[llvm-commits] [PATCH] Add support for functions with VLAs and dynamic stack realignment on x86

[Jakob Stoklund Olesen]

On Jun 1, 2012, at 10:13 AM, Chad Rosier <mcrosier at apple.com> wrote:

>> Index: lib/Target/X86/X86RegisterInfo.cpp
>> 
>> +    if (CC == CallingConv::GHC) {
>> +      report_fatal_error(
>> 
>> Could you check for the actual feature here, instead of testing specifically for GHC? Check out TRI::getCallPreservedMask() and the static MO::clobbersPhysReg().
> 
> I'm not sure I follow.  Could I call TRI::getCalleeSavedRegs() and then check to see if this is a non-empty set?  Then I could select one of these registers or would it be better to define the register statically (currently, X86::RBX)?

The base pointer must be a callee-saved register in the current calling convention, that is why GHC won't work. You can check that like this:

  if (MachineOperand::clobbersPhysReg(getCallPreservedMask(CC), getBaseRegister()))
    report_fatal_error;

I guess it would be even better to pick a different base pointer register, but I don't think that is necessary in the first patch.

>> +    Reserved.set(X86::RBX);
>> +    Reserved.set(X86::EBX);
>> +    Reserved.set(X86::BX);
>> +    Reserved.set(X86::BL);
>> +    Reserved.set(X86::BH);
>> 
>> What about the configurable getBaseRegister()? Look at MCSubRegIterator.
> 
> I'm using the MCSubRegIterator now, but I'm not sure this addresses your comment.  Is that what you were suggesting?

What if getBaseRegister() returns something other than RBX? You shouldn't hardcode RBX, that's all I meant.

/jakob