about the following ?

static inline bool CPLDetectSSE3()
{
    int cpuinfo[4] = { 0, 0, 0, 0 };
    CPL_CPUID(0, cpuinfo);
    if( cpuinfo[REG_EAX] <= 2 )
        return false;
    CPL_CPUID(0x80000000, cpuinfo);
    if( static_cast<unsigned>(cpuinfo[REG_EAX]) <= 1 )
        return false;
    CPL_CPUID(1, cpuinfo);
    fprintf(stderr, "ECX = 0x%X\n", cpuinfo[REG_ECX]);
    return (cpuinfo[REG_ECX] & (1 << CPUID_SSSE3_ECX_BIT)) != 0;
}

> 
> On 29/12/2019 22:15, Momtchil Momtchev wrote:
> >     I get the same wrong results with and without Docker, with and
> > without the SSE/SSE3/AVX when using the configure script. I also tried
> > removing -O2. Is the SSE3 unpacking in GDAL or in libgeotiff?
> > 
> > On 29/12/2019 21:51, Even Rouault wrote:
> >>>       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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