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Hello, LLVM generates two additional instructions for 128->256 bit typecasts (e.g. _mm256_castsi128_si256()) to clear out the upper 128 bits of YMM register corresponding to source XMM register. vxorps xmm2,xmm2,xmm2 vinsertf128 ymm0,ymm2,xmm0,0x0 Most of the industry-standard C/C++ compilers (GCC, Intel's compiler, Visual Studio compiler) don't generate any extra moves for 128-bit->256-bit typecast intrinsics. None of these compilers zero-extend the upper 128 bits of the 256...

On Tue, Sep 9, 2014 at 11:39 PM, Chandler Carruth <chandlerc at google.com> wrote: > Awesome, thanks for all the information! > > See below: > > On Tue, Sep 9, 2014 at 6:13 AM, Andrea Di Biagio <andrea.dibiagio at gmail.com> > wrote: >> >> You have already mentioned how the new shuffle lowering is missing >> some features; for example, you explicitly