[LLVMdev] MemoryDependenceAnalysis Bug or Feature?

[Marc de Kruijf]

Yes, I'm not arguing that there is a dependence, just that it's not a
clobber dependence.  The case of a load is already considered earlier in
that function and with isLoad == false it returns MemDepResult::getDef().
 My question is:  why should a read-only call (which yields
AliasAnalysis::Ref and is handled in this code fragment) be any different
from e.g. a load.  Isn't a read-only call effectively just a series of loads
from a memory-dependence perspective?

In other words, why does this code fragment return
MemDepResult::getClobber() instead of MemDepResult::getDef() for a read-only
call?

On Sat, Jul 17, 2010 at 6:18 PM, Eugene Toder <eltoder at gmail.com> wrote:

> 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
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
> >
>
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