Re: [PATCH] Powerpc, fix vla-optimized-out.exp test

[Joel Brobecker <brobecker@adacore.com>]

Hi everyone,

On Thu, Jun 09, 2022 at 10:23:58AM -0700, Carl Love wrote:
> Joel:
> 
> I have been talking with the gcc developers to understand more about
> how DWARF reports the type and size of the variable in the DWARF output
> to answer your question below.

Thanks Carl.

Could someone else take a look and help Carl? I've been meaning to
get to this, but haven't been able to get to this, not even on weekends,
and I don't see myself having time for quite a while, unfortunately :-(.

> 
> On Sun, 2022-04-17 at 08:19 -0700, Joel Brobecker wrote:
> > Hello Carl,
> > 
> > Thanks for the patch.
> > 
> > > Powerpc, fix vla-optimized-out.exp test
> > > 
> > > The size of the variable a is not optimized out on Intel inspite of
> > > the
> > > use of the variable use being optimized out.
> > > 
> > > On Powerpc, the use of variable a and the size of variable a are
> > > both
> > > optimized out.
> > 
> > For me, it would be useful to include an annotated copy of the DWARF
> > info being produced on x86_64-linux where the size is available,
> > vs the DWARF being produced on PowerPC. This will help understand
> > what's happening and confirm that it is indeed valid for GDB to
> > print that the size has been optimized out
> 
> I am still learning the DWARF format but here is what I have from my
> discussions.  The dwarf is for the vla-optimized-out.c program compiled
> with -O1 which corresponds to the case where the <optimized out>
> message gets printed.  
> 
> Contents of the .debug_info section:
> 
>   Compilation Unit @ offset 0x0:
>    Length:        0xef (32-bit)
>    Version:       5
>    Unit Type:     DW_UT_compile (1)
>    Abbrev Offset: 0x0
>    Pointer Size:  8
>  <0><c>: Abbrev Number: 2 (DW_TAG_compile_unit)
>     <d>   DW_AT_producer    : (indirect string, offset: 0x3e): GNU C17 11.2.1 20220127 (Red Hat 11.2.1-9) -msecure-plt -mtune=power9 -mcpu=power9 -g -O1 -fno-stack-protector
>     <11>   DW_AT_language    : 29	(C11)
>     <12>   DW_AT_name        : (indirect string, offset: 0xb2): /.../gdb/testsuite/gdb.base/vla-optimized-out.c
>     <16>   DW_AT_comp_dir    : (indirect string, offset: 0x0): /.../gdb/testsuite
>     <1a>   DW_AT_low_pc      : 0x1000068c
>     <22>   DW_AT_high_pc     : 0x5c
>     <2a>   DW_AT_stmt_list   : 0x0
>  <1><2e>: Abbrev Number: 3 (DW_TAG_subprogram)
>     <2f>   DW_AT_external    : 1
>     <2f>   DW_AT_name        : (indirect string, offset: 0xad): main
>     <33>   DW_AT_decl_file   : 1
>     <34>   DW_AT_decl_line   : 37
>     <35>   DW_AT_decl_column : 1
>     <36>   DW_AT_prototyped  : 1
>     <36>   DW_AT_type        : <0x7d>
>     <3a>   DW_AT_low_pc      : 0x100006a0
>     <42>   DW_AT_high_pc     : 0x48
>     <4a>   DW_AT_frame_base  : 1 byte block: 9c 	(DW_OP_call_frame_cfa)
>     <4c>   DW_AT_call_all_calls: 1
>     <4c>   DW_AT_sibling     : <0x7d>
>  <2><50>: Abbrev Number: 4 (DW_TAG_variable)
>     <51>   DW_AT_name        : j
>     <53>   DW_AT_decl_file   : 1
>     <54>   DW_AT_decl_line   : 39
>     <55>   DW_AT_decl_column : 16
>     <56>   DW_AT_type        : <0x84>
>     <5a>   DW_AT_location    : 2 byte block: 91 70 	(DW_OP_fbreg: -16)
>  <2><5d>: Abbrev Number: 5 (DW_TAG_variable)
>     <5e>   DW_AT_name        : i
>     <60>   DW_AT_decl_file   : 1
>     <61>   DW_AT_decl_line   : 40
>     <62>   DW_AT_decl_column : 7
>     <63>   DW_AT_type        : <0x7d>
>     <67>   DW_AT_location    : 0x10 (location list)
>     <6b>   DW_AT_GNU_locviews: 0xc
>  <2><6f>: Abbrev Number: 6 (DW_TAG_call_site)
>     <70>   DW_AT_call_return_pc: 0x100006c4
>     <78>   DW_AT_call_origin : <0x89>
>  <2><7c>: Abbrev Number: 0
>  <1><7d>: Abbrev Number: 7 (DW_TAG_base_type)
>     <7e>   DW_AT_byte_size   : 4
>     <7f>   DW_AT_encoding    : 5	(signed)
>     <80>   DW_AT_name        : int
>  <1><84>: Abbrev Number: 8 (DW_TAG_volatile_type)
>     <85>   DW_AT_type        : <0x7d>
>  <1><89>: Abbrev Number: 9 (DW_TAG_subprogram)
>     <8a>   DW_AT_external    : 1
>     <8a>   DW_AT_name        : f1
>     <8d>   DW_AT_decl_file   : 1
>     <8e>   DW_AT_decl_line   : 29
>     <8f>   DW_AT_decl_column : 1
>     <90>   DW_AT_prototyped  : 1
>     <90>   DW_AT_type        : <0x7d>
>     <94>   DW_AT_low_pc      : 0x1000068c
>     <9c>   DW_AT_high_pc     : 0x14
>     <a4>   DW_AT_frame_base  : 1 byte block: 9c 	(DW_OP_call_frame_cfa)
>     <a6>   DW_AT_call_all_calls: 1
>     <a6>   DW_AT_sibling     : <0xc7>
>  <2><aa>: Abbrev Number: 10 (DW_TAG_formal_parameter)
>     <ab>   DW_AT_name        : i
>     <ad>   DW_AT_decl_file   : 1
>     <ae>   DW_AT_decl_line   : 29
>     <af>   DW_AT_decl_column : 9
>     <b0>   DW_AT_type        : <0x7d>
>     <b4>   DW_AT_location    : 0x20 (location list)
>     <b8>   DW_AT_GNU_locviews: 0x1c
>  <2><bc>: Abbrev Number: 11 (DW_TAG_variable)
>     <bd>   DW_AT_name        : a           <- Variable a, array of (unsigned char)
>     <bf>   DW_AT_decl_file   : 1
>     <c0>   DW_AT_decl_line   : 31
>     <c1>   DW_AT_decl_column : 8
>     <c2>   DW_AT_type        : <0xc7>      <- a is type entry c7
>     
>  <2><c6>: Abbrev Number: 0
>  <1><c7>: Abbrev Number: 12 (DW_TAG_array_type)   <- array type entry
>     <c8>   DW_AT_type        : <0xeb>             <- array element type eb
>     <cc>   DW_AT_sibling     : <0xe4>
>  <2><d0>: Abbrev Number: 13 (DW_TAG_subrange_type)
>     <d1>   DW_AT_type        : <0xe4>
> 
>         From what I was told, the following DW_AT_upper_bound gives
>         the upper bound for calculating the size of the array.  I believe
>         the size is given by (upper-bound (13) + 1 - lower bound(default 0))
> 	          * size of base type (8) = 112
> 	The program declares char a[6] so I would expect the size to be 6 bytes not
> 	112.  Either way, it does look like the info is there, not that I claim
> 	to completely understand the DWARF output.  Am I missing something
> 	which results in gdb saying the size is optimized out?
> 					       
>     <d5>   DW_AT_upper_bound : 13 byte block: a3 1 53 23 1 8 20 24 8 20 26 31 1c 	(DW_OP_entry_value: (DW_OP_reg3 (r3)); DW_OP_plus_uconst: 1; DW_OP_const1u: 32; DW_OP_shl; DW_OP_const1u: 32; DW_OP_shra; DW_OP_lit1; DW_OP_minus)
>  <2><e3>: Abbrev Number: 0
>  <1><e4>: Abbrev Number: 1 (DW_TAG_base_type)
>     <e5>   DW_AT_byte_size   : 8
>     <e6>   DW_AT_encoding    : 7	(unsigned)
>     <e7>   DW_AT_name        : (indirect string, offset: 0x2c): long unsigned int
>  <1><eb>: Abbrev Number: 1 (DW_TAG_base_type)        Here is the base type
>     <ec>   DW_AT_byte_size   : 1
>     <ed>   DW_AT_encoding    : 8	(unsigned char)
>     <ee>   DW_AT_name        : (indirect string, offset: 0x127): char
>  <1><f2>: Abbrev Number: 0
> 
>                     Carl
> 
> 

-- 
Joel