[PATCH] [SCCP] Calculate Null Pointer Comparisons

[Sanjoy Das]

I'm not sure that this is doing the right thing -- I don't think simply //producing// `undef` implies undefined behavior.

For instance, in

  declare void @g()
  
  define i1 @test_postdominate(i8* %mem) {
    %test = icmp eq i8* %mem, null
    br i1 %test, label %a, label %b
  a:
    call void @g()
    br label %c
  b:
    br label %c
  c:
    %join = phi i8* [ %mem, %a ], [ null, %b ]
    %gep = getelementptr inbounds i8, i8* %join, i64 4
    ret i1 %test
  }

if `%mem` is `null`, then we have to execute the call to `@g`, but this pass make the branch into `a` dead (so `@f` never calls `@g`).  The fact that `%gep` is `undef` (or poison) does not make this program undefined.  In fact, if `inbounds` GEP instructions could invoke UB, then we would not be able to hoist such instructions out of loops.


http://reviews.llvm.org/D9634

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