[Dwarf-Discuss] FDEs involving predicated instructions

[Christopher Mead]

Hi,

In the FDE one could, in theory, emit a DW_CFA_def_cfa_expression for the ADDNE and BEQ along the lines of (modulo bugs):
  ULEB128 <size of following DW_OP_* "instructions">

  ;Extract the mode bits from the CPSR (or wherever your processor stores condition flags)
  DW_OP_bregx CPSR, 0   ; If the CPSR isn't numbered (e.g. in ARM DWARF ABI) then you'll
                        ; need to recompute the CPSR or equivalent bits here
  ; Pull out the bit that controls the NE predication and mask them off
  DW_OP_shr #30         ; Or wherever the relevant bits are
  DW_OP_lit1
  DW_OP_and #0x1
  DW_OP_bra LABEL       ; Conditionally branch on NE predicate
  .
  .                     ; Value of CFA is SP +- some offset
  .
  DW_OP_skip END
LABEL:
  .
  .                     ; Value of CFA is SP +- some other offset
  .
END:
  ; When we get here the right value for the CFA is on top of the operand stack.

I may have put some illegal expression operators (6.4.2 in DWARF v4).

Although whether any consumers actually support DW_CFA_def_cfa_expression (or even DW_CFA_val_expression or DW_CFA_expression for that matter?) and have actually been tested for this is a different matter.  This approach was broadly enough describe unwinding through ARM (hardware, not C++) exception vector handlers where the return address is stored in different physical register to the usual LR as per the procedure calling standard.

Kind regards,

Chris Mead.
===========================================
Chris Mead - Senior Software Engineer - ARM
Phone: +44 1223 406 317
Email: christopher.mead at arm.com

> -----Original Message-----
> From: dwarf-discuss-bounces at lists.dwarfstd.org [mailto:dwarf-discuss-
> bounces at lists.dwarfstd.org] On Behalf Of Michael Eager
> Sent: 08 May 2013 09:19
> To: Humphreys, Jonathan
> Cc: dwarf-discuss at lists.dwarfstd.org
> Subject: Re: [Dwarf-Discuss] FDEs involving predicated instructions
>
> Anyone have any suggestions for Jonathan?
>
> On 04/22/2013 11:47 AM, Humphreys, Jonathan wrote:
> > Sure, I'll use ARM as it's a pretty well-known processor.
> >
> > Suppose I have an epilog that consists of stack deallocation and a
> combined SOE register save and return.  And suppose that the
> predecessor block will conditionally fall through to the epilog.  So we
> have:
> >
> > continue:
> >          ....
> >          BEQ continue
> > Epilog:
> >          ...
> >          ADD       SP, SP, #52
> >          POP       {V1, V2, V3, V4, V5, V6, V7, V8, PC}
> >
> > Let's then say that the optimizer sees a stall in the predecessor
> block where it can place the stack deallocation instruction.  But to
> move it into the predecessor block, it must predicate the instruction
> so that it only executes on the fall through path.  We then have:
> >
> > continue:
> > ....
> >          ADDNE     SP, SP, #52
> >          BEQ continue
> > Epilog:
> >          ...
> >          POP       {V1, V2, V3, V4, V5, V6, V7, V8, PC}
> >
> > In the unoptimized case, the compiler would put a DWARF directive
> after the ADD instruction to indicate the CFA has changed by 52.  I
> cannot do the same in the optimized case without also predicating the
> DWARF directive somehow.
> >
> > So my question is how do we support DWARF directives on predicated
> instructions where the effect of the instruction may not happen,
> depending upon some predicate or status register.
> >
> > I hope this helps.
> > Thanks
> > Jon
> >
> > -----Original Message-----
> > From: Michael Eager [mailto:eager at eagercon.com]
> > Sent: Friday, April 19, 2013 6:05 PM
> > To: Humphreys, Jonathan
> > Cc: dwarf-discuss at lists.dwarfstd.org
> > Subject: Re: [Dwarf-Discuss] FDEs involving predicated instructions
> >
> > On 04/19/2013 03:50 PM, Humphreys, Jonathan wrote:
> >> I?m wondering the best way to handle predicated prolog/epilog
> >> instructions in the Call Frame Information.
> >>
> >> Suppose you have an epilog instruction that restores an SOE register
> >> and it is predicated and scheduled in a predecessor block.  How do
> you
> >> handle emitting an FDE rule that describes that a register is
> restored when a predicate or status register is a certain value?
> >>
> >> I can give an example if this isn?t clear.
> >
> > Yes, an example would be helpful.
> >
> >
>
>
> --
> Michael Eager  eager at eagercon.com
> 1960 Park Blvd., Palo Alto, CA 94306  650-325-8077
>
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