https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77476
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Most likely:
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r239672 | uros | 2016-08-22 12:13:45 -0700 (Mon, 22 Aug 2016) | 6 lines
* config/i386/i386.md (*zero_extendsidi2): Add (*r,*k) alternative.
(zero_extend<mode>di2): Ditto.
(*zero_extend<mode>si2): Ditto.
(*zero_extendqihi2): Ditto.
Which added k alternatives.
(define_register_constraint "k" "TARGET_AVX512F ? MASK_REGS : NO_REGS"
"@internal Any mask register.")
Oh it looks like these alternatives should be disabled for KNL targets which
does not have support for the full AVX512F ISA :).
