[LLVMdev] possible addrspacecast problem

[Junjie Gu]

Given a pointer,  does any addrspacecast affect the pointer's
dereferenceablity ?  For example,
    %pm = addrspaacecast float addrspacecast(n)* %pn to float
addrspacecast(m)*
    %r  = load float addrspace(m)* %pm
In another word. the question is whether the following is true ?
    isDereferenceablePointer(pn) == isDereferenceablePointer(pm)

  [Note that the function is defined as
   Value.cpp:isDereferenceaablePointer(const Value*, ...) ]


The reason I am asking this is that the current LLVM thinks the answer is
yes
(they are the same).  But for the following case, it will make
non-speculative
load to be moved out of its guard.  For example (assume that
isDereferenceablePointer(p)
is true),

The following code:

     %pm = addrspaacecast float* %p to float addrspacecast(m)*
     if (p is in addrspace(m))
        // BB_then
        %r0 = load float,  float addrspace(m)* %pm;
     else  // p is in default addrspace
        // BB_else
        %r1 = load float,  float* p;
     %r = phi float [r0, BB_then], [r1, BB_else]

will be transformed to

     %pm = addrspaacecast float* %p to float addrspacecast(m)*
     %r0 = load float addrspace(m)* %pm;
     %r1 = load float* p;
     %r = select i1 (p is in adrspace(m)) float %r0, float %r1

which is wrong as "%r0 = load float addrspace(m)* %pm" would be illegal
if (p is in addrspace(m)) is false.

If we agree that the answer to the above question is no, then fix
is simple (fix diff is attached). I also have a small test to show
the problem,  simplifyCFG actually does the above transformation
(to reproduce, use "opt -simplifycfg -S test3.ll -o tt.ll).

Any suggestions/comments ?


thanks
Junjie
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Index: Value.cpp
===================================================================
--- Value.cpp	(revision 232165)
+++ Value.cpp	(working copy)
@@ -574,8 +574,8 @@
       return isDereferenceablePointer(RelocateInst.derivedPtr(), DL, Visited);
     }
 
-  if (const AddrSpaceCastInst *ASC = dyn_cast<AddrSpaceCastInst>(V))
-    return isDereferenceablePointer(ASC->getOperand(0), DL, Visited);
+//  if (const AddrSpaceCastInst *ASC = dyn_cast<AddrSpaceCastInst>(V))
+//    return isDereferenceablePointer(ASC->getOperand(0), DL, Visited);
 
   // If we don't know, assume the worst.
   return false;

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