[PATCH] D11758: [Unroll] Implement a conservative and monotonically increasing cost tracking system during the full unroll heuristic analysis that avoids counting any instruction cost until that instruction becomes "live" through a side-effect or use outside the...

[Michael Zolotukhin via llvm-commits]

mzolotukhin added a comment.

Hi Chandler,

This patch got stuck, but finally I got some time and investigated it more carefully. I found a bug in current implementation that explains everything, and as a consequence I constructed a simple test-case to demonstrate the problem:

  int foo(char *a) {
    int i = 0;
    for (i = 0; i < 500; i++)
      if (a[i] == 0)
        return i;
    return 0;
  }

The problem is that the only instructions we consider live are those having side effects. However, they might have no side effects (by LLVM's definition of side effects: `mayWriteToMemory() || mayThrow() || !mayReturn()`) and still affect program behavior, as in the case above.

Thanks,
Michael


================
Comment at: lib/Transforms/Scalar/LoopUnrollPass.cpp:752-753
@@ -620,1 +751,4 @@
+        // leading up to it.
+        if (I.mayHaveSideEffects())
+          AddCostRecursively(I, Iteration);
 
----------------
Should we also take into account instructions with out-of-the-loop uses and early-exit instructions? Are there any other cases we want not to miss?


http://reviews.llvm.org/D11758