[llvm-dev] Data-dependent cycles in IR without cycles in source?

[Matthew O'Connor via llvm-dev]

Hi,

I'm attempting to understand why in the following C++ code LLVM produes
cycles in the IR (for `cycle1` and `cycle2`, that do not have
data-dependent cycles), but `nocycle` is not. In the first two cases,
`Count` has an undefined value when `Count + 1` is evaluated and the value
is scoped to the body of the loop so can't (or shouldn't) hold values from
one iteration to the next, but in `clang++ -std=c++11 sample.cpp -S
-emit-llvm -OX -o sample.ll` where X = any of 0,1,2,3,s,z, a data-dependent
cycle is created - like below:

%for.body:
  Count.012 = phi i32 [ undef, %entry ], [ %cond.i, %for.body ]
  %add = add nsw i32 %Count.012, 1
  %cond.i = select i1 %call, i32 %0, i32 %add

Thanks,
Matt

int select(bool P, int True, int False) { return P ? True : False; }

bool random(void);

void cycle1(int (&Input)[100], int (&Output)[100]) {
  for (int I = 0, N = 100; I < N; ++I) {
    int Count;

    Count = select(random(), Input[I], Count + 1);

    Output[I] = Count;
  }
}

void cycle2(int (&Input)[100], int (&Output)[100]) {
  for (int I = 0, N = 100; I < N; ++I) {
    int Count = select(random(), Input[I], Count + 1);

    Output[I] = Count;
  }
}

void nocycle1(int (&Input)[100], int (&Output)[100]) {
  for (int I = 0, N = 100; I < N; ++I) {
    int Count = 0;

    Count = select(random(), Input[I], Count + 1);

    Output[I] = Count;
  }
}
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