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

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

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

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
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