[PATCH] readelf: allow build with LLVM/clang

[Timm Baeder]

On 14/07/2021 19:40, Mark Wielaard wrote:
>>   AC_CACHE_CHECK([whether gcc supports __attribute__((visibility()))],
>>   	ac_cv_visibility, [dnl
>> diff --git a/src/readelf.c b/src/readelf.c
>> index 161d7e65..3d6f263e 100644
>> --- a/src/readelf.c
>> +++ b/src/readelf.c
>> @@ -8763,13 +8763,17 @@ print_debug_line_section (Dwfl_Module *dwflmod, Ebl *ebl, GElf_Ehdr *ehdr,
>>         /* Apply the "operation advance" from a special opcode
>>   	 or DW_LNS_advance_pc (as per DWARF4 6.2.5.1).  */
>>         unsigned int op_addr_advance;
>> -      inline void advance_pc (unsigned int op_advance)
>> -      {
>> -	op_addr_advance = minimum_instr_len * ((op_index + op_advance)
>> -					       / max_ops_per_instr);
>> -	address += op_addr_advance;
>> -	op_index = (op_index + op_advance) % max_ops_per_instr;
>> -      }
>> +      bool show_op_index;
>> +      #define advance_pc(op_advance_arg) \
>> +      ( { \
>> +        unsigned int op_advance = op_advance_arg; \
>> +		op_addr_advance = minimum_instr_len * ((op_index + (op_advance)) \
>> +			/ max_ops_per_instr); \
>> +		address += (op_advance); \
>> +		show_op_index = (op_index > 0 || \
>> +			(op_index + (op_advance)) % max_ops_per_instr > 0); \
>> +		op_index = (op_index + (op_advance)) % max_ops_per_instr; \
>> +      } )
> 
> This doesn't compile with gcc:
> 
> readelf.c: In function ‘print_debug_line_section’:
> readelf.c:8766:12: error: variable ‘show_op_index’ set but not used [-Werror=unused-but-set-variable]
>   8766 |       bool show_op_index;
>        |            ^~~~~~~~~~~~~
> cc1: all warnings being treated as errors
> 
> Removing the unused show_op_index makes two testcases fail:
> 
> FAIL: run-readelf-line.sh
> =========================
> 
> --- readelf.out	2021-07-14 19:37:00.962372827 +0200
> +++ -	2021-07-14 19:37:00.971161054 +0200
> @@ -42,13 +42,13 @@
>    [    36] set prologue end flag
>    [    37] special opcode 19: address+0 = 0x100005a4 <main>, line+1 = 6
>    [    38] set column to 8
> - [    3a] special opcode 47: address+8 = 0x100005a6 <main+0x2>, line+1 = 7
> + [    3a] special opcode 47: address+8 = 0x100005ac <main+0x8>, line+1 = 7
>    [    3b] set 'is_stmt' to 0
>    [    3c] advance line by constant -7 to 0
> - [    3e] special opcode 32: address+4 = 0x100005a7 <main+0x3>, line+0 = 0
> + [    3e] special opcode 32: address+4 = 0x100005b0 <main+0xc>, line+0 = 0
>    [    3f] set column to 3
>    [    41] set 'is_stmt' to 1
> - [    42] special opcode 108: address+24 = 0x100005ad <main+0x9>, line+6 = 6
> - [    43] special opcode 76: address+16 = 0x100005b1 <main+0xd>, line+2 = 8
> - [    44] advance address by 32 to 0x100005b9 <main+0x15>
> + [    42] special opcode 108: address+24 = 0x100005c8 <main+0x24>, line+6 = 6
> + [    43] special opcode 76: address+16 = 0x100005d8 <main+0x34>, line+2 = 8
> + [    44] advance address by 32 to 0x100005f8
>    [    46] extended opcode 1:  end of sequence
> FAIL run-readelf-line.sh (exit status: 1)
> 
> FAIL: run-readelf-multi-noline.sh
> =================================
> 
> --- readelf.out	2021-07-14 19:37:10.054186557 +0200
> +++ -	2021-07-14 19:37:10.062074795 +0200
> @@ -112,6 +112,6 @@
>    [    6e] extended opcode 2:  set address to +0x724 <main>
>    [    79] copy
>    [    7a] set column to 15
> - [    7c] special opcode 32: address+4 = +0x725 <main+0x1>, line+0 = 1
> - [    7d] advance address by 4 to +0x726 <main+0x2>
> + [    7c] special opcode 32: address+4 = +0x728 <main+0x4>, line+0 = 1
> + [    7d] advance address by 4 to +0x72c
>    [    7f] extended opcode 1:  end of sequence
> FAIL run-readelf-multi-noline.sh (exit status: 1)

FWIW, there is a different version of this patch at
https://sourceware.org/pipermail/elfutils-devel/2021q1/003674.html
that doesn't have those problems as far as I remember.