[llvm-commits] sret-demotion for large struct returns working on x86!

[Dan Gohman]

On Nov 8, 2009, at 6:37 PM, Kenneth Uildriks wrote:

> If a struct can't be returned in registers, have SelectionDAGBuild
> insert a hidden sret parameter and have return instructions store the
> return value through the hidden pointer.  Formal arguments, return
> instructions, and calls are all updated as needed.
> 
> Only x86 has the code to actually check whether sret-demotion is
> needed.... other platforms invariably report that they can return
> values in registers, even when they can't.

Hello,

Overall this patch looks good. Thanks for working on this! I
have a few comments.

> +  // True if the function needs to be sret-demoted
> +  bool CantLowerReturn;
> 
> +  // Virtual register to hold the sret pointer for sret-demotion
> +  unsigned DemoteRegister;

Would it be possible to put this field in FunctionLoweringInfo
instead of MachineFunction? It's information that's only really
relevant during lowering to the SelectionDAG.

> -    if (F->paramHasAttr(0, Attribute::SExt))
> -      ExtendKind = ISD::SIGN_EXTEND;
> -    else if (F->paramHasAttr(0, Attribute::ZExt))
> -      ExtendKind = ISD::ZERO_EXTEND;

I'm unclear on why this code is being removed. Can you explain
what's happening here?

> +  if (!CanLowerReturn)
> +  {

In general, LLVM style has the opening brace on the same line as the
if, else, for, etc.

Dan