[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

3 files changed, 27 insertions, 0 deletions

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);