[PATCH] D69514: [InstCombine] Expand usub_sat patterns to handle constants

[Dave Green via Phabricator via llvm-commits]

dmgreen updated this revision to Diff 228272.
dmgreen added a comment.

Sorry for the delay. I was trying to look into the "off by one" patterns, the last two of these:

  Name: Forwards
  Pre: C1 == -C2
    %cmp = icmp ugt i8 %a, C1
    %sub = add i8 %a, C2
    %sel = select i1 %cmp, i8 %sub, i8 0
  =>
    %sel = usub_sat %a, C1
  
  Name: Backwards
  Pre: C1 == -C2
    %cmp = icmp ult i8 %a, C1
    %sub = add i8 %a, C2
    %sel = select i1 %cmp, i8 %sub, i8 0
  =>
    %x = usub_sat C1, %a
    %sel = sub 0, %x
  
  Name: Forwards_off
  Pre: C1 + 1 == -C2 && C2 != 0
    %cmp = icmp ugt i8 %a, C1
    %sub = add i8 %a, C2
    %sel = select i1 %cmp, i8 %sub, i8 0
  =>
    %sel = usub_sat %a, C1 + 1
  
  Name: Backwards_off
  Pre: C1 - 1 == -C2 && C1 != 0
    %cmp = icmp ult i8 %a, C1
    %sub = add i8 %a, C2
    %sel = select i1 %cmp, i8 %sub, i8 0
  =>
    %x = usub_sat C1 - 1, %a
    %sel = sub 0, %x

They are a bit fiddly though and deserve to be their own commit.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D69514/new/

https://reviews.llvm.org/D69514

Files:
  llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
  llvm/test/Transforms/InstCombine/builtin-dynamic-object-size.ll
  llvm/test/Transforms/InstCombine/unsigned_saturated_sub.ll

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