[llvm-commits] [PATH] Fix unwind info in x86 JIT for functions without stack space

[Zoltan Varga]

Hi,

  Attached an .ll file which reproduces this on x86-64. Compile it using:

llvm-as -f empty.ll && llc -f empty.bc && as empty.s

The prolog of the 'stelemref' function looks like this:

0000000000000000 <stelemref>:
   0:   41 57                   push   %r15
   2:   41 56                   push   %r14
   4:   53                      push   %rbx

While the unwind info looks like this:

00000018 00000014 0000001c FDE cie=00000000 pc=00000000..00000045
  DW_CFA_advance_loc: 5 to 00000005
  DW_CFA_offset: r3 (rbx) at cfa-32
  DW_CFA_offset: r14 (r14) at cfa-24
  DW_CFA_offset: r15 (r15) at cfa-16

Notice that there is no DW_CFA_def_cfa_offset instruction.

                    Zoltan


On Sun, Aug 2, 2009 at 1:48 PM, Bill Wendling <isanbard at gmail.com> wrote:

> On Jul 31, 2009, at 10:56 AM, Zoltan Varga wrote:
> > Hi,
> >
> > This is a resend of a patch sent earlier which got lost in the ml
> > traffic.
> >
> >   The x86 jit doesn't generate a def_cfa_offset unwind instruction
> > after the
> > pushes in the function prolog if the function doesn't
> >
> > have any stack space, i.e. for a prolog like:
> >
> > 0x40011870:     push %r15
> >
> > 0x40011872:     push %r14
> >
> > 0x40011874:     push %rbx
> >
> > The attached patch fixes this.
> >
> Hi Zoltan,
>
> (Sorry it took me a while to look at this email.)
>
> I don't think that this patch is the correct solution. The unwind info
> for callee-saved registers should be emitted in this code:
>
>   // Skip the callee-saved push instructions.
>   bool RegsSaved = false;
>   while (MBBI != MBB.end() &&
>          (MBBI->getOpcode() == X86::PUSH32r ||
>           MBBI->getOpcode() == X86::PUSH64r)) {
>     RegsSaved = true;
>     ++MBBI;
>   }
>
>   if (RegsSaved && needsFrameMoves) {
>     // Mark end of callee-saved push instructions.
>     unsigned LabelId = MMI->NextLabelID();
>     BuildMI(MBB, MBBI, DL, TII.get(X86::DBG_LABEL)).addImm(LabelId);
>
>     // Emit DWARF info specifying the offsets of the callee-saved
> registers.
>     emitCalleeSavedFrameMoves(MF, LabelId, HasFP ? FramePtr :
> StackPtr);
>   }
>
> The code you modified is placing a label/machine-move after the "subl
> X, %esp" instruction when there isn't a frame pointer. (N.B., this
> instruction won't exist if NumBytes == 0.)
>
> Do you have a code example for me to look at?
>
> -bw
>
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