Re: MIPS stack tracing

[Daniel Jacobowitz <drow at mvista dot com>]

On Wed, Feb 06, 2002 at 12:40:18PM -0500, Don Bowman wrote:
> 
> > From: Don Bowman 
> >
> > I'm going to give dwarf2 a try (as suggested in the thread 
> > @ http://sources.redhat.com/ml/crossgcc/2001-12/msg00039.html)
> > 
> 
> To summarise:
> 
> So the dwarf-2 gives the same affect, stack traces do not work
> on mips with gcc 3.0.3 and binutils 2.11.2. The root of the
> problem is that the multiple returns per function exist as
> of the new version of gcc, and there is no .mdebug section 
> anymore. gdb doesn't read the .pdr section which is emitted
> with the information on how to unwind the stack, so it
> switches to its heuristic, which is now broken because of the
> multiple returns.
> 
> Upon examination of gas, the .pdr section is only emitted if
> MIPS_STABS_ELF is defined.  Am I to assume that if I'm using
> DWARF2 this won't occur? The code which actually emits it
> seems to be in ecoff.c.

No.  It should be emitted unless we are emitting .mdebug, which we
don't do any more for mips-*-linux.

> Am I correct that the .pdr section is an array of:
> 
> /*
>  * Procedure Descriptor
>  *
>  * There is one of these for EVERY TEXT LABEL.
>  * If a procedure is in a file with full symbols, then isym
>  * will point to the PROC symbols, else it will point to the
>  * global symbol for the label.
>  */
> 
> typedef struct pdr {
>         bfd_vma adr;            /* memory address of start of procedure */
>         long    isym;           /* start of local symbol entries */
>         long    iline;          /* start of line number entries*/
>         long    regmask;        /* save register mask */
>         long    regoffset;      /* save register offset */
>         long    iopt;           /* start of optimization symbol entries*/
>         long    fregmask;       /* save floating point register mask */
>         long    fregoffset;     /* save floating point register offset */
>         long    frameoffset;    /* frame size */
>         short   framereg;       /* frame pointer register */
>         short   pcreg;          /* offset or reg of return pc */
>         long    lnLow;          /* lowest line in the procedure */
>         long    lnHigh;         /* highest line in the procedure */
>         bfd_vma cbLineOffset;   /* byte offset for this procedure from the
> fd base */
>         /* These fields are new for 64 bit ECOFF.  */
>         unsigned gp_prologue : 8; /* byte size of GP prologue */
>         unsigned gp_used : 1;   /* true if the procedure uses GP */
>         unsigned reg_frame : 1; /* true if register frame procedure */
>         unsigned prof : 1;      /* true if compiled with -pg */
>         unsigned reserved : 13; /* reserved: must be zero */
>         unsigned localoff : 8;  /* offset of local variables from vfp */
>         } PDR, *pPDR;
> #define cbPDR sizeof(PDR)

I don't think that's the right one.  The code in gas to emit them:

    subseg_set (pdr_seg, 0);

    /* Write the symbol.  */
    exp.X_op = O_symbol;
    exp.X_add_symbol = p;
    exp.X_add_number = 0;
    emit_expr (&exp, 4);

    fragp = frag_more (7 * 4);

    md_number_to_chars (fragp,      (valueT) cur_proc_ptr->reg_mask, 4);
    md_number_to_chars (fragp +  4, (valueT) cur_proc_ptr->reg_offset, 4);
    md_number_to_chars (fragp +  8, (valueT) cur_proc_ptr->fpreg_mask, 4);
    md_number_to_chars (fragp + 12, (valueT) cur_proc_ptr->fpreg_offset, 4);
    md_number_to_chars (fragp + 16, (valueT) cur_proc_ptr->frame_offset, 4);
    md_number_to_chars (fragp + 20, (valueT) cur_proc_ptr->frame_reg, 4);
    md_number_to_chars (fragp + 24, (valueT) cur_proc_ptr->pc_reg, 4);

    subseg_set (saved_seg, saved_subseg);

So that is: symbol, regmask, regoffset, fpregmask, fpregoffset,
frameoffset, framereg, pcreg, and broken for 64-bit targets.  Feh, I'll
have to fix that last one.

> This doesn't seem right to me, if I dump my .pdr section I get:
> Contents of section .pdr:
>  0000 00400080 00000000 00000000 00000000  .@..............
>  0010 00000000 00000000 00000000 00000000  ................
>  0020 74430080 00000000 00000000 00000000  tC..............
>  0030 00000000 00000000 00000000 00000000  ................
>  0040 b8430080 00000000 00000000 00000000  .C..............
>  ...
> 
> But all of my addresses start @ 0x80000000.

Careful, it has relocations if you look at it in an object.  Also
careful, you have a host-target endian mismatch.  That first word is
0x80000400.


-- 
Daniel Jacobowitz                           Carnegie Mellon University
MontaVista Software                         Debian GNU/Linux Developer