[all-commits] [llvm/llvm-project] fbcf8a: [ConstraintElim] Add (UGE, var, 0) to unsigned sys...

[Florian Hahn via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: fbcf8a8cbb2461730bfd0603b396842925a88ef2
      https://github.com/llvm/llvm-project/commit/fbcf8a8cbb2461730bfd0603b396842925a88ef2
  Author: Florian Hahn <flo at fhahn.com>
  Date:   2023-12-23 (Sat, 23 Dec 2023)

  Changed paths:
    M llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
    M llvm/test/Transforms/ConstraintElimination/add-nuw.ll
    M llvm/test/Transforms/ConstraintElimination/and-implied-by-operands.ll
    M llvm/test/Transforms/ConstraintElimination/gep-arithmetic-add.ll
    M llvm/test/Transforms/ConstraintElimination/gep-arithmetic.ll
    M llvm/test/Transforms/ConstraintElimination/geps-pointers-to-structs.ll
    M llvm/test/Transforms/ConstraintElimination/large-constant-ints.ll
    M llvm/test/Transforms/ConstraintElimination/loops-bottom-tested-pointer-cmps.ll
    M llvm/test/Transforms/ConstraintElimination/loops-header-tested-base.ll
    M llvm/test/Transforms/ConstraintElimination/loops-header-tested-pointer-cmps.ll
    M llvm/test/Transforms/ConstraintElimination/max-row-limit.ll
    M llvm/test/Transforms/ConstraintElimination/mul.ll
    M llvm/test/Transforms/ConstraintElimination/or-implied-by-operands.ll
    M llvm/test/Transforms/ConstraintElimination/or.ll
    M llvm/test/Transforms/ConstraintElimination/reason-about-add-operands.ll
    M llvm/test/Transforms/ConstraintElimination/reproducer-remarks-debug.ll
    M llvm/test/Transforms/ConstraintElimination/reproducer-remarks.ll
    M llvm/test/Transforms/ConstraintElimination/shl.ll
    M llvm/test/Transforms/ConstraintElimination/sub-nuw.ll
    M llvm/test/Transforms/PhaseOrdering/runtime-check-removal.ll

  Log Message:
  -----------
  [ConstraintElim] Add (UGE, var, 0) to unsigned system for new vars. (#76262)

The constraint system used for ConstraintElimination assumes all
varibles to be signed. This can cause missed optimization in the
unsigned system, due to missing the information that all variables are
unsigned (non-negative).

Variables can be marked as non-negative by adding Var >= 0 for all
variables. This is done for arguments on ConstraintInfo construction and
after adding new variables. This handles cases like the ones outlined in
https://discourse.llvm.org/t/why-does-llvm-not-perform-range-analysis-on-integer-values/74341

The original example shared above is now handled without this change,
but adding another variable means that instcombine won't be able to
simplify examples like https://godbolt.org/z/hTnra7zdY

Adding the extra variables comes with a slight compile-time increase
https://llvm-compile-time-tracker.com/compare.php?from=7568b36a2bc1a1e496ec29246966ffdfc3a8b87f&to=641a47f0acce7755e340447386013a2e086f03d9&stat=instructions:u

stage1-O3    stage1-ReleaseThinLTO    stage1-ReleaseLTO-g  stage1-O0-g
 +0.04%           +0.07%                   +0.05%           +0.02%
stage2-O3    stage2-O0-g    stage2-clang
  +0.05%         +0.05%        +0.05%

https://github.com/llvm/llvm-project/pull/76262