[GHC] #8279: bad alignment in code gen yields substantial perf issue

[GHC]

#8279: bad alignment in code gen  yields substantial perf issue
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        Reporter:  carter                   |            Owner:
            Type:  bug                      |           Status:  new
        Priority:  highest                  |        Milestone:
       Component:  Compiler                 |          Version:  7.7
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  Runtime performance bug  |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:  #8082
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Comment (by schyler):

 Little bit of research uncovers that Intel recommends aligning code and
 branch targets on 16-byte boundaries: `3.4.1.5 - Assembly/Compiler Coding
 Rule 12. (M impact, H generality)
 All branch targets should be 16-byte aligned.`

 The reasons for this are as follows;
 * Aligning to a 16-byte boundary means that it's more likely loops will
 fall inside a single cacheline rather than inside 2. Falling on a boundary
 in a loop has a really noticeable negative performance impact.
 * Forward small relative jumps that lie inside the same cache line are
 handled more efficiently by the pipeline than far jumps

 Note that we have to be really careful not to turn a small relative jump
 into a larger jump by altering alignment slightly, because these tend to
 be slower (see pt. 2).

 Tail calls may benefit from aligning both the top of tables (for aligned
 reading) and the top of actual function entry points (for aligned
 iteration) to 16 bytes. This requires experimentation and benchmarking.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8279#comment:9>
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