ShiftChecker: Avoid unneeded checks

Do not instrument shift operations with constant shift operations that
are smaller than the type size.

1 files changed, 10 insertions, 0 deletions

diff --git a/lib/Module/Checks.cpp b/lib/Module/Checks.cpp
index bd9a8bd3..53ba4266 100644
--- a/lib/Module/Checks.cpp
+++ b/lib/Module/Checks.cpp
@@ -104,6 +104,16 @@ bool OvershiftCheckPass::runOnModule(Module &M) {
         if (opcode != Instruction::Shl && opcode != Instruction::LShr &&
             opcode != Instruction::AShr)
           continue;
+
+        // Check if the operand is constant and not zero, skip in that case
+        auto operand = binOp->getOperand(1);
+        if (auto coOp = dyn_cast<llvm::ConstantInt>(operand)) {
+          auto typeWidth =
+              binOp->getOperand(0)->getType()->getScalarSizeInBits();
+          // If the constant shift is positive and smaller,equal the type width,
+          // we can ignore this instruction
+          if (!coOp->isNegative() && coOp->getZExtValue() < typeWidth)
+            continue;
         }
 
         shiftInstructions.push_back(binOp);