Avoid generating vzeroupper instruction

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Hello,

is there a way to tell gcc to not emit the `vzeroupper` instruction
while using -march=corei7avx? I do realize that this instruction is
neccessary if legacy 128 bit SSE instructions are intermixed with AVX
128/256 bit code, like calling a libc function which makes use of SSE in
a non-AVX glibc or the other way around.

But in a pure AVX code environment I noticed it actually does hurt
performance. So how can I tell gcc to not emit vzeroupper while
generating AVX instructions?

- Gunther


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