Re: [PATCH v3 27/28] Add DW_OP_LLVM_extend DWARF operation

[Zoran Zaric <zoran.zaric@amd.com>]

On 11/1/21 9:48 PM, Lancelot SIX wrote:
> [CAUTION: External Email]
> 
> Hi,
> 
> On Thu, Oct 14, 2021 at 10:32:34AM +0100, Zoran Zaric via Gdb-patches wrote:
>> Previous changes allow a new set of more complex DWARF expression
>> operations to be added which are very usefull for SIMD/SIMT like
>> architectures. First of which is the DW_OP_LLVM_extend operation
>> that pops one stack element (which must be a location description)
>> and treat it as a number of pieces of a new composite location
>> description.
>>
>> This means that a resulting composite location contains a given
>> number of pieces of a given bit size, where all the pieces are
>> described by the same location description found on top of the stack.
>>
>> gdb/ChangeLog:
>>
>>          * compile/compile-loc2c.c (compute_stack_depth_worker): Add
>>          new DW_OP_LLVM_extend operation support.
>>          * dwarf2/expr.c (dwarf_expr_context::create_extend_composite):
>>          New method that creates the extend composite.
>>          (dwarf_expr_context::execute_stack_op): Add new
>>          DW_OP_LLVM_extend operation support.
>>          * dwarf2/loc.c (dwarf2_get_symbol_read_needs): Add new
>>          DW_OP_LLVM_extend operation support.
>>          (disassemble_dwarf_expression): Add new DW_OP_LLVM_extend
>>          operation support.
>>
>> include/ChangeLog:
>>
>>          * dwarf2.def: Add new DW_OP_LLVM_extend enumeration.
>>
>> gdb/testsuite/ChangeLog:
>>
>>          * gdb.dwarf2/dw2-llvm-extend.exp: New test.
>>          * lib/dwarf.exp: Add new DW_OP_LLVM_extend operation support.
>> ---
>>   gdb/compile/compile-loc2c.c                  |   4 +
>>   gdb/dwarf2/expr.c                            |  44 ++++++
>>   gdb/dwarf2/loc.c                             |  12 ++
>>   gdb/testsuite/gdb.dwarf2/dw2-llvm-extend.exp | 148 +++++++++++++++++++
>>   gdb/testsuite/lib/dwarf.exp                  |   5 +
>>   include/dwarf2.def                           |   1 +
>>   6 files changed, 214 insertions(+)
>>   create mode 100644 gdb/testsuite/gdb.dwarf2/dw2-llvm-extend.exp
>>
>> diff --git a/gdb/compile/compile-loc2c.c b/gdb/compile/compile-loc2c.c
>> index ba177726d4f..2c5e13cfb6c 100644
>> --- a/gdb/compile/compile-loc2c.c
>> +++ b/gdb/compile/compile-loc2c.c
>> @@ -366,6 +366,10 @@ compute_stack_depth_worker (int start, int *need_tempvar,
>>          ++stack_depth;
>>          break;
>>
>> +     case DW_OP_LLVM_extend:
>> +       stack_depth -= 2;
> 
> If DW_OP_LLVM_extend pops one element from the stack and pushes one
> composite value back, shouldn't the net result be 0i stack_depth wise?
> 
> Best,
> Lancelot.
> 

Good point.

Thanks,
Zoran