[InstCombine] Inefficient pattern for high-bits checking (PR38708)

Summary:
It is sometimes important to check that some newly-computed value
is non-negative and only `n` bits wide (where `n` is a variable.)
There are **many** ways to check that:
https://godbolt.org/z/o4RB8D
The last variant seems best?
(I'm sure there are some other variations i haven't thought of..)

Let's handle the second variant first, since it is much simpler.
https://rise4fun.com/Alive/LYjY

https://bugs.llvm.org/show_bug.cgi?id=38708

Reviewers: spatel, craig.topper, RKSimon

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D51985

llvm-svn: 342067

1 files changed, 38 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 875d6d9f053..d0673d952d0 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -4623,6 +4623,41 @@ static Instruction *canonicalizeICmpBool(ICmpInst &I,
   }
 }
 
+// Transform pattern like:
+//   (1 << Y) u<= X
+//   (1 << Y) u>  X
+// Into:
+//   (X l>> Y) != 0
+//   (X l>> Y) == 0
+static Instruction *foldICmpWithHighBitMask(ICmpInst &Cmp,
+                                            InstCombiner::BuilderTy &Builder) {
+  ICmpInst::Predicate Pred;
+  Value *X, *Y;
+  if (!match(&Cmp,
+             m_c_ICmp(Pred, m_OneUse(m_Shl(m_One(), m_Value(Y))), m_Value(X))))
+    return nullptr;
+
+  // We want X to be the icmp's second operand, so swap predicate if it is not.
+  if (Cmp.getOperand(0) == X)
+    Pred = Cmp.getSwappedPredicate();
+
+  ICmpInst::Predicate NewPred;
+  switch (Pred) {
+  case ICmpInst::ICMP_ULE:
+    NewPred = ICmpInst::ICMP_NE;
+    break;
+  case ICmpInst::ICMP_UGT:
+    NewPred = ICmpInst::ICMP_EQ;
+    break;
+  default:
+    return nullptr;
+  }
+
+  Value *NewX = Builder.CreateLShr(X, Y, X->getName() + ".highbits");
+  Constant *Zero = Constant::getNullValue(NewX->getType());
+  return CmpInst::Create(Instruction::ICmp, NewPred, NewX, Zero);
+}
+
 static Instruction *foldVectorCmp(CmpInst &Cmp,
                                   InstCombiner::BuilderTy &Builder) {
   // If both arguments of the cmp are shuffles that use the same mask and
@@ -4913,6 +4948,9 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
       return foldICmpAddOpConst(X, *C, I.getSwappedPredicate());
   }
 
+  if (Instruction *Res = foldICmpWithHighBitMask(I, Builder))
+    return Res;
+
   if (I.getType()->isVectorTy())
     if (Instruction *Res = foldVectorCmp(I, Builder))
       return Res;