[GHC] #16052: Core optimizations for memset on a small range

[GHC]

#16052: Core optimizations for memset on a small range
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        Reporter:  andrewthad        |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.9
       Component:  Compiler          |              Version:  8.6.3
      Resolution:                    |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by bgamari):

 * keywords:   => newcomer
 * milestone:   => 8.9


Comment:

 Actually, now that I think of it, I believe the problem is that we don't
 know enough about the `ByteArray#`'s alignment for the inline memset logic
 to fire, given the current implementation. The current implementation is
 (found in `compiler/nativeGen/X86/CodeGen.hs`) is too strict in its
 condition:
 {{{#!hs
 genCCall dflags _ (PrimTarget (MO_Memset align)) _
          [dst,
           CmmLit (CmmInt c _),
           CmmLit (CmmInt n _)]
          _
     | fromInteger insns <= maxInlineMemsetInsns dflags && align .&. 3 == 0
 = do
 }}}

 Another problem is that the codegen logic (`StgCmmPrim.doSetByteArrayOp`)
 makes too weak a claim about the alignment. It claims that the region is
 merely byte-aligned, even if the offset is aligned. Given that we know
 that the beginning of the bytearray is aligned to 16-bytes, we should be
 able to do better here (and the copy primops).

 Fixing this would be a nice project.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16052#comment:3>
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