[Dwarf-Discuss] compilers generating ABI non-compliant function calls?

[David Blaikie]

On Tue, Mar 9, 2021 at 1:28 PM Paul Robinson via Dwarf-Discuss <
dwarf-discuss at lists.dwarfstd.org> wrote:

> (re-sending because outlook omitted the group address)
>
> > -----Original Message-----
> > From: Dwarf-Discuss <dwarf-discuss-bounces at lists.dwarfstd.org> On Behalf
> > Of Jakub Jelinek via Dwarf-Discuss
> > Sent: Tuesday, March 9, 2021 10:16 AM
> > To: Andrew Cagney <andrew.cagney at gmail.com>
> > Cc: DWARF Discussion <dwarf-discuss at lists.dwarfstd.org>
> > Subject: Re: [Dwarf-Discuss] compilers generating ABI non-compliant
> > function calls?
> >
> > On Tue, Mar 09, 2021 at 10:05:04AM -0500, Andrew Cagney via Dwarf-Discuss
> > wrote:
> > > Is anyone aware of a compiler doing this (I figure with LTO there's a
> > > strong incentive)?  And if so, how is this described to the debugger.
> > > The ABI / calling-convention is no longer on hand for filling in the
> > > blanks.
> >
> > Sure, GCC does that.  On many architectures, IPA-RA might keep data
> > live across a function call even in registers that are per the ABI
> > officially call clobbered (if it can prove the particular callee does not
> > clobber it).  This isn't expressed in DWARF I believe.
>
>
> DWARF doesn't describe clobbering or non-clobbering; it describes where
> values live.


Right - but that lack of knowledge may be problematic in some situations
I'm trying to describe. (ie: a "gap" where  current DWARF encoding + ABI
knowledge is insufficient for a consumer to behave correctly)


> If something gets clobbered by a call, the location list
> should reflect that the location changes (or doesn't exist) as of the
> instruction after the call; if it doesn't get clobbered, the location
> range should correctly span the call instruction.  The debugger does not
> need to know the ABI in order to trust that location lists are correct.
>

Agreed that the debugger doesn't need to know the clobber/callee
save/caller save ABI to use a location list correctly.


> The producer is responsible for emitting correct location lists that
> don't depend on ABI knowledge by the consumer.
>
> AFAIK the main reason to annotate a subprogram with a calling convention
> is so that a debugger can manufacture a call correctly, in response to a
> user command.
>

Right - and generally the debugger would expect to call this function using
the ABI - except that with internalized functions (either static CU-local
functions, or with any function that LTO optimizations can internalize -
everything other than the specified entry points (in the worst case, only
"main")) the ABI may be violated (because the compiler can see the
implementation of the function and all its callers - well, it thinks it can
see all the callers, but it doesn't know about the debugger making calls).

So, for instance, the internalized function may use callee save registers
without restoring them - if a debugger then tried to call this function
expecting it to conform to the ABI and restore those registers before
returning, the debugger may malfunction/break as the register values would
not be restored. I think?

- Dave


> It might also be necessary to identify the location of the return value,
> as someone else mentioned.
> --paulr
>
> >
> > On x86, GCC can use different register calling conventions for local
> > functions (basically automatic regparm and/or sseregparm calling
> > conventions
> > when possible).  I think this is reflected in the debug info, the
> > DW_TAG_formal_parameter locations should match those.
> >
> >       Jakub
> >
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