[all-commits] [llvm/llvm-project] ccf1a5: [InstCombine] dropRedundantMaskingOfLeftShiftInput...

[Roman Lebedev via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: ccf1a5f4bbe680f20e26c29774d62bec6cb226da
      https://github.com/llvm/llvm-project/commit/ccf1a5f4bbe680f20e26c29774d62bec6cb226da
  Author: Roman Lebedev <lebedev.ri at gmail.com>
  Date:   2019-11-05 (Tue, 05 Nov 2019)

  Changed paths:
    M llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
    M llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-after-truncation-variant-a.ll
    M llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-after-truncation-variant-b.ll
    M llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-after-truncation-variant-c.ll
    M llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-after-truncation-variant-d.ll
    M llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-after-truncation-variant-e.ll
    M llvm/test/Transforms/InstCombine/redundant-left-shift-input-masking-after-truncation-variant-a.ll
    M llvm/test/Transforms/InstCombine/redundant-left-shift-input-masking-after-truncation-variant-b.ll
    M llvm/test/Transforms/InstCombine/redundant-left-shift-input-masking-after-truncation-variant-c.ll
    M llvm/test/Transforms/InstCombine/redundant-left-shift-input-masking-after-truncation-variant-d.ll
    M llvm/test/Transforms/InstCombine/redundant-left-shift-input-masking-after-truncation-variant-e.ll
    M llvm/test/Transforms/InstCombine/redundant-left-shift-input-masking-after-truncation-variant-f.ll

  Log Message:
  -----------
  [InstCombine] dropRedundantMaskingOfLeftShiftInput(): truncation (PR42563)

Summary:
That fold keeps growing and growing :(
I think this may be one of the last pieces for it.

Since D67677/D67725, the fold knowns the general form
of the pattern - where some masking is needed:
https://rise4fun.com/Alive/F5R
https://rise4fun.com/Alive/gslRa

But there is one more huge piece missing - if you are extracting some bits,
it is not impossible that the origin is wider than the extraction,
i.e. there may be a truncation. And we don't deal with that yet.

But we can, and the generalization remains fully identical:
https://rise4fun.com/Alive/Uar
https://rise4fun.com/Alive/5SW

After a preparatory cleanup i think the diff looks rather clean.

One missing piece is that in some patterns (especially pat. b),
`-1` only needs to be `-1` in final type, but that is for later..

https://bugs.llvm.org/show_bug.cgi?id=42563

Reviewers: spatel, nikic

Reviewed By: spatel

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69125