simd: stop the AVX2 rolling checksum reading past its buffer

The loop is software-pipelined: each iteration folds in the 64 bytes it preloaded last time and preloads the next 64. Nothing stopped the final iteration doing that preload, so it always read the 64 bytes after the region it was asked to checksum. Not an edge case. The assembly processes len&~63 and leaves the remainder to the caller, so the remainder is by construction under 64 bytes and the preload passed buf+len on every call, by 64 minus the remainder. It normally landed in slack inside the map_ptr() window and nothing noticed. Where the buffer ended near an unmapped page it was a SIGSEGV in the middle of a transfer -- reported on macOS x86-64 by Roland Kletzing, whose `partial` run died with "connection unexpectedly closed" because the generator had crashed. A guard page reproduces it on Linux too, so it was latent there, not absent. Run the pipelined loop one block short and finish the last block in .last, which does the same arithmetic without the preload. No per-iteration cost, and checksums are bit-identical -- simdtest compares every implementation against the C reference. The earlier fix for that report, "lib: use .balign in md5 x86-64 asm", was to the md5 assembly. It addressed the linker alignment warning that appeared alongside, not this.

ATAndrew Tridgell committed 3 months ago