InstSimplify: Move a transform from InstCombine to InstSimplify

Several combines involving icmp (shl C2, %X) C1 can be simplified
without introducing any new instructions.  Move them to InstSimplify;
while we are at it, make them more powerful.

llvm-svn: 216642

1 files changed, 35 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 9f3b2876afe..2642ffffd77 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -2388,6 +2388,41 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
       return getTrue(ITy);
   }
 
+  // handle:
+  //   CI2 << X == CI
+  //   CI2 << X != CI
+  //
+  //   where CI2 is a power of 2 and CI isn't
+  if (auto *CI = dyn_cast<ConstantInt>(RHS)) {
+    const APInt *CI2Val, *CIVal = &CI->getValue();
+    if (LBO && match(LBO, m_Shl(m_APInt(CI2Val), m_Value())) &&
+        CI2Val->isPowerOf2()) {
+      if (!CIVal->isPowerOf2()) {
+        // CI2 << X can equal zero in some circumstances,
+        // this simplification is unsafe if CI is zero.
+        //
+        // We know it is safe if:
+        // - The shift is nsw, we can't shift out the one bit.
+        // - The shift is nuw, we can't shift out the one bit.
+        // - CI2 is one
+        // - CI isn't zero
+        if (LBO->hasNoSignedWrap() || LBO->hasNoUnsignedWrap() ||
+            *CI2Val == 1 || !CI->isZero()) {
+          if (Pred == ICmpInst::ICMP_EQ)
+            return ConstantInt::getFalse(RHS->getContext());
+          if (Pred == ICmpInst::ICMP_NE)
+            return ConstantInt::getTrue(RHS->getContext());
+        }
+      }
+      if (CIVal->isSignBit() && *CI2Val == 1) {
+        if (Pred == ICmpInst::ICMP_UGT)
+          return ConstantInt::getFalse(RHS->getContext());
+        if (Pred == ICmpInst::ICMP_ULE)
+          return ConstantInt::getTrue(RHS->getContext());
+      }
+    }
+  }
+
   if (MaxRecurse && LBO && RBO && LBO->getOpcode() == RBO->getOpcode() &&
       LBO->getOperand(1) == RBO->getOperand(1)) {
     switch (LBO->getOpcode()) {