[PATCH] D39345: SCEV: preserve debug information attached to PHI nodes.

[Adrian Prantl via Phabricator via llvm-commits]

aprantl added a comment.

I moved the logic over to `indvars` now, and that seems to fit naturally into `WidenIV::createWideIV()` with about the same functionality as above.

But I'm having trouble generating more interesting testcases. For example: The top-level comment in IndVarSimplify claims that the pass would

  //   1. The exit condition for the loop is canonicalized to compare the
  //      induction value against the exit value.  This turns loops like:
  //        'for (i = 7; i*i < 1000; ++i)' into 'for (i = 0; i != 25; ++i)'

But I can't reproduce this:

  char *a;
  void f(int j) {
    for (int i = 7; i*i < 1000; i+=1)
        a[i] = i;
  }
  
  clang -O3 -c -S -emit-llvm -o - /tmp/loop.c -fno-unroll-loops -mllvm -print-after-all -g 2>&1
  
  *** IR Dump After Induction Variable Simplification ***
  for.body:                                         ; preds = %entry, %for.body
    %indvars.iv = phi i64 [ 7, %entry ], [ %indvars.iv.next, %for.body ]

Has the functionality bitrotted, or is the comment out-of-date, or is there now a different transformation that would rewrite a loop like int that comment? Or can I change my testcase somehow?

I then put an assert that IVDiff (the diff between SCEV for the old and new phi in WidenIV::createWideIV()) is not a constant zero and check-llvm-transform still passes. That's not the end of the world, handling the constant-zero case would already be a nice improvement, but I'm wondering if I'm missing the big picture here.


https://reviews.llvm.org/D39345