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

[paul.robinson@sony.com]

(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.  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.
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.

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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