[LLVMdev] MemoryDependenceAnalysis Bug or Feature?

[Eugene Toder]

Since isLoad == false means we're looking at a store, what this does
is making the store *p depend on the load *p. This is correct -- you
can't move store before load, otherwise load will start returning a
different value.

Eugene

On Fri, Jul 16, 2010 at 5:43 PM, Marc de Kruijf <dekruijf at cs.wisc.edu> wrote:
> Hello,
>
> I'm taking a really good look at the MemoryDependenceAnalysis pass, but I'm
> slightly confused about one little thing.  I think it's a bug but I'm not
> confident enough to submit a bug report.
>
> Is there a reason why read-only calls are considered to "clobber" pointers
> in the non-load case (which is what gets returned due to the fall-through in
> the switch -- see below).  It seems this should be returning a "def"
> instead.  My thinking is the code should look like this (from line 288 on,
> +++ marks addition):
>
>      // See if this instruction (e.g. a call or vaarg) mod/ref's the
> pointer.
>      switch (AA->getModRefInfo(Inst, MemPtr, MemSize)) {
>      case AliasAnalysis::NoModRef:
>        // If the call has no effect on the queried pointer, just ignore it.
>        continue;
>      case AliasAnalysis::Mod:
>        // If we're in an invariant region, we can ignore calls that ONLY
>        // modify the pointer.
>        if (InvariantTag) continue;
>        return MemDepResult::getClobber(Inst);
>      case AliasAnalysis::Ref:
>        // If the call is known to never store to the pointer, and if this is
> a
>        // load query, we can safely ignore it (scan past it).
>        if (isLoad)
>          continue;
> +++    return MemDepResult::getDef(Inst);
>      default:
>        // Otherwise, there is a potential dependence.  Return a clobber.
>        return MemDepResult::getClobber(Inst);
>      }
>
> If this seems right to you too,  I've attached the patch.  If this isn't
> right, can someone please explain the logic?
>
> Thanks,
> Marc
>
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