[PATCH] 32-bit PowerPC ELF Position Independent Code

[Justin Hibbits]

Hi Hal,

On Jul 16, 2014 8:21 PM, "Hal Finkel" <hfinkel at anl.gov> wrote:
>
> Hi Justin,
>
> Thanks for working on this!
>
> As Bill said, we need test cases. In addition, a few comments:

I have only one more test case to provide for this and I think it should be
ready.

>
> +  const MCExpr *tocExpr =
MCBinaryExpr::CreateAdd(MCSymbolRefExpr::Create(CurrentPos, OutContext),
>
> +
 MCConstantExpr::Create(0x8000, OutContext),
>
> +                                                  OutContext);
>
> what is 0x8000? Please provide a comment.

I will provide a comment on this.  The GOT pointer points to the middle of
the GOT which is 64k in total size.

>
>        BuildMI(FirstMBB, MBBI, dl, TII.get(PPC::MFLR), GlobalBaseReg);
>
> +      if (PPCSubTarget->isTargetELF()) {
>
> +        unsigned TempReg =
RegInfo->createVirtualRegister(&PPC::GPRCRegClass);
>
> +        BuildMI(FirstMBB, MBBI, dl, TII.get(PPC::GetGBRO),
TempReg).addReg(GlobalBaseReg);
>
> +        BuildMI(FirstMBB, MBBI, dl,
TII.get(PPC::UpdateGBR)).addReg(GlobalBaseReg).addReg(TempReg);
>
> +        MF->getInfo<PPCFunctionInfo>()->setUsesPICBase(true);
>
> It is okay here for TempReg to be R0?

Since it is active for only two instructions r0 should be fine.

>
>          // automatically synthesizes these stubs.
>
> -        OpFlags = PPCII::MO_DARWIN_STUB;
>
> +        OpFlags = PPCII::MO_PLT_STUB;
>
> is this going to break Darwin?

No. I overrode this constant to serve the same purpose in ELF as it serves
in Darwin.

>
>        // unless we're building with the leopard linker or later, which
>
>        // automatically synthesizes these stubs.
>
> -      OpFlags = PPCII::MO_DARWIN_STUB;
>
> +      OpFlags = PPCII::MO_PLT_STUB;
>
> this too? Darwin breaking?

Again nope.

>
> -  explicit PPCFunctionInfo(MachineFunction &MF)
> +  explicit PPCFunctionInfo(MachineFunction &MFC)
> ...
> +      MF(MFC),
>
> This is unnecessary; because of the way that C++ name lookup works,
MF(MF) will be fine here.

Gotcha. My c++fu is weak, didn't know this.

Thanks for the review. I will have another patch for you layer tonight or
tomorrow.

- Justin

>
> Thanks again,
> Hal
>
> ----- Original Message -----
> > From: "Justin Hibbits" <chmeeedalf at gmail.com>
> > To: llvm-commits at cs.uiuc.edu
> > Sent: Friday, July 11, 2014 7:54:08 PM
> > Subject: [PATCH] 32-bit PowerPC ELF Position Independent Code
> >
> > Hi,
> >
> > For the last few weeks I've been working off and on with ppc32 PIC
> > for
> > ELF/SysVR4, and now have something to show for it.  The patch is
> > attached.  It passes all existing tests, and I haven't yet added new
> > tests for this.  I also tested by compiling zlib on FreeBSD/PowerPC,
> > and running some commands linked against it.
> >
> > Differences from GCC:
> >
> > * GCC sets aside r30 as the PIC register, this will use any available
> >   register.
> > * GNU AS writes a real difference to the function pre-word, whereas
> > the
> >   LLVM assembler writes 0, with a relocation.  This appears to be
> >   irrelevant, as it works correctly in my test case.
> >
> > - Justin
> > _______________________________________________
> > llvm-commits mailing list
> > llvm-commits at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> >
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
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