llvm dev - May 2012 - [LLVMdev] Inserting a branch in PPCTargetLowering::LowerFormalArguments_SVR4

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

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

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. 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.
-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