[llvm-bugs] [Bug 39903] New: Vectorization generates unaligned moves for properly aligned data

via llvm-bugs llvm-bugs at lists.llvm.org
Thu Dec 6 06:38:08 PST 2018


            Bug ID: 39903
           Summary: Vectorization generates unaligned moves for properly
                    aligned data
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: jamesmatta at gmail.com
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

Created attachment 21198
  --> https://bugs.llvm.org/attachment.cgi?id=21198&action=edit
Code that shows the problem in compiler explorer.

A simple FMA loop operating on data that is aligned with `alignas(64)` is
generating `vmovups` instructions when it could, and should, be generating

Since the nehalem architecture the cost of a `vmovups` relative to `vmovaps` is
not as large as it once was, but it is noticeable and if the data is guaranteed
to be aligned, then `vmovaps` should be the instruction used.


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