X86 Calls marked to clobber RSP. Intended?

[Frédéric Riss]

Hi,

The debug info code and more specifically the code in 
llvm/lib/CodeGen/AsmPrinter/DbgValueHistoryCalculator.cpp 
tries to infer debug variables live ranges using the MachineInstr operand clobbers. Under some circumstances it fails to prove that spilled variables are live during the whole function because those variables location is described in terms of stack pointer+offset but the call MIs are marked to clobber the stack pointer. Which IMHO doesn’t make much sense.

I tested this on X86, don’t know if other backends are affected.

The code that marks RSP as clobbered is in 
lib/CodeGen/MachineInstr.cpp:void MachineInstr::setPhysRegsDeadExcept():

The X86 calls have a RegMask argument that trigger this piece of code in the above function:
  // This is a call with a register mask operand.
  // Mask clobbers are always dead, so add defs for the non-dead defines.
  if (HasRegMask)
    for (ArrayRef<unsigned>::iterator I = UsedRegs.begin(), E = UsedRegs.end();
         I != E; ++I)
      addRegisterDefined(*I, &TRI);

The call instructions have an implicit RSP use which is turned into an implicit def by the above piece of code.

This doesn’t happen every time as calls emitted by FastISel don’t have the aforementioned clobber added to them (Looks like FastISel doesn’t add the implicit uses before calling setPhysRegsDeadExcept).

For this simple code:

int bar();
int foo() {
	return bar();
}

if compiled without options I get:

	CALL64pcrel32 <ga:@bar>, <regmask>, %RSP<imp-use>, %AL<imp-use,kill>, %EAX<imp-def>

When using -mllvm -fast-isel=0 I get:

	CALL64pcrel32 <ga:@bar>, <regmask>, %RSP<imp-use>, %AL<imp-use,kill>, %RSP<imp-def>, %EAX<imp-def>

Any thoughts? Does the RSP clobber make sense? Could it be removed? I’d happily work on a patch, if I could figure out the exact expected semantics of the above code...

Thanks!
Fred