llvm dev - Nov 2011 - [LLVMdev] Why there is no unary operator in LLVM?

Hi llvmdev,

I've noticed that there is no unary operator in LLVM. For unary operator
such as Neg or Or operator, the IR builder just creates a binary operation
with one dummy operand,

01823
<http://llvm.org/doxygen/classllvm_1_1BinaryOperator.html#a073c092ce74a122e898e435e60e84599>
BinaryOperator <http://llvm.org/doxygen/classllvm_1_1BinaryOperator.html>
*BinaryOperator::CreateNeg
<http://llvm.org/doxygen/classllvm_1_1BinaryOperator.html#a073c092ce74a122e898e435e60e84599>(Value
<http://llvm.org/doxygen/classllvm_1_1Value.html> *Op, const Twine
<http://llvm.org/doxygen/classllvm_1_1Twine.html> &Name,01824
                                 BasicBlock
<http://llvm.org/doxygen/classllvm_1_1BasicBlock.html> *InsertAtEnd)
{01825   Value <http://llvm.org/doxygen/classllvm_1_1Value.html> *zero
= ConstantFP::getZeroValueForNegation
<http://llvm.org/doxygen/classllvm_1_1ConstantFP.html#ab4d218c572245abd0b1c895d603cba36>(Op->getType
<http://llvm.org/doxygen/classllvm_1_1Value.html#a0cf3748dba54f931bb1241ae4adc76bc>());01826
  return new BinaryOperator
<http://llvm.org/doxygen/classllvm_1_1BinaryOperator.html#acf242c32d995c5276931f545aa2bdd9c>(Instruction::Sub,01827
                            zero, Op,01828
Op->getType
<http://llvm.org/doxygen/classllvm_1_1Value.html#a0cf3748dba54f931bb1241ae4adc76bc>(),
Name
<http://llvm.org/doxygen/classllvm_1_1BinaryOperator.html#a274586caf47dcc28ed878ad21af28152>,
InsertAtEnd);



However, I think it's more natural to describe this as unary operation, and
may bring benefits to the optimiser and code generator.

Can anyone tell me the reason?

Thanks!
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On Thu, Nov 3, 2011 at 3:45 AM, Fan Dawei <fandawei.s at gmail.com>
wrote:
> Hi llvmdev,
>
> I've noticed that there is no unary operator in LLVM. For unary
operator
> such as Neg or Or operator, the IR builder just creates a binary operation
> with one dummy operand,
>
> 01823 BinaryOperator *BinaryOperator::CreateNeg(Value *Op, const Twine
> &Name,
> 01824                                           BasicBlock *InsertAtEnd) {
> 01825   Value *zero =
ConstantFP::getZeroValueForNegation(Op->getType());
> 01826   return new BinaryOperator(Instruction::Sub,
> 01827                             zero, Op,
> 01828                             Op->getType(), Name, InsertAtEnd);
>
>
> However, I think it's more natural to describe this as unary operation,
and
> may bring benefits to the optimiser and code generator.
>
> Can anyone tell me the reason?
We describe it as a binary operation simply to reduce the number of
instructions in the IR.  There are helper functions for things which
are really looking for unary not etc.

-Eli