[LLVMdev] Inserting a branch in PPCTargetLowering::LowerFormalArguments_SVR4

[Hal Finkel]

On Thu, 10 May 2012 12:34:03 -0700
Jim Grosbach <grosbach at apple.com> wrote:

> Hi Hal,
> 
> Yes, you could definitely expand them later. Custom inserters are
> effectively just a very early expand pseudo instruction. You can even
> expand them very late in the MC lowering (asmprinter) if you really
> need to. That could get tricky, as you'll effectively be fibbing to
> the register allocator.

This sounds dangerous ;) -- I suppose that so long as no additional
registers (that might be live at that point) are used by the branching,
then it might be okay.

ARM uses quite a lot of late expanded
> pseudo-instructions, for example, though generally not w/ branching
> involved, so the regalloc implications are more easily modeled.
> 
> From the context, I gather that CR6 changes state dynamically over
> the course of a program's execution? It's not a compile time
> determinable thing? If it were compile time, you could deal with this
> as a different calling convention instead, which changes which regs
> are call clobbered vs. call preserved. Sounds like this is more
> complicated than that, though.

This is an unfortunate special case, and it applies to va_arg functions.
The PPC32 ABI spec says:

> A caller of a function that takes a variable argument list shall set
> condition register bit 6 to 1 if it passes one or more arguments in
> the floating-point registers. It is strongly recommended that the
> caller set the bit to 0 otherwise, using the creqv 6, 6, 6 (set to 1)
> or crxor 6, 6, 6 (set to 0) instruction.

> The motivation for using the
> condition register bit is twofold. First, a function that takes a
> variable argument list may test condition register bit 6 to determine
> whether or not to store the floating- point argument registers in
> memory, thereby making execution of such functions more efficient
> when there are no floating-point arguments. Second, programs that do
> not otherwise use floating point need not acquire a floating-point
> state, with the attendant saving and restoring of the state on
> context switches, merely because they call functions with variable
> argument lists.

It seems that kernel-mode code (on FreeBSD and perhaps other systems as
well) depends on this behavior.

 -Hal

> 
> -Jim
> 
> 
> On May 10, 2012, at 12:16 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> > Jim,
> > 
> > Thanks! Might it be possible to do this another way? For example,
> > could I insert some special pseudo-instructions and then turn them
> > into branches later on?
> > 
> > -Hal
> > 
> > On Thu, 10 May 2012 11:39:10 -0700
> > Jim Grosbach <grosbach at apple.com> wrote:
> > 
> >> Hi Hal,
> >> 
> >> For lowering code that requires inserting branches, you need to
> >> use a custom inserter, yes. Theoretically, that does indeed sound
> >> like what you want to do here.
> >> 
> >> It's complicated by the general structure of argument passing,
> >> though. In particular, there's lots of assumptions about the call
> >> sequence stuff. I don't know if things are smart enough (EH in
> >> particular worries me) to handle this sort of thing. On the plus
> >> side, there are copious assert() bits in there to catch it if the
> >> compiler gets itself confused, so you should know sooner rather
> >> than later if something goes wonky.
> >> 
> >> -Jim
> >> 
> >> On May 10, 2012, at 11:25 AM, Hal Finkel <hfinkel at anl.gov> wrote:
> >> 
> >>> I hate to be bothersome, but can someone please comment on this?
> >>> 
> >>> Thanks again,
> >>> Hal
> >>> 
> >>> On Thu, 3 May 2012 14:49:04 -0500
> >>> Hal Finkel <hfinkel at anl.gov> wrote:
> >>> 
> >>>> Hello,
> >>>> 
> >>>> The current code in PPCTargetLowering::LowerFormalArguments_SVR4
> >>>> contains a FIXME over the code which saves the live
> >>>> floating-point registers to the stack. The FIXME states that
> >>>> this should only be done if CR bit 6 is set. I've been told that
> >>>> the lack of this check is preventing clang/LLVM from compiling a
> >>>> functional FreeBSD kernel on PPC.
> >>>> 
> >>>> Is is possible to insert another branch in LowerFormalArguments?
> >>>> Some of the atomic instructions insert branches, but those are
> >>>> handled in EmitInstrWithCustomInserter. Can branches be inserted
> >>>> inside LowerFormalArguments in the same way?
> >>>> 
> >>>> Thanks in advance,
> >>>> Hal
> >>>> 
> >>> 
> >>> 
> >>> 
> >>> -- 
> >>> Hal Finkel
> >>> Postdoctoral Appointee
> >>> Leadership Computing Facility
> >>> Argonne National Laboratory
> >>> _______________________________________________
> >>> LLVM Developers mailing list
> >>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> >>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >> 
> > 
> > 
> > 
> > -- 
> > Hal Finkel
> > Postdoctoral Appointee
> > Leadership Computing Facility
> > Argonne National Laboratory
> 



-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory