Re: [PATCH 4/5] ARM: read_pieced_value do big endian processing only in case of valid gdb_regnum

[Yao Qi <yao@codesourcery.com>]

Victor Kamensky <victor.kamensky@linaro.org> writes:

> In both little endian and big endian cases compiler generate DW_OP_reg29-
> DW_OP_reg31 something like this.
>
>  <2><792>: Abbrev Number: 10 (DW_TAG_formal_parameter)
>     <793>   DW_AT_name        : u
>     <795>   DW_AT_decl_file   : 1
>     <796>   DW_AT_decl_line   : 115
>     <797>   DW_AT_type        : <0x57c>
>     <79b>   DW_AT_location    : 6 byte block: 6d 93 4 6c 93 4
> (DW_OP_reg29 (r29); DW_OP_piece: 4; DW_OP_reg28 (r28); DW_OP_piece: 4)
>

This is quite illustrative.

> I strongly suspect that it is compiler error, but more accurately
> it is hard to say, because I never saw a document where for given CPU
> mapping from registers to DWARF reg numbers is defined. Have you
> seen such document for example for ARM V7? In any case for this
> test case Gdb believes that those register numbers are wrong. I.e we
> can say for sure that gcc and gdb are disagrees.

You need doc "DWARF for the ARM Architecture", which has a table about
the mapping between dwarf reg numbers and processor registers. For the
table, we can see that dwarf register 16 to 63 doesn't map to any
processor registers.

>
>
> (gdb) file /wd1/gdb/20140930/build-v7le/gdb/testsuite/gdb.base/store
> Reading symbols from /wd1/gdb/20140930/build-v7le/gdb/testsuite/gdb.base/store...done.
> (gdb) tbreak wack_double
> Temporary breakpoint 1 at 0x1076c: file ../../../binutils-gdb/gdb/testsuite/gdb.base/store.c, line 117.
> (gdb) run
> Starting program: /wd1/gdb/20140930/build-v7le/gdb/testsuite/gdb.base/store 
>
> Temporary breakpoint 1, wack_double (u=
> ../../binutils-gdb/gdb/regcache.c:177: internal-error: register_size: Assertion `regnum >= 0 && regnum < (gdbarch_num_regs (gdbarch) + gdbarch_num_pseudo_regs (gdbarch))' failed.
> A problem internal to GDB has been detected,
> further debugging may prove unreliable.
>

This is quite useful too.

>
> BE Dump
> =======
>
>
>  <1><779>: Abbrev Number: 12 (DW_TAG_subprogram)
>     <77a>   DW_AT_external    : 1       
>     <77a>   DW_AT_name        : (indirect string, offset: 0x3c9): wack_double   
>     <77e>   DW_AT_decl_file   : 1       
>     <77f>   DW_AT_decl_line   : 115     
>     <780>   DW_AT_prototyped  : 1       
>     <780>   DW_AT_type        : <0x57c> 
>     <784>   DW_AT_low_pc      : 0x10758 
>     <788>   DW_AT_high_pc     : 0x40    
>     <78c>   DW_AT_frame_base  : 1 byte block: 9c        (DW_OP_call_frame_cfa)
>     <78e>   DW_AT_GNU_all_tail_call_sites: 1    
>     <78e>   DW_AT_sibling     : <0x7d7> 
>  <2><792>: Abbrev Number: 10 (DW_TAG_formal_parameter)
>     <793>   DW_AT_name        : u       
>     <795>   DW_AT_decl_file   : 1       
>     <796>   DW_AT_decl_line   : 115     
>     <797>   DW_AT_type        : <0x57c> 
>     <79b>   DW_AT_location    : 6 byte block: 6d 93 4 6c 93 4   (DW_OP_reg29 (r29); DW_OP_piece: 4; DW_OP_reg28 (r28); DW_OP_piece: 4)
>  <2><7a2>: Abbrev Number: 10 (DW_TAG_formal_parameter)
>     <7a3>   DW_AT_name        : v       
>     <7a5>   DW_AT_decl_file   : 1       
>     <7a6>   DW_AT_decl_line   : 115     
>     <7a7>   DW_AT_type        : <0x57c> 
>     <7ab>   DW_AT_location    : 6 byte block: 6f 93 4 6e 93 4   (DW_OP_reg31 (r31); DW_OP_piece: 4; DW_OP_reg30 (r30); DW_OP_piece: 4)
>  <2><7b2>: Abbrev Number: 13 (DW_TAG_variable)
>     <7b3>   DW_AT_name        : l       
>     <7b5>   DW_AT_decl_file   : 1       
>     <7b6>   DW_AT_decl_line   : 117     
>     <7b7>   DW_AT_type        : <0x57c> 
>     <7bb>   DW_AT_location    : 8 byte block: 90 21 93 4 90 20 93 4     (DW_OP_regx: 33 (r33); DW_OP_piece: 4; DW_OP_regx: 32 (r32); DW_OP_piece: 4)
>  <2><7c4>: Abbrev Number: 13 (DW_TAG_variable)
>     <7c5>   DW_AT_name        : r       
>     <7c7>   DW_AT_decl_file   : 1       
>     <7c8>   DW_AT_decl_line   : 117     
>     <7c9>   DW_AT_type        : <0x57c> 
>     <7cd>   DW_AT_location    : 8 byte block: 90 23 93 4 90 22 93 4     (DW_OP_regx: 35 (r35); DW_OP_piece: 4; DW_OP_regx: 34 (r34); DW_OP_piece: 4)
>

However, we don't need to copy the whole DIE here, instead, we can only
copy one DW_TAG_formal_parameter, which is should be illustrative enough
for the problem.

> Backtrace when it failed to get reg number
> ==========================================

We don't need to copy the full stack back trace here.

-- 
Yao (齐尧)