[llvm-commits] [PATCH] Teach IRBuilder about simplifying BinOp(Value, Constant)

[To"ro"k Edwin]

On 2008-12-29 14:34, Duncan Sands wrote:
> Hi Edwin,
>
>   
>> We now have a NoopFolder, ConstantFolder, and TargetFolder.
>>
>> Should the simplifications be part of ConstantFolder, or should there be
>> a 4th "folder" like this:
>> NoopFolder, ConstantFolder, SimplifyingConstantFolder (derived from
>> ConstantFolder), TargetFolder (derived from SimplifyingConstantFolder)?
>>     
>
> I think it should be part of ConstantFolder.  Of course, TargetFolder
> should do at least as much as ConstantFolder.
>   

Ok.

>   
>> Yes, that looks like a good place. Can I then just make ConstantFolder
>> use ConstantFoldInstruction?
>>     
>
> It's probably more efficient to directly call ConstantExpr::get if all
> operands are constants rather than building an instruction only to have
> ConstantFoldInstruction maybe throw it away in favour of a ConstantExpr.
>   

Agreed, I don't want to create an instruction just to throw it away later.
I thought to have a new variant of ConstantFoldInstOperands that takes a
Value** instead of a Constant**. Then ConstantFolder could call that
without actually creating the instruction, and if both are constants, 
it would return ConstantExpr::get. However there is still the issue that
the new ConstantFoldInstOperands won't be inlined, whereas previously
the ConstantExpr::get were inline.

So how about calling the ConstantExpr::get through the folder (as now),
and also add a call to Folder.simplify, something like:

  Value *CreateAnd(Value *LHS, Value *RHS, const char *Name = "") {
    Constant *LC  = dyn_cast<Constant>(LHS);
    Constant *RC = dyn_cast<Constant>(RHS);
    if (LC && RC)
        return Folder.CreateAnd(LC, RC);
    if (LC || RC)
        if (Value *S = Folder.SimplifyBinOp(Instruction::And, LHS, RHS,
LC, RC, InsertPt))
          return S;
    return Insert(BinaryOperator::CreateAnd(LHS, RHS), Name);
  }

ConstantFoldInstruction (and a variant of ConstantFoldInstOperands) will
also use this Folder.

Best regards,
--Edwin