[InstCombine] Fold (X / C) cmp X and (X >> C) cmp X into X cmp 0

Author: Poseydon42

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -7103,6 +7103,34 @@ Instruction *InstCombinerImpl::foldICmpCommutative(ICmpInst::Predicate Pred,
   if (Value *V = foldICmpWithLowBitMaskedVal(Pred, Op0, Op1, Q, *this))
     return replaceInstUsesWith(CxtI, V);
 
+  // Folding (X / Y) < X => X > 0 for some constant Y other than 0 or 1
+  {
+    const APInt *Divisor;
+    Value *Dividend;
+    if (match(Op0, m_UDiv(m_Value(Dividend), m_APInt(Divisor))) &&
+        Op1 == Dividend && Divisor->ugt(1)) {
+      return new ICmpInst(ICmpInst::getSwappedPredicate(Pred), Dividend,
+                          Constant::getNullValue(Dividend->getType()));
+    }
+
+    if (match(Op0, m_SDiv(m_Value(Dividend), m_APInt(Divisor))) &&
+        Op1 == Dividend && Divisor->ugt(1) && !ICmpInst::isUnsigned(Pred)) {
+      return new ICmpInst(ICmpInst::getSwappedPredicate(Pred), Dividend,
+                          Constant::getNullValue(Dividend->getType()));
+    }
+  }
+
+  // Another case of this fold is (X >> Y) < X => X > 0 if Y != 0
+  {
+    const APInt *Shift;
+    Value *V;
+    if (match(Op0, m_LShr(m_Value(V), m_APInt(Shift))) && Op1 == V &&
+        Shift->ugt(0) && !ICmpInst::isUnsigned(Pred)) {
+      return new ICmpInst(ICmpInst::getInversePredicate(Pred), V,
+                          Constant::getNullValue(V->getType()));
+    }
+  }
+
   return nullptr;
 }
 

llvm/test/Transforms/InstCombine/icmp-div-constant.ll

@@ -381,8 +381,7 @@ define i1 @sdiv_eq_smin_use(i32 %x, i32 %y) {
 
 define i1 @sdiv_x_by_const_cmp_x(i32 %x) {
 ; CHECK-LABEL: @sdiv_x_by_const_cmp_x(
-; CHECK-NEXT:    [[V:%.*]] = udiv i32 [[X:%.*]], 13
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i32 [[V]], [[X]]
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i32 [[X:%.*]], 0
 ; CHECK-NEXT:    ret i1 [[TMP1]]
 ;
   %v = udiv i32 %x, 13
@@ -392,8 +391,7 @@ define i1 @sdiv_x_by_const_cmp_x(i32 %x) {
 
 define i1 @udiv_x_by_const_cmp_x(i32 %x) {
 ; CHECK-LABEL: @udiv_x_by_const_cmp_x(
-; CHECK-NEXT:    [[TMP2:%.*]] = udiv i32 [[X:%.*]], 123
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp slt i32 [[TMP2]], [[X]]
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp sgt i32 [[X:%.*]], 0
 ; CHECK-NEXT:    ret i1 [[TMP1]]
 ;
   %1 = udiv i32 %x, 123
@@ -405,8 +403,7 @@ define i1 @udiv_x_by_const_cmp_x(i32 %x) {
 
 define i1 @lshr_x_by_const_cmp_x(i32 %x) {
 ; CHECK-LABEL: @lshr_x_by_const_cmp_x(
-; CHECK-NEXT:    [[V:%.*]] = lshr i32 [[X:%.*]], 1
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i32 [[V]], [[X]]
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ne i32 [[X:%.*]], 0
 ; CHECK-NEXT:    ret i1 [[TMP1]]
 ;
   %v = lshr i32 %x, 1