This is a meta bug to track the port status of all ports that have not yet been updated to handle CVE-2017-5753 (Spectre variant 1). If your port does (or might in the future) need to mitigate against unsafe speculation, then you will need to define (at a minimum) a "speculation_barrier" insn. If your port does not (and never will in the future) need to mitigate against unsafe speculation, you can simply add #undef TARGET_HAVE_SPECULATION_SAFE_VALUE #define TARGET_HAVE_SPECULATION_SAFE_VALUE speculation_safe_value_not_needed. to your target defines.
Original patch series and discussion can be found here: https://gcc.gnu.org/ml/gcc-patches/2018-07/msg01700.html
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MicroBlaze does not do speculative execution. No action needed.