Qt 5.3 Might Depend On SSE2 CPUs

Thiago Macieira of Intel is coordinating a proposal to enable SSE2 by default for the 32-bit builds of the Qt tool-kit beginning with the Qt 5.3 release. This likely change is due to the V4 JIT engine generating floating-point math for Qt's QML only using SSE x86 registers.

Beyond the fact that the V4 JIT engine is generaBy making this change the developers assume that anyone using Qt 5.3+ will basically be on an x86 CPU from 2004 or later -- basically the Pentium 4 and Pentium M era or AMD Athlon 64. Those older users would be forced to switching over to a source-based build of Qt where they can manually disable the SSE2+ support.ting floating-point instructions in SSE registers for x86, Microsoft with Visual Studio 2012 is also using SSE2 instructions by default. This proposal is about having the Qt Project Linux 32-bit packages having SSE2 optimizations by default.

This change to use SSE2 by default would also affect source builds of the Qt source code, but the compile-time options would remain for disabling SSE2 instructions. There's also SSSE3 optimizations in Qt that are run-time checked. The proposed change isn't for the imminent release of Qt 5.2 but is anticipated for the release of Qt 5.3 in Q2'2014.

Enabling SSE2 by default in the 32-bit Linux builds will also mean all floating-point math done by Qt will be done in SSE rather than the slower x87 stack, drawhelpers are unconditionally enabled, and SSE2-based code for the Latin1 and UTF-8 codecs will always be enabled.

By making this change the developers assume that anyone using Qt 5.3+ will basically be on an x86 CPU from 2004 or later -- basicxally the Pentium 4 and Pentium M era or AMD Athlon 64. Those older users would be forced to switching over to a source-based build of Qt where they can manually disable the SSE2+ support at compile-time.

If you are a 32-bit Linux user with a CPU more than one decade old that would be affected by this change, see the SSE2 change proposal for more details.