I would like to raise a compatibility and documentation concern regarding the CPU requirements of the prebuilt Qt binaries.

I recently encountered this when running a Python application using PySide6 on an older x86-64 system. The CPU supports SSE4.1, but not SSE4.2 or POPCNT.

With PySide6 6.9.3 (Qt 6.9.3), the application runs normally. With PySide6 6.10.0 (Qt 6.10.0), importing QtCore immediately terminates with:

Incompatible processor. This Qt build requires the following features:
    sse4.2 popcnt
Aborted

I compared the actual Qt libraries from the two PySide6 distributions. The Qt 6.9.3 libQt6Core.so.6 contains no POPCNT instructions, whereas the Qt 6.10.0 library contains actual POPCNT instructions.

This therefore appears to be an intentional change in the CPU baseline of the Qt 6.10 binaries rather than an application or Python problem.

I understand the motivation for this change. The Qt development discussion from 2022 proposed raising the default x86-64 target to x86-64-v2 in order to enable additional optimizations in QtCore and QtGui. x86-64-v2 includes SSE4.2 and POPCNT. This is a reasonable engineering decision if the performance benefits justify it.

However, there is an important compatibility consequence.

An application which updates from Qt 6.9 to Qt 6.10 can become completely unusable on an otherwise functional x86-64 system. This is particularly relevant for long-lived Linux installations and older computers, where x86-64 itself is still supported by the operating system.

This is not merely a case of an optional optimization being unavailable. The application does not start at all.

I have also encountered reports of the same problem in other Qt applications, including Linphone. This suggests that the impact is not limited to PySide6 applications.

My main concern is therefore not that Qt has chosen to raise the CPU baseline. Rather, it is that this change does not appear to be clearly communicated in the supported-platform documentation.

The Qt documentation continues to describe Linux x86-64 as a supported architecture, but does not prominently state that the prebuilt Qt binaries require an x86-64-v2-capable processor. For users maintaining older hardware, this distinction is significant.

I would therefore suggest:

1. Clearly documenting the minimum CPU instruction set required by the prebuilt Qt binaries for each Qt release.
2. Clearly identifying the Qt release in which the x86-64 CPU baseline was raised.
3. Considering whether an x86-64-v1 build could remain available for users who need compatibility with older processors.

I am not arguing that Qt should never raise its CPU requirements. I am asking that the compatibility impact of such a change be made explicit, because otherwise users may reasonably interpret "x86_64" as meaning the traditional x86-64 baseline.

For reference, the practical boundary I observed is:

Qt 6.9.3   — works on an SSE4.1 / non-POPCNT CPU
Qt 6.10.0  — requires SSE4.2 + POPCNT

This makes the change particularly easy to reproduce and verify.