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Our initial work with int128 neglected alignment considerations, an oversight that came back to bite us in bug #14897 from Vincent Lachenal. It is unsurprising that int128 might have a 16-byte alignment requirement; what's slightly more surprising is that even notoriously lax Intel chips sometimes enforce that. Raising MAXALIGN seems out of the question: the costs in wasted disk and memory space would be significant, and there would also be an on-disk compatibility break. Nor does it seem very practical to try to allow some data structures to have more-than-MAXALIGN alignment requirement, as we'd have to push knowledge of that throughout various code that copies data structures around. The only way out of the box is to make type int128 conform to the system's alignment assumptions. Fortunately, gcc supports that via its __attribute__(aligned()) pragma; and since we don't currently support int128 on non-gcc-workalike compilers, we shouldn't be losing any platform support this way. Although we could have just done pg_attribute_aligned(MAXIMUM_ALIGNOF) and called it a day, I did a little bit of extra work to make the code more portable than that: it will also support int128 on compilers without __attribute__(aligned()), if the native alignment of their 128-bit-int type is no more than that of int64. Add a regression test case that exercises the one known instance of the problem, in parallel aggregation over a bigint column. This will need to be back-patched, along with the preparatory commit 91aec93e6. But let's see what the buildfarm makes of it first. Discussion: https://postgr.es/m/20171110185747.31519.28038@wrigleys.postgresql.org
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