[PATCH] D67677: [InstCombine] dropRedundantMaskingOfLeftShiftInput(): pat. a/b with mask (PR42563)

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri created this revision.
lebedev.ri added reviewers: spatel, nikic, xbolva00.
lebedev.ri added a project: LLVM.
Herald added a subscriber: hiraditya.

And this is **finally** the interesting part of that fold!

If we have a pattern `(x & (~(-1 << maskNbits))) << shiftNbits`,
we already know (have a fold) that will drop the `& (~(-1 << maskNbits))`
mask iff `(maskNbits+shiftNbits) u>= bitwidth(x)`.
But that is actually ignorant, there's more general fold here:

In this pattern, `(maskNbits+shiftNbits)` actually correlates
with the number of low bits that will remain in the final value.
So even if `(maskNbits+shiftNbits) u< bitwidth(x)`, we can still
fold, we will just need to apply a **constant** mask afterwards:

  Name: a, normal+mask
    %onebit = shl i32 -1, C1
    %mask = xor i32 %onebit, -1
    %masked = and i32 %mask, %x
    %r = shl i32 %masked, C2
  =>
    %n0 = shl i32 %x, C2
    %n1 = add i32 C1, C2
    %n2 = zext i32 %n1 to i64
    %n3 = shl i64 -1, %n2
    %n4 = xor i64 %n3, -1
    %n5 = trunc i64 %n4 to i32
    %r = and i32 %n0, %n5

https://rise4fun.com/Alive/F5R

Naturally, old `%masked` will have to be one-use.
Similar fold exists for patterns c,d,e, will post patch later.

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


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D67677

Files:
  llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
  llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-a.ll
  llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-b.ll

-------------- next part --------------
A non-text attachment was scrubbed...
Name: D67677.220559.patch
Type: text/x-patch
Size: 11201 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20190917/cff2f026/attachment.bin>