Recently merged vector primops support only 16 bytes operands - Int32 x 4, Double x 2 and so on. Current AVX instructions support 256 bit operands and with simple cut'n'paste work it's possible to support at least Double x 4 operands. I made those changes and GHC generates (using llvm) proper AVX code using ymm registers. Also it might make sense to support primops for vector types larger than any currently supported primitive types - I have those changes in my branch as well and llvm generates pretty good code as well - those changes might be useful to provide access for llvm shufflevector instruction or writing high performance processing of large vectors - with less potential overhead. Do we want to support larger vectors directly or ghc should be made smart enough to fuse operations with vector primops performed in parallel into larger vectors/registers for llvm? Do we want to provide access to llvm shufflevector instruction?