[PATCH] D40375: Use MemorySSA in LICM to do sinking and hoisting.

[Daniel Berlin via llvm-commits]

>
>
> ================
> Comment at: lib/Transforms/Scalar/LICM.cpp:1650
> +        // if the Source is a combination of liveOnEntry and
> +        // volatile loads (treated as Defs), it's not invalidated.
> +        SmallPtrSet<MemoryAccess *, 4> Processed;
> ----------------
> This seems fishy -- if `volatile` loads do not invalidate pointers then
> why are they treated as defs by MSSA?
>
> We've gone back and forth on this as to whether volatile loads should be
defs or not.

We don't represent ordering chains separately from memory chains.

" // The isOrdered check is used to ensure that volatiles end up as defs
   // (atomics end up as ModRef right now anyway).  Until we separate the
   // ordering chain from the memory chain, this enables people to see at
least
   // some relative ordering to volatiles.  Note that
getClobberingMemoryAccess
   // will still give an answer that bypasses other volatile loads.  TODO:
   // Separate memory aliasing and ordering into two different chains so
that we
   // can precisely represent both "what memory will this read/write/is
      clobbered
   // by" and "what instructions can I move this past". "
bool Def = bool(ModRef & MRI_Mod) || isOrdered(I);
   bool Use = bool(ModRef & MRI_Ref);
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