[InstCombine] add folds for unsigned-overflow compares

Name: op_ugt_sum
  %a = add i8 %x, %y
  %r = icmp ugt i8 %x, %a
  =>
  %notx = xor i8 %x, -1
  %r = icmp ugt i8 %y, %notx

Name: sum_ult_op
  %a = add i8 %x, %y
  %r = icmp ult i8 %a, %x
  =>
  %notx = xor i8 %x, -1
  %r = icmp ugt i8 %y, %notx

https://rise4fun.com/Alive/ZRxI

AFAICT, this doesn't interfere with any add-saturation patterns
because those have >1 use for the 'add'. But this should be
better for IR analysis and codegen in the basic cases.

This is another fold inspired by PR14613:
https://bugs.llvm.org/show_bug.cgi?id=14613

llvm-svn: 342004

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index e8e9e86d08b..875d6d9f053 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3047,6 +3047,18 @@ Instruction *InstCombiner::foldICmpBinOp(ICmpInst &I) {
     return nullptr;
 
   const CmpInst::Predicate Pred = I.getPredicate();
+  Value *X;
+
+  // Convert add-with-unsigned-overflow comparisons into a 'not' with compare.
+  // (Op1 + X) <u Op1 --> ~Op1 <u X
+  // Op0 >u (Op0 + X) --> X >u ~Op0
+  if (match(Op0, m_OneUse(m_c_Add(m_Specific(Op1), m_Value(X)))) &&
+      Pred == ICmpInst::ICMP_ULT)
+    return new ICmpInst(Pred, Builder.CreateNot(Op1), X);
+  if (match(Op1, m_OneUse(m_c_Add(m_Specific(Op0), m_Value(X)))) &&
+      Pred == ICmpInst::ICMP_UGT)
+    return new ICmpInst(Pred, X, Builder.CreateNot(Op0));
+
   bool NoOp0WrapProblem = false, NoOp1WrapProblem = false;
   if (BO0 && isa<OverflowingBinaryOperator>(BO0))
     NoOp0WrapProblem =