Re: (ITS#8819) LMDB seg fault with MDB_DUPSORT on -O3

[nic@nicwatson.org]

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Summary of findings so far:

Attached is a make script that applies the minimal set of
optimizations (at least on gcc 7) that still triggers the seg fault.

It fails the first time that address is executed, when NUMKEYS(fp)
hits 10 or 11 (I *think* depending on the alignment of fp).

It looks like the autovectorization logic is split in three parts: one
unaligned non-SSE copy, an aligned copy that uses SSE, and then an
unaligned non-SSE copy. It makes sense that the SSE logic path is only
triggered when the size is greater than some threshold.

We should coordinate filing a gcc bug report.

Nic Watson


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