[InstCombine] Fold selection between less than zero and one

Author: elhewaty

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

@@ -3415,6 +3415,20 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
                                 TrueVal);
   }
 
+  // select (icmp eq a, 0), 1, (lshr a, 31) -> icmp sle a, 0,
+  // which is then converted to icmp sle a, 1
+  CmpInst::Predicate Pred;
+  Value *A;
+  const APInt *C;
+  if (match(CondVal, m_Cmp(Pred, m_Value(A), m_Zero())) &&
+      match(TrueVal, m_One()) &&
+      match(FalseVal, m_LShr(m_Specific(A), m_APInt(C))) &&
+      Pred == ICmpInst::ICMP_EQ && *C == 31) {
+    auto *Cond = Builder.CreateICmpSLE(A,
+                                       ConstantInt::getNullValue(A->getType()));
+    return new ZExtInst(Cond, A->getType());
+  }
+
   if (Instruction *R = foldSelectOfBools(SI))
     return R;
 

llvm/test/Transforms/InstCombine/icmp-select.ll

@@ -7,9 +7,8 @@ declare i8 @llvm.umin.i8(i8, i8)
 
 define i32 @test_icmp_select_lte_0(i32 %0) {
 ; CHECK-LABEL: @test_icmp_select_lte_0(
-; CHECK-NEXT:    [[CML:%.*]] = icmp eq i32 [[TMP0:%.*]], 0
-; CHECK-NEXT:    [[LSHR:%.*]] = lshr i32 [[TMP0]], 31
-; CHECK-NEXT:    [[RE:%.*]] = select i1 [[CML]], i32 1, i32 [[LSHR]]
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp slt i32 [[TMP0:%.*]], 1
+; CHECK-NEXT:    [[RE:%.*]] = zext i1 [[TMP2]] to i32
 ; CHECK-NEXT:    ret i32 [[RE]]
 ;
   %cml = icmp eq i32 %0, 0