Re: MIPS: Handle manual calls of MIPS16 functions with a call stub

[Daniel Jacobowitz <drow@false.org>]

On Fri, Feb 01, 2008 at 03:34:04PM +0000, Maciej W. Rozycki wrote:
> On Fri, 1 Feb 2008, Daniel Jacobowitz wrote:
> 
> > >  The other way round -- the minimal symbol points to the actual entry 
> > > point, but the stub precedes it and is included in the DWARF-2 block 
> > > together with the MIPS16 function body.  Here's an example that triggers a 
> > > failure in the test suite (generated from gdb.base/call-ar-st.c by GCC 
> > > 4.2.2):
> > 
> > Then why aren't we calling the instruction at the start of the block,
> > i.e. the stub?  In which case not using the MIPS16 convention is
> > correct.  I don't see why you'd want to call
> > __fn_stub_print_ten_doubles as a MIPS16 function.
> 
>  Well, GDB does not ever seem to call the stub.

That's because I didn't notice the section switch in your example.
The stub is not part of the block described in the DWARF-2 table;
it's physically between the labels in the assembly file, but the
labels are in the text section and the stub isn't.  So that's why we
call the real function.  One confusion solved.

>  <1><de6>: Abbrev Number: 16 (DW_TAG_subprogram)
>   <de7>     DW_AT_external    : 1
>   <de8>     DW_AT_name        : print_ten_doubles
>   <dfa>     DW_AT_decl_file   : 1
>   <dfb>     DW_AT_decl_line   : 664
>   <dfd>     DW_AT_low_pc      : 0x80020a90
>   <e01>     DW_AT_high_pc     : 0x80020b00
>   <e05>     DW_AT_frame_base  : 0x306   (location list)
>   <e09>     DW_AT_sibling     : <e9d>
> 
>  I have done a little more research of this matter now and it looks like 
> the reason this is happening is a likely bug somewhere in GAS.  For 
> comparison, here are the unrelocated DWARF-2 records for 
> print_ten_doubles() and a nearby function that has no stub:
> 
>  <1><bab>: Abbrev Number: 16 (DW_TAG_subprogram)
>     <bac>   DW_AT_external    : 1
>     <bad>   DW_AT_name        : init_small_structs
>     <bc0>   DW_AT_decl_file   : 1
>     <bc1>   DW_AT_decl_line   : 613
>     <bc3>   DW_AT_low_pc      : 0x790
>     <bc7>   DW_AT_high_pc     : 0x900
>     <bcb>   DW_AT_frame_base  : 0x2db   (location list)
>     <bcf>   DW_AT_sibling     : <0xcaf>
> 
>  <1><caf>: Abbrev Number: 16 (DW_TAG_subprogram)
>     <cb0>   DW_AT_external    : 1
>     <cb1>   DW_AT_name        : print_ten_doubles
>     <cc3>   DW_AT_decl_file   : 1
>     <cc4>   DW_AT_decl_line   : 664
>     <cc6>   DW_AT_low_pc      : 0x900
>     <cca>   DW_AT_high_pc     : 0x97c
>     <cce>   DW_AT_frame_base  : 0x306   (location list)
>     <cd2>   DW_AT_sibling     : <0xd66>
> 
> And here are the relevant relocation records:
> 
> 00000bc3  00003c02 R_MIPS_32         00000790   .LFB23
> 00000bc7  00000202 R_MIPS_32         00000000   .text
> 
> 00000cc6  00000202 R_MIPS_32         00000000   .text
> 00000cca  00000202 R_MIPS_32         00000000   .text
> 
> Notice that the DWARF-2 record at 0xbc3 is relocated against .LFB23 and 
> one at 0xcc6 -- against .text, rather than .LFB20 as it should be.  I 
> presume this is because of the section switch happening inbetween.  Or 
> could it be because of ".set nomips16" actually preceding the section 
> switch?  Thiemo, can you perhaps make any comments about this?

Does the low bit end up set in the DWARF for init_small_structs?  From
what you're saying, I think it does.  My experience with Thumb makes
me think that's the actual GAS bug here.  Those are code addresses,
not symbols, and I don't think they should encode MIPS16-ness -
see the recent discussion of values in .debug_line, which we agreed
should not have assorted flag bits set either.  The code starts at
0x900, not 0x901.

-- 
Daniel Jacobowitz
CodeSourcery