[InstCombine] Fold "x ?% y ==/!= 0" to "x & (y-1) ==/!= 0" iff y is power-of-two

Summary:
I have stumbled into this by accident while preparing to extend backend `x s% C ==/!= 0` handling.

While we did happen to handle this fold in most of the cases,
the folding is indirect - we fold `x u% y` to `x & (y-1)` (iff `y` is power-of-two),
or first turn `x s% -y` to `x u% y`; that does handle most of the cases.
But we can't turn `x s% INT_MIN` to `x u% -INT_MIN`,
and thus we end up being stuck with `(x s% INT_MIN) == 0`.

There is no such restriction for the more general fold:
https://rise4fun.com/Alive/IIeS

To be noted, the fold does not enforce that `y` is a constant,
so it may indeed increase instruction count.
This is consistent with what `x u% y`->`x & (y-1)` already does.
I think it makes sense, it's at most one (simple) extra instruction,
while `rem`ainder is really much more un-simple (and likely **very** costly).

Reviewers: spatel, RKSimon, nikic, xbolva00, craig.topper

Reviewed By: RKSimon

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65046

llvm-svn: 367322

5 files changed, 54 insertions, 13 deletions

diff --git a/llvm/include/llvm/IR/PatternMatch.h b/llvm/include/llvm/IR/PatternMatch.h
index 9d51bcd5d71..2a9c3dd8c20 100644
--- a/llvm/include/llvm/IR/PatternMatch.h
+++ b/llvm/include/llvm/IR/PatternMatch.h
@@ -1003,6 +1003,12 @@ struct is_idiv_op {
   }
 };
 
+struct is_irem_op {
+  bool isOpType(unsigned Opcode) {
+    return Opcode == Instruction::SRem || Opcode == Instruction::URem;
+  }
+};
+
 /// Matches shift operations.
 template <typename LHS, typename RHS>
 inline BinOpPred_match<LHS, RHS, is_shift_op> m_Shift(const LHS &L,
@@ -1038,6 +1044,13 @@ inline BinOpPred_match<LHS, RHS, is_idiv_op> m_IDiv(const LHS &L,
   return BinOpPred_match<LHS, RHS, is_idiv_op>(L, R);
 }
 
+/// Matches integer remainder operations.
+template <typename LHS, typename RHS>
+inline BinOpPred_match<LHS, RHS, is_irem_op> m_IRem(const LHS &L,
+                                                    const RHS &R) {
+  return BinOpPred_match<LHS, RHS, is_irem_op>(L, R);
+}
+
 //===----------------------------------------------------------------------===//
 // Class that matches exact binary ops.
 //
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 3a4283ae540..ddbc17075ca 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1317,6 +1317,27 @@ static Instruction *processUGT_ADDCST_ADD(ICmpInst &I, Value *A, Value *B,
   return ExtractValueInst::Create(Call, 1, "sadd.overflow");
 }
 
+/// If we have:
+///   icmp eq/ne (urem/srem %x, %y), 0
+/// iff %y is a power-of-two, we can replace this with a bit test:
+///   icmp eq/ne (and %x, (add %y, -1)), 0
+Instruction *InstCombiner::foldIRemByPowerOfTwoToBitTest(ICmpInst &I) {
+  // This fold is only valid for equality predicates.
+  if (!I.isEquality())
+    return nullptr;
+  ICmpInst::Predicate Pred;
+  Value *X, *Y, *Zero;
+  if (!match(&I, m_ICmp(Pred, m_OneUse(m_IRem(m_Value(X), m_Value(Y))),
+                        m_CombineAnd(m_Zero(), m_Value(Zero)))))
+    return nullptr;
+  if (!isKnownToBeAPowerOfTwo(Y, /*OrZero*/ true, 0, &I))
+    return nullptr;
+  // This may increase instruction count, we don't enforce that Y is a constant.
+  Value *Mask = Builder.CreateAdd(Y, Constant::getAllOnesValue(Y->getType()));
+  Value *Masked = Builder.CreateAnd(X, Mask);
+  return ICmpInst::Create(Instruction::ICmp, Pred, Masked, Zero);
+}
+
 // Handle  icmp pred X, 0
 Instruction *InstCombiner::foldICmpWithZero(ICmpInst &Cmp) {
   CmpInst::Predicate Pred = Cmp.getPredicate();
@@ -1335,6 +1356,9 @@ Instruction *InstCombiner::foldICmpWithZero(ICmpInst &Cmp) {
     }
   }
 
+  if (Instruction *New = foldIRemByPowerOfTwoToBitTest(Cmp))
+    return New;
+
   // Given:
   //   icmp eq/ne (urem %x, %y), 0
   // Iff %x has 0 or 1 bits set, and %y has at least 2 bits set, omit 'urem':
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
index 327e7ab4fe4..fc0a1e90d38 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
+++ b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
@@ -839,6 +839,7 @@ private:
   Instruction *foldICmpInstWithConstantNotInt(ICmpInst &Cmp);
   Instruction *foldICmpBinOp(ICmpInst &Cmp);
   Instruction *foldICmpEquality(ICmpInst &Cmp);
+  Instruction *foldIRemByPowerOfTwoToBitTest(ICmpInst &I);
   Instruction *foldICmpWithZero(ICmpInst &Cmp);
 
   Instruction *foldICmpSelectConstant(ICmpInst &Cmp, SelectInst *Select,
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index 30861bfe6c5..7939bbf5ca6 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -1319,6 +1319,8 @@ Instruction *InstCombiner::visitURem(BinaryOperator &I) {
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
   Type *Ty = I.getType();
   if (isKnownToBeAPowerOfTwo(Op1, /*OrZero*/ true, 0, &I)) {
+    // This may increase instruction count, we don't enforce that Y is a
+    // constant.
     Constant *N1 = Constant::getAllOnesValue(Ty);
     Value *Add = Builder.CreateAdd(Op1, N1);
     return BinaryOperator::CreateAnd(Op0, Add);
diff --git a/llvm/test/Transforms/InstCombine/rem.ll b/llvm/test/Transforms/InstCombine/rem.ll
index ea95072e8c1..c833acc1685 100644
--- a/llvm/test/Transforms/InstCombine/rem.ll
+++ b/llvm/test/Transforms/InstCombine/rem.ll
@@ -201,8 +201,8 @@ define <2 x i19> @weird_vec_power_of_2_constant_splat_divisor(<2 x i19> %A) {
 
 define i1 @test3a(i32 %A) {
 ; CHECK-LABEL: @test3a(
-; CHECK-NEXT:    [[B1:%.*]] = and i32 [[A:%.*]], 7
-; CHECK-NEXT:    [[C:%.*]] = icmp ne i32 [[B1]], 0
+; CHECK-NEXT:    [[TMP1:%.*]] = and i32 [[A:%.*]], 7
+; CHECK-NEXT:    [[C:%.*]] = icmp ne i32 [[TMP1]], 0
 ; CHECK-NEXT:    ret i1 [[C]]
 ;
   %B = srem i32 %A, -8
@@ -212,8 +212,8 @@ define i1 @test3a(i32 %A) {
 
 define <2 x i1> @test3a_vec(<2 x i32> %A) {
 ; CHECK-LABEL: @test3a_vec(
-; CHECK-NEXT:    [[B1:%.*]] = and <2 x i32> [[A:%.*]], <i32 7, i32 7>
-; CHECK-NEXT:    [[C:%.*]] = icmp ne <2 x i32> [[B1]], zeroinitializer
+; CHECK-NEXT:    [[TMP1:%.*]] = and <2 x i32> [[A:%.*]], <i32 7, i32 7>
+; CHECK-NEXT:    [[C:%.*]] = icmp ne <2 x i32> [[TMP1]], zeroinitializer
 ; CHECK-NEXT:    ret <2 x i1> [[C]]
 ;
   %B = srem <2 x i32> %A, <i32 -8, i32 -8>
@@ -681,8 +681,8 @@ define <2 x i1> @test24_vec(<2 x i32> %A) {
 
 define i1 @test25(i32 %A) {
 ; CHECK-LABEL: @test25(
-; CHECK-NEXT:    [[B:%.*]] = srem i32 [[A:%.*]], -2147483648
-; CHECK-NEXT:    [[C:%.*]] = icmp ne i32 [[B]], 0
+; CHECK-NEXT:    [[TMP1:%.*]] = and i32 [[A:%.*]], 2147483647
+; CHECK-NEXT:    [[C:%.*]] = icmp ne i32 [[TMP1]], 0
 ; CHECK-NEXT:    ret i1 [[C]]
 ;
   %B = srem i32 %A, 2147483648 ; signbit
@@ -692,8 +692,8 @@ define i1 @test25(i32 %A) {
 
 define <2 x i1> @test25_vec(<2 x i32> %A) {
 ; CHECK-LABEL: @test25_vec(
-; CHECK-NEXT:    [[B:%.*]] = srem <2 x i32> [[A:%.*]], <i32 -2147483648, i32 -2147483648>
-; CHECK-NEXT:    [[C:%.*]] = icmp ne <2 x i32> [[B]], zeroinitializer
+; CHECK-NEXT:    [[TMP1:%.*]] = and <2 x i32> [[A:%.*]], <i32 2147483647, i32 2147483647>
+; CHECK-NEXT:    [[C:%.*]] = icmp ne <2 x i32> [[TMP1]], zeroinitializer
 ; CHECK-NEXT:    ret <2 x i1> [[C]]
 ;
   %B = srem <2 x i32> %A, <i32 2147483648, i32 2147483648>
@@ -703,9 +703,10 @@ define <2 x i1> @test25_vec(<2 x i32> %A) {
 
 define i1 @test26(i32 %A, i32 %B) {
 ; CHECK-LABEL: @test26(
-; CHECK-NEXT:    [[C:%.*]] = shl nuw i32 1, [[B:%.*]]
-; CHECK-NEXT:    [[D:%.*]] = srem i32 [[A:%.*]], [[C]]
-; CHECK-NEXT:    [[E:%.*]] = icmp ne i32 [[D]], 0
+; CHECK-NEXT:    [[NOTMASK:%.*]] = shl nsw i32 -1, [[B:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = xor i32 [[NOTMASK]], -1
+; CHECK-NEXT:    [[TMP2:%.*]] = and i32 [[TMP1]], [[A:%.*]]
+; CHECK-NEXT:    [[E:%.*]] = icmp ne i32 [[TMP2]], 0
 ; CHECK-NEXT:    ret i1 [[E]]
 ;
   %C = shl i32 1, %B ; not a constant
@@ -729,8 +730,8 @@ define i1 @test27(i32 %A, i32* %remdst) {
 
 define i1 @test28(i32 %A) {
 ; CHECK-LABEL: @test28(
-; CHECK-NEXT:    [[B:%.*]] = srem i32 [[A:%.*]], -2147483648
-; CHECK-NEXT:    [[C:%.*]] = icmp eq i32 [[B]], 0
+; CHECK-NEXT:    [[TMP1:%.*]] = and i32 [[A:%.*]], 2147483647
+; CHECK-NEXT:    [[C:%.*]] = icmp eq i32 [[TMP1]], 0
 ; CHECK-NEXT:    ret i1 [[C]]
 ;
   %B = srem i32 %A, 2147483648 ; signbit