[InstCombine] Fold a shifty implementation of clamp-to-zero.

Summary:
Fold
and(ashr(subNSW(Y, X), ScalarSizeInBits(Y)-1), X)
into
X s> Y ? X : 0

https://rise4fun.com/Alive/lFH

Fold shift into select enables more optimization,
e.g., vmax generation for ARM target.

Reviewers: lebedev.ri, efriedma, spatel, kparzysz, bcahoon

Reviewed By: lebedev.ri

Subscribers: xbolva00, andreadb, craig.topper, RKSimon, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67799

llvm-svn: 372676

1 files changed, 14 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
index b0cf81209db..490faddaefb 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
@@ -1926,6 +1926,20 @@ Instruction *InstCombiner::visitAnd(BinaryOperator &I) {
       A->getType()->isIntOrIntVectorTy(1))
     return SelectInst::Create(A, Op0, Constant::getNullValue(I.getType()));
 
+  // and(ashr(subNSW(Y, X), ScalarSizeInBits(Y)-1), X) --> X s> Y ? X : 0.
+  {
+    Value *X, *Y;
+    const APInt *ShAmt;
+    Type *Ty = I.getType();
+    if (match(&I, m_c_And(m_OneUse(m_AShr(m_NSWSub(m_Value(Y), m_Value(X)),
+                                          m_APInt(ShAmt))),
+                          m_Deferred(X))) &&
+        *ShAmt == Ty->getScalarSizeInBits() - 1) {
+      Value *NewICmpInst = Builder.CreateICmpSGT(X, Y);
+      return SelectInst::Create(NewICmpInst, X, ConstantInt::getNullValue(Ty));
+    }
+  }
+
   return nullptr;
 }