Eliminate a bitwise 'not' op of 'not' min/max by inverting the min/max.

Bitwise 'not' of the min/max could be eliminated in the pattern:

%notx = xor i32 %x, -1
%cmp1 = icmp sgt[slt/ugt/ult] i32 %notx, %y
%smax = select i1 %cmp1, i32 %notx, i32 %y
%res = xor i32 %smax, -1

https://rise4fun.com/Alive/lCN

Reviewers: spatel

Reviewed by: spatel

Subscribers: a.elovikov, llvm-commits

Differential Revision: https://reviews.llvm.org/D45317

llvm-svn: 329791

1 files changed, 30 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
index 04a85c7a3e7..9ba92f7a637 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
@@ -2696,5 +2696,35 @@ Instruction *InstCombiner::visitXor(BinaryOperator &I) {
     return SelectInst::Create(Cmp, Builder.CreateNeg(A), A);
   }
 
+  // Eliminate a bitwise 'not' op of 'not' min/max by inverting the min/max:
+  //
+  //   %notx = xor i32 %x, -1
+  //   %cmp1 = icmp sgt i32 %notx, %y
+  //   %smax = select i1 %cmp1, i32 %notx, i32 %y
+  //   %res = xor i32 %smax, -1
+  // =>
+  //   %noty = xor i32 %y, -1
+  //   %cmp2 = icmp slt %x, %noty
+  //   %res = select i1 %cmp2, i32 %x, i32 %noty
+  //
+  // Same is applicable for smin/umax/umin.
+  {
+    Value *LHS, *RHS;
+    SelectPatternFlavor SPF = matchSelectPattern(Op0, LHS, RHS).Flavor;
+    if (Op0->hasOneUse() && SelectPatternResult::isMinOrMax(SPF) &&
+        match(Op1, m_AllOnes())) {
+
+      Value *X;
+      if (match(RHS, m_Not(m_Value(X))))
+        std::swap(RHS, LHS);
+
+      if (match(LHS, m_Not(m_Value(X)))) {
+        Value *NotY = Builder.CreateNot(RHS);
+        return SelectInst::Create(
+            Builder.CreateICmp(getInverseMinMaxPred(SPF), X, NotY), X, NotY);
+      }
+    }
+  }
+
   return Changed ? &I : nullptr;
 }