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

[Luis Machado]

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
>>>
>>>
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