A lean way for getting the size of the instruction at a given address

[Zied Guermazi]

hi Luis,

thanks for your support. To experiment the impact of removing the 
printing of the instruction on the overall performance, I commented out 
setting and using the print function pointer in print_insn (bfd_vma pc, 
struct disassemble_info *info, bfd_boolean little) in opcodes/arm-dis.c, 
and the result was very interesting: The time needed to process the 
traces dropped down from 12 minutes to 34 seconds for 64 MB of traces.

now that we have a proof that the bottleneck was printing, we can think 
about a way to provide a clean implementation.

Kind Regards

Zied Guermazi


On 05.04.21 18:40, Luis Machado wrote:
> On 4/5/21 1:17 PM, Zied Guermazi wrote:
>> hi Luis
>>
>> A new member function in "class gdb_disassembler" to calculate the 
>> instruction length only will be a good solution. In fact a big 
>> overhead is added by the printing of instruction disassembly, which 
>> is not needed at all. On aarch64, the decoder is optimized to issue 
>> many instruction in one trace element, and here calculating the size 
>> consumes more than 80% of the time. On arm, the decoder issues one 
>> instruction after another and here getting the size consumes 50% of 
>> the time. Considering the amount of traces this can sum up to a dozen 
>> of minutes in some cases (64MB of traces)
>
> Indeed, that doesn't sound good.
>
>>
>> Calculating the instruction size per se, on arm is a "rapid" 
>> operation and consists of checking few bits in the opcode. So the 
>> time can be drastically decreased by having a function to calculate 
>> the size only.
>>
>>
>> gdb_print_insn can be then changed as following (pseudo code):
>>
>> int
>> gdb_print_insn (struct gdbarch *gdbarch, CORE_ADDR memaddr,
>>          struct ui_file *stream, int *branch_delay_insns)
>> {
>>
>>    gdb_disassembler di (gdbarch, stream);
>>
>>    if ( di.get_insn_size != 0)
>>
>>     return di.get_insn_size(memaddr);
>>
>>    else
>>
>>     return di.print_insn (memaddr, branch_delay_insns);
>> }
>>
>> Is there a function in aarch64-tdep or arm-tdep doing job of 
>> disassembly ( the lower layer handling the opcode)? are we relaying 
>> on the bfd library for it? can someone give me a hint of where to 
>> find those functions?
>
> The gdbarch hooks in arm-tdep.c (gdb_print_insn_arm) and 
> aarch64-tdep.c (aarch64_gdb_print_insn) are more like helper functions 
> and do some initial setup, but the code to disassemble lies in 
> opcodes/arm-dis.c (print_insn) and opcodes/aarch64-dis.c 
> (print_insn_aarch64).
>
> If you go with the route of changing "class gdb_disassembler", then 
> you'll probably need to touch binutils/opcodes.
>
> If you decide to have a gdbarch hook (in arm-tdep/aarch64-tdep), then 
> you only need to change GDB.
>>
>>
>> Kind Regards
>>
>> Zied Guermazi
>>
>>
>> On 05.04.21 15:01, Luis Machado wrote:
>>> Hi Zied,
>>>
>>> On 4/4/21 4:59 AM, Zied Guermazi wrote:
>>>> hi
>>>>
>>>> I need to get the size of the instruction at a given address. I am 
>>>> currently using gdb_insn_length (struct gdbarch *gdbarch, CORE_ADDR 
>>>> addr) which calls gdb_print_insn (struct gdbarch *gdbarch, 
>>>> CORE_ADDR memaddr, struct ui_file *stream, int 
>>>> *branch_delay_insns). and this is consuming a huge time, 
>>>> considering that this is used in branch tracing and this gets 
>>>> repeated up to few millions times.
>>>>
>>>>
>>>> Is there a lean way for getting the size of the instruction at a 
>>>> given address, I am using it for aarch64 and arm targets.
>>>
>>> At the moment I don't think there is an optimal solution for this. 
>>> The instruction length is calculated as part of the disassemble 
>>> process, and is tied to the function that prints instructions.
>>>
>>> One way to speed things up is to have a new member function in 
>>> "class gdb_disassembler" to calculate the instruction length only.
>>>
>>> Another way is to have a new gdbarch hook that calculates the size 
>>> of an instruction based on the current PC, mapping symbols etc.
>>>
>>>>
>>>> Kind Regards
>>>>
>>>> Zied Guermazi
>>>>
>>>>
>>
>>