[PATCH] D97122: [SCEV] Improve handling of pointer compares involving subtractions (WIP).

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.
fhahn added reviewers: reames, mkazantsev, sanjoy.google, nikic.
Herald added a subscriber: hiraditya.
fhahn requested review of this revision.
Herald added a project: LLVM.

WIP as I still need to do further testing and re-collecting &
applying the loop guards every time can be quite expensive in terms of
compile time. But I thought it would be good to share early, in case I
missed anything.

This patch improves handling of pointer comparisons involving
subtractions, by using knowledge that the involved variables are != 0
and signed positive.

It uses applyLoopGuards to add != 0 info to the involved expressions (by
re-writing them as umax expression. It also adds a new fold to move zext
into umax expressions and to move udiv expression inside subtraction, if
the operands guarantee a positive result.

The motivating code for this is eliminating range checks for code
like below from PR48965.

  #include <assert.h>
  
  void foo(const int *y, unsigned y_len) {
    const int *y_end = y + y_len;
    unsigned c = 0;
    for (const int *y_iter = y; y_iter != y_end; y_iter++, c++) {
      assert(c < y_len);
    }
    assert(c == y_len);
  }

Proof for zext/umax fold: https://alive2.llvm.org/ce/z/t75T53

Proof for getUDivExpr extension:a https://alive2.llvm.org/ce/z/H_G2Q0

Proof for isKnownPredicateSubIdiom: https://alive2.llvm.org/ce/z/Gfe8mS


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D97122

Files:
  llvm/include/llvm/Analysis/ScalarEvolution.h
  llvm/lib/Analysis/ScalarEvolution.cpp
  llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
  llvm/test/Transforms/IndVarSimplify/simplify-pointer-arithmetic.ll

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