Re: [ppc64-linux]: skip linkage functions

Daniel Jacobowitz
2003-06-06T21:40:08Z

On Fri, Jun 06, 2003 at 03:31:58PM -0500, Jim Blandy wrote:
> Daniel Jacobowitz <drow@mvista.com> writes:
>
> > On Fri, Jun 06, 2003 at 03:10:09PM -0500, Jim Blandy wrote:
> > > Daniel Jacobowitz <drow@mvista.com> writes:
> > >
> > > > On Fri, Jun 06, 2003 at 01:22:27AM -0500, Jim Blandy wrote:
> > > > > Daniel Jacobowitz <drow@mvista.com> writes:
> > > > >
> > > > > > On Thu, Jun 05, 2003 at 06:54:57PM -0500, Jim Blandy wrote:
> > > > > > >
> > > > > > > 2003-06-05 Jim Blandy <jimb@redhat.com>
> > > > > > >
> > > > > > > Recognize and skip 64-bit PowerPC Linux linkage functions.
> > > > > > > * ppc-linux-tdep.c (insn_d, insn_ds, insn_xfx, read_insn, struct
> > > > > > > insn_pattern, insns_match_pattern, d_field, ds_field): New
> > > > > > > functions, macros, and types for working with PPC instructions.
> > > > > > > (ppc64_standard_linkage, PPC64_STANDARD_LINKAGE_LEN,
> > > > > > > ppc64_in_solib_call_trampoline, ppc64_standard_linkage_target,
> > > > > > > ppc64_skip_trampoline_code): New functions, variables, and macros
> > > > > > > for recognizing and skipping linkage functions.
> > > > > > > (ppc_linux_init_abi): Use ppc64_in_solib_call_trampoline and
> > > > > > > ppc64_skip_trampoline_code for the 64-bit PowerPC Linux ABI.
> > > > > >
> > > > > > Hmm. Probably not good enough for our needs, but is the
> > > > > > DW_AT_trampoline attribute useful here?
> > > > >
> > > > > I'll say it, so nobody else has to feel bad saying it: that patch is
> > > > > complete shite. I just can't see any other way to do it with the info
> > > > > I have.
> > > > >
> > > > > DW_AT_trampoline would allow me to implement in_solib_call_trampoline
> > > > > and skip_trampoline_code simply by consulting the debugging info,
> > > > > which would be eons better. And in generic code, to boot. The only
> > > > > thing is, the trampolines are generated by the linker, not the
> > > > > compiler. Could the linker contribute its own Dwarf compilation unit
> > > > > to .debug_info and .debug_abbrev? How should it decide which
> > > > > debugging format to use, and whether to emit anything at all?
> > > > >
> > > > > If we could get this working, we could start using it on other
> > > > > architectures, too.
> > > >
> > > > Hmm. I believe it could be done. It would probably require adding
> > > > a --gdwarf2 to the linker, matching the one added to GAS. It's
> > > > certainly practical for the linker to add a CU.
> > > >
> > > > As always, it wouldn't free us from the need to grub through assembly
> > > > trampolines by hand. There's always something without debug info. But
> > > > it would make that code a little less important...
> > >
> > > I haven't been living with CFI long enough to know how these stories
> > > turn out, but my gut feeling is that replacing these heuristic
> > > techniques like prologue unwinding with real debug info has got to be
> > > the Right Thing.
> >
> > The only problem is that DW_AT_trampoline doesn't live in the CFI - it
> > lives in the .debug_info section with the full debug info. Some
> > architectures are moving to always providing CFI, but debug info is
> > more than we can count on.
>
> Oh, I know where DW_AT_trampoline lives. I was referring to the
> general trend of providing debug info for stuff GDB previously had to
> guess about, e.g., CFI and location lists replacing prologue analysis.
> You can never really get rid of the old heuristics, but since they
> won't be used every day any more, they're going to bit-rot. Even if
> we included actual binaries in the test suite to make sure the
> prologue analyzers continued to recognize what we'd once taught them,
> compilers will continue to change the prologues they emit. In the
> end, if it rots badly enough, is it worth keeping it at all?
>
> > What blows up if we don't recognize the trampolines though?
>
> On the PPC64, 'next' blows up.

Oh, I hit this problem on another platform recently. And I had to do a
similar hack for trampolines.

It seems that there shold be a simpler solution than trying to parse
the trampoline...

--
Daniel Jacobowitz
MontaVista Software Debian GNU/Linux Developer