[all-commits] [llvm/llvm-project] 36cfea: [X86] Add tests for inverting `(x * (Pow2_Ceil(C1)...

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  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 36cfea5a98fd09e61f91937547c7796318f2c026
      https://github.com/llvm/llvm-project/commit/36cfea5a98fd09e61f91937547c7796318f2c026
  Author: Noah Goldstein <goldstein.w.n at gmail.com>
  Date:   2023-05-13 (Sat, 13 May 2023)

  Changed paths:
    A llvm/test/CodeGen/X86/undo-mul-and.ll

  Log Message:
  -----------
  [X86] Add tests for inverting `(x * (Pow2_Ceil(C1) - (1 << C0))) & C1` -> `(-x << C0) & C1`; NFC

Differential Revision: https://reviews.llvm.org/D150293


  Commit: 6c3bf364bf95209925b1e884077ec79cca274dc7
      https://github.com/llvm/llvm-project/commit/6c3bf364bf95209925b1e884077ec79cca274dc7
  Author: Noah Goldstein <goldstein.w.n at gmail.com>
  Date:   2023-05-13 (Sat, 13 May 2023)

  Changed paths:
    M llvm/lib/Target/X86/X86ISelLowering.cpp
    M llvm/test/CodeGen/X86/undo-mul-and.ll

  Log Message:
  -----------
  [X86] Invert transforming `(x * (Pow2_Ceil(C1) - (1 << C0))) & C1` -> `(-x << C0) & C1`

We can detect the case under the following circumstances:
Take `(Pow2_Ceil(C1) - (1 << C0))` as `C2`.
    1) `C2` is NOT a power of 2.
    2) `C2 + LeastSignificantBit(C2)` is a nonzero power of 2.
    3) `C2 u>= C1`

The motivation is the middle end transforms:
    `(-x << C0) & C1`
to
    `(x * (Pow2_Ceil(C1) - (1 << C2))) & C1`

As it saves IR instructions. On X86 the two instruction, `sub` and
`shl`, and better than the `mul` so we want to undo the transform.

This comes up when shifting a bit-mask by a byte-misalignment i.e:
    `y << ((-(uintptr)x * 8) & 63)`

Alive2 Proofs (including all cases with undefs in the vector):
https://alive2.llvm.org/ce/z/f-65b6

Reviewed By: RKSimon, pengfei

Differential Revision: https://reviews.llvm.org/D150294


  Commit: ab1b42ac5cb82cd7d541caa06e7929825b4fed84
      https://github.com/llvm/llvm-project/commit/ab1b42ac5cb82cd7d541caa06e7929825b4fed84
  Author: Noah Goldstein <goldstein.w.n at gmail.com>
  Date:   2023-05-13 (Sat, 13 May 2023)

  Changed paths:
    M llvm/lib/Analysis/InstructionSimplify.cpp
    A llvm/test/Transforms/InstCombine/div-i1.ll

  Log Message:
  -----------
  [InstCombine] Add simplifications for div/rem with `i1` operands; PR62607

This is generally handled already in early CSE.

If a specialized pipeline is used, however, its possible for `i1`
operand with known-zero denominator to slip through. Generally the
known-zero denominator is caught and poison is returned, but if it is
indirect enough (known zero through a phi node) we can miss this case
in `InstructionSimplify` and then miss handling `i1`. This is because
`i1` is current handled with the following check:
    `if(Known.countMinLeadingZeros() == Known.getBitWidth() - 1)`

which only works on the assumption we don't know the denominator to be
zero. If we know the denominator to be zero, this check fails:
https://github.com/llvm/llvm-project/issues/62607

This patch simply adds an explicit `if(Known.isZero) return poison;`
which fixes the issue.

Alive2 Link for tests:
    https://alive2.llvm.org/ce/z/VTw54n

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D150142


  Commit: da9f306739a60508f9ab10f2cea939b7a6e2d328
      https://github.com/llvm/llvm-project/commit/da9f306739a60508f9ab10f2cea939b7a6e2d328
  Author: Noah Goldstein <goldstein.w.n at gmail.com>
  Date:   2023-05-13 (Sat, 13 May 2023)

  Changed paths:
    M llvm/include/llvm/CodeGen/SelectionDAG.h
    M llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

  Log Message:
  -----------
  [SelectionDAG] Limit max recursion in `isKnownNeverZero` and `isKnownToBeAPowerOfTwo`

Both of these functions recursively call themselves so it makes sense
to limit that upper bound.

Differential Revision: https://reviews.llvm.org/D149195


  Commit: e36caaeeb25fafba0851e4a1905c7ceb08c337a8
      https://github.com/llvm/llvm-project/commit/e36caaeeb25fafba0851e4a1905c7ceb08c337a8
  Author: Noah Goldstein <goldstein.w.n at gmail.com>
  Date:   2023-05-13 (Sat, 13 May 2023)

  Changed paths:
    M llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
    M llvm/test/CodeGen/ARM/cttz_vector.ll

  Log Message:
  -----------
  [SelectionDAG] Use `computeKnownBits` if `Op` is not recognized by `isKnownNeverZero`

The current logic is pretty limitted unless the `Op` is a
constant. This at least covers more obvious cases.

Reviewed By: craig.topper, foad

Differential Revision: https://reviews.llvm.org/D149196


Compare: https://github.com/llvm/llvm-project/compare/94f7c961c78d...e36caaeeb25f