[llvm] [InstCombine] fold "icmp eq (X + (V - 1)) & -V, X" to "icmp eq (and X, V - 1), 0"

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -1320,6 +1320,67 @@ Instruction *InstCombinerImpl::foldICmpWithZero(ICmpInst &Cmp) {
   return nullptr;
 }
 
+// Fold icmp eq (num + (val - 1)) & -val, num
+//      to
+//      icmp eq 0, (and num, val - 1)
+// For value being power of two
+Instruction *InstCombinerImpl::foldNextMultiply(ICmpInst &Cmp) {
+  Value *Op0 = Cmp.getOperand(0), *Op1 = Cmp.getOperand(1);
+  Value *Neg, *Add, *Num, *Mask, *Value;
+  CmpInst::Predicate Pred = Cmp.getPredicate();
+  const APInt *NegConst, *MaskConst, *NumCost;
+
+  if (Pred != ICmpInst::ICMP_EQ)
+    return nullptr;
+
+  // Match num + neg
+  if (!match(Op0, m_And(m_Value(Add), m_Value(Neg))))
+    return nullptr;
+
+  // Match num & mask
+  if (!match(Add, m_Add(m_Value(Num), m_Value(Mask))))
+    return nullptr;
+
+  // Check the constant case
+  if (match(Neg, m_APInt(NegConst)) && match(Mask, m_APInt(MaskConst))) {
+    // Mask + 1 should be a power-of-two
+    if (!(*MaskConst + 1).isPowerOf2())
+      return nullptr;
+
+    // Neg = -(Mask + 1)
+    if (*NegConst != -(*MaskConst + 1))
+      return nullptr;
+  } else {
+    // Match neg = sub 0, val
+    if (!match(Neg, m_Sub(m_Zero(), m_Value(Value))))
+      return nullptr;
+
+    // mask = %val - 1, which can be represented as sub %val, 1 or add %val, -1
+    if (!match(Mask, m_Add(m_Value(Value), m_AllOnes())) &&
+        !match(Mask, m_Sub(m_Value(Value), m_One())))
+      return nullptr;
+
+    // Value should be a known power-of-two.
+    if (!isKnownToBeAPowerOfTwo(Value, false, &Cmp))
+      return nullptr;
+  }
+
+  // Guard against weird special-case where Op1 gets optimized to constant.
+  // Leave it constant fonder.
+  if (match(Op1, m_APInt(NumCost)))
+    return nullptr;
+
+  if (!match(Op1, m_Value(Num)))
+    return nullptr;
+
+  // Create new icmp eq (num & (val - 1)), 0
+  auto NewAnd = Builder.CreateAnd(Num, Mask);
+  auto Zero = llvm::Constant::getNullValue(Num->getType());
+  auto ICmp = Builder.CreateICmp(CmpInst::ICMP_EQ, NewAnd, Zero);
+
+  return replaceInstUsesWith(Cmp, ICmp);
+}
+
 /// Fold icmp Pred X, C.
 /// TODO: This code structure does not make sense. The saturating add fold
 /// should be moved to some other helper and extended as noted below (it is also
@@ -7644,6 +7705,9 @@ Instruction *InstCombinerImpl::visitICmpInst(ICmpInst &I) {
   if (Instruction *Res = foldICmpUsingKnownBits(I))
     return Res;
 
+  if (Instruction *Res = foldNextMultiply(I))
+    return Res;
+
   // Test if the ICmpInst instruction is used exclusively by a select as
   // part of a minimum or maximum operation. If so, refrain from doing
   // any other folding. This helps out other analyses which understand

llvm/lib/Transforms/InstCombine/InstCombineInternal.h

@@ -721,6 +721,7 @@ class LLVM_LIBRARY_VISIBILITY InstCombinerImpl final
   Instruction *foldICmpUsingKnownBits(ICmpInst &Cmp);
   Instruction *foldICmpWithDominatingICmp(ICmpInst &Cmp);
   Instruction *foldICmpWithConstant(ICmpInst &Cmp);
+  Instruction *foldNextMultiply(ICmpInst &Cmp);
   Instruction *foldICmpUsingBoolRange(ICmpInst &I);
   Instruction *foldICmpInstWithConstant(ICmpInst &Cmp);
   Instruction *foldICmpInstWithConstantNotInt(ICmpInst &Cmp);

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

@@ -3300,3 +3300,93 @@ entry:
   %cmp = icmp ult i32 %add, 253
   ret i1 %cmp
 }
+
+define i1 @val_is_aligend_sub(i32 %num, i32 %val) {
+; CHECK-LABEL: @val_is_aligend_sub(
+; CHECK-NEXT:    [[TMP1:%.*]] = tail call range(i32 1, 33) i32 @llvm.ctpop.i32(i32 [[NUM:%.*]])
+; CHECK-NEXT:    [[POW:%.*]] = icmp eq i32 [[TMP1]], 1
+; CHECK-NEXT:    call void @llvm.assume(i1 [[POW]])
+; CHECK-NEXT:    [[NEG:%.*]] = add i32 [[NUM]], -1
+; CHECK-NEXT:    [[_2_SROA_0_0:%.*]] = and i32 [[NUM_BIASED:%.*]], [[NEG]]
+; CHECK-NEXT:    [[_0:%.*]] = icmp eq i32 [[_2_SROA_0_0]], 0
+; CHECK-NEXT:    ret i1 [[_0]]
+;
+  %1 = tail call range(i32 1, 33) i32 @llvm.ctpop.i32(i32 %val)
+  %pow = icmp eq i32 %1, 1
+  call void @llvm.assume(i1 %pow)
+
+  %mask = sub i32 %val, 1
+  %neg = sub nsw i32 0, %val
+
+  %num.biased = add i32 %num, %mask
+  %_2.sroa.0.0 = and i32 %num.biased, %neg
+  %_0 = icmp eq i32 %_2.sroa.0.0, %num
+  ret i1 %_0
+}
+
+define i1 @val_is_aligend_add(i32 %num, i32 %val) {
+; CHECK-LABEL: @val_is_aligend_add(
+; CHECK-NEXT:    [[TMP1:%.*]] = tail call range(i32 1, 33) i32 @llvm.ctpop.i32(i32 [[NUM:%.*]])
+; CHECK-NEXT:    [[POW:%.*]] = icmp eq i32 [[TMP1]], 1
+; CHECK-NEXT:    call void @llvm.assume(i1 [[POW]])
+; CHECK-NEXT:    [[NEG:%.*]] = add i32 [[NUM]], -1
+; CHECK-NEXT:    [[_2_SROA_0_0:%.*]] = and i32 [[NUM_BIASED:%.*]], [[NEG]]
+; CHECK-NEXT:    [[_0:%.*]] = icmp eq i32 [[_2_SROA_0_0]], 0
+; CHECK-NEXT:    ret i1 [[_0]]
+;
+  %1 = tail call range(i32 1, 33) i32 @llvm.ctpop.i32(i32 %val)
+  %pow = icmp eq i32 %1, 1
+  call void @llvm.assume(i1 %pow)
+
+  %mask = add i32 %val, -1
+  %neg = sub nsw i32 0, %val
+
+  %num.biased = add i32 %num, %mask
+  %_2.sroa.0.0 = and i32 %num.biased, %neg
+  %_0 = icmp eq i32 %_2.sroa.0.0, %num
+  ret i1 %_0
+}
+
+define i1 @val_is_aligend_const_pow2(i32 %num) {
+; CHECK-LABEL: @val_is_aligend_const_pow2(
+; CHECK-NEXT:    [[TMP1:%.*]] = and i32 [[NUM:%.*]], 4095
+; CHECK-NEXT:    [[_0:%.*]] = icmp eq i32 [[TMP1]], 0
+; CHECK-NEXT:    ret i1 [[_0]]
+;
+  %num.biased = add i32 %num, 4095
+  %_2.sroa.0.0 = and i32 %num.biased, -4096
+  %_0 = icmp eq i32 %_2.sroa.0.0, %num
+  ret i1 %_0
+}
+
+; Should not work for non-power-of-two cases
+define i1 @val_is_aligend_const_non-pow2(i32 %num) {
+; CHECK-LABEL: @val_is_aligend_const_non-pow2(
+; CHECK-NEXT:    [[NUM_BIASED:%.*]] = add i32 [[NUM:%.*]], 6
+; CHECK-NEXT:    [[_2_SROA_0_0:%.*]] = and i32 [[NUM_BIASED]], -7
+; CHECK-NEXT:    [[_0:%.*]] = icmp eq i32 [[_2_SROA_0_0]], [[NUM]]
+; CHECK-NEXT:    ret i1 [[_0]]
+;
+  %num.biased = add i32 %num, 6
+  %_2.sroa.0.0 = and i32 %num.biased, -7
+  %_0 = icmp eq i32 %_2.sroa.0.0, %num
+  ret i1 %_0
+}
+
+define i1 @val_is_aligend_non_pow(i32 %num, i32 %val) {
+; CHECK-LABEL: @val_is_aligend_non_pow(
+; CHECK-NEXT:    [[MASK:%.*]] = add i32 [[VAL:%.*]], -1
+; CHECK-NEXT:    [[NEG:%.*]] = sub nsw i32 0, [[VAL]]
+; CHECK-NEXT:    [[NUM_BIASED:%.*]] = add i32 [[NUM:%.*]], [[MASK]]
+; CHECK-NEXT:    [[_2_SROA_0_0:%.*]] = and i32 [[NUM_BIASED]], [[NEG]]
+; CHECK-NEXT:    [[_0:%.*]] = icmp eq i32 [[_2_SROA_0_0]], [[NUM]]
+; CHECK-NEXT:    ret i1 [[_0]]
+;
+  %mask = add i32 %val, -1
+  %neg = sub nsw i32 0, %val
+
+  %num.biased = add i32 %num, %mask
+  %_2.sroa.0.0 = and i32 %num.biased, %neg
+  %_0 = icmp eq i32 %_2.sroa.0.0, %num
+  ret i1 %_0
+}