RFC: Tailcall Improvement

Here is a patch to improve argument lowering for tail calls. Before this patch all outgoing arguments were move to the stack slot where they would go on a normal function call and from there moved back to the tail call stack slot to prevent overwriting of values.
After this patch only arguments that source from callers arguments (formal_arguments) are lowered this way.

I moved some code that would otherwise be duplicated to a new function 'GetMemCpyWithFlags'.

Okay to commit?

regards arnold

tailcall-improvement.patch (10.4 KB)

1. IsPossiblyOverriddenArgumentOfTailCall() should be marked static?

2. Please start all function level comments this way:
// IsPossiblyOverriddenArgumentOfTailCall - Check if ...

+ if (IsPossiblyOverriddenArgumentOfTailCall(Arg)){
+ // Copy from stack slots to stack slot of a tail called function. This
+ // needs to be done because if we would lower the arguments directly
+ // to their real stack slot we might end up overwriting each other.
+ // Get source stack slot.
+ Source = DAG.getConstant(VA.getLocMemOffset(),
+ getPointerTy());
Funky indentation?

+ if (StackPtr.Val == 0)
+ StackPtr = DAG.getRegister(getStackPtrReg(), getPointerTy());

This should be a CopyFromReg which reads the stack pointer register.


+ SDOperand Source;
+ if (IsPossiblyOverriddenArgumentOfTailCall(Arg)){
+ } else {
+ Source=Arg;
+ }

I prefer this:

           SDOperand Source = Arg;
           if (IsPossiblyOverriddenArgumentOfTailCall(Arg)) {
      if (!MemOpChains2.empty())
        Chain = DAG.getNode(ISD::TokenFactor, MVT::Other,
- &MemOpChains2[0], MemOpChains.size());
+ &MemOpChains2[0], MemOpChains2.size());

This looks like a bug fix? Please check this part in first.