Re: flag to know that we are compiling GDB for an arm target

Zied Guermazi
2021-03-22T15:13:41Z

hi

On 22.03.21 16:06, Simon Marchi wrote:
>
> On 2021-03-22 10:53 a.m., Zied Guermazi wrote:
>> hi
>>
>> I will put different solutions together with advantages disadvantages
>>
>> - add a vector of registers to each instruction
>>
>> advantage: close to the logical model: a function is a set of instructions, an instruction changes a set of registers.
>>
>> disadvantage: consumes much memory (3 additional pointers, for an empty vector)
>>
>> - extend the instruction class
>>
>> advantage: targets not needing the registers are not heavily impacted
>>
>> disadvantages: still an additional pointer is added
>>
>> - Infer the ISA mode from mapping symbols
>>
>> advantages: no overhead in the data structure
>>
>> disadvantages: dwarf info are not always available.
> The mapping symbols (if I understand correctly, $a and $t) are not in
> the DWARF, they are ELF symbols. However, is it possible to record
> execution when you don't event have and ELF file, just connect to a
> target and record it? In that case, you might not even have ELF
> symbols, so that's perhaps not sufficient.
>
> There's also the case of self-modifying or JIT-ed code, where mapping
> symbols from the ELF file are of no use. So I think it's better to just
> always rely on CPSR.
>
>> - use additional bits in btrace_insn_flag
>>
>> advantages: no memory overhead
>>
>> disadvantages: encode architecture specific info.
> Since we already pay the cost of having space for flags, we might as
> well use it. It sounds like a good solution for your current problem,
> since there is very little info you need to keep (the execution mode).
> That doesn't solve the problem for when you'll want to record data
> though, a more flexible solution will be needed.
>
>> I will go for using additional bits (bit 1, bit 2 and bit 3) in btrace_insn_flag to encode the isa for armv7 as folowing
>>
>> ocsd_isa_arm as 0x02
>>
>> ocsd_isa_thumb2 as 0x04
>>
>> ocsd_isa_tee as 0x06
>>
>> ocsd_isa_jazelle as 0x08
> Is that 0x06 really what you want, it's a OR of arm and thumb2?
that is not a big issue, I can mask the flags value with 0x0E and then
compare against the value of the isa (the 3bits are handled as one
flag). it will be one switch case.
>
> Simon
/Zied