Re: [PATCH v4 1/2] Add back gdb_pretty_print_insn

[Simon Marchi <simon.marchi@polymtl.ca>]

On 2017-02-01 13:09, Simon Marchi wrote:
> On 2017-01-31 19:30, Pedro Alves wrote:
>> ui_file_rewind is a ui_file method that only really works with mem
>> buffer files, and is a nop on other ui_file types.  It'd be desirable
>> to eliminate it from the base ui_file interface, and move it to the
>> "mem_fileopen" subclass of ui_file instead.  A following patch does
>> just that.
>> 
>> Unfortunately, there are a couple references to ui_file_rewind inside
>> gdb_disassembler::pretty_print_insn that were made harder to eliminate
>> with the recent addition of the gdb_disassembler wrapper.
>> 
>> Before the gdb_disassembler wrapper was added, in commit
>> e47ad6c0bd7aa3 ("Refactor disassembly code"), gdb_pretty_print_insn
>> used to be passed a ui_file pointer as argument, and it was simple to
>> adjust that pointer be a "mem_fileopen" ui_file pointer instead, since
>> there's only one gdb_pretty_print_insn caller.
>> 
>> That commit made gdb_pretty_print_insn be a method of
>> gdb_disassembler, and removed the method's ui_file parameter at the
>> same time, replaced by referencing the gdb_disassembler's stream
>> instead.  The trouble is that a gdb_disassembler can be instantiated
>> with a pointer any kind of ui_file.  Casting the gdb_disassembler's
>> stream to a mem_fileopen ui_file inside
>> gdb_disassembler::pretty_print_insn in order to call the reset method
>> would be gross hack.
>> 
>> The fix here is to:
>> 
>>  - make gdb_disassembler::pretty_print_insn a be free function again
>>    instead of a method of gdb_disassembler.  I.e., bring back
>>    gdb_pretty_print_insn.
>> 
>>  - but, don't add back the ui_file * parameter.  We'd always be
>>    passing in a fresh mem_fileopen anyway, so move the mem_fileopen
>>    allocation inside.  That is a better interface, given that the
>>    ui_file is only ever used as temporary scratch buffer as an
>>    implementation detail of gdb_pretty_print_insn.  The function's
>>    real "where to send output" parameter is the ui_out pointer.
>> 
>>  - don't add back a disassemble_info pointer either.  That used to be
>>    necessary for this bit:
>> 
>> 	  err = m_di.read_memory_func (pc, &data, 1, &m_di);
>> 	  if (err != 0)
>> 	    m_di.memory_error_func (err, pc, &m_di);
>> 
>>    ... but AFAIK, it's not really necessary.  We can replace those
>>    three lines with a call to read_code.  This seems to fix a
>>    regression even, because before commit d8b49cf0c891d0 ("Don't throw
>>    exception in dis_asm_memory_error"), that memory_error_func call
>>    would throw an error/exception, but now it only records the error
>>    in the gdb_disassembler's m_err_memaddr field.  (read_code throws
>>    on error.)
>> 
>> With all these, gdb_pretty_print_insn is completely layered on top of
>> gdb_disassembler only using the latter's public API.
> 
> I don't think I understand the situation fully, but what you suggest
> looks good to me.  I was confused by the fact that the
> gdb_disassembler constructor accepts a stream, but the
> pretty_print_insn method takes a uiout.  Which one is used for
> printing then?  I think that your patch clears that up.
> 
> The only possible issue I can see is that in your version, one
> gdb_disassembler and one string_file object are constructed for each
> disassembled instruction, rather than re-using them for as long as we
> need to disassemble.  I don't know how much impact it has on the
> performance (probably negligible), but something to keep in mind.

I'll just mention it so we hopefully don't duplicate work, looking at 
your patch has prompted me to start a patch adding a "disassembly_flags" 
enum type.

https://github.com/simark/binutils-gdb/tree/disas-flags