Re: [rfc] btrace: change record instruction-history /m

[Doug Evans <dje@google.com>]

On Fri, Aug 14, 2015 at 4:37 AM, Markus Metzger
<markus.t.metzger@intel.com> wrote:
> The /m modifier interleaves source lines with the disassembly of recorded
> instructions.  This calls disasm.c's do_mixed_source_and_assembly once for
> each recorded instruction to be printed.
>
> The latter really does a source print with intermixed disassembly.  To that
> purpose, it may reorder instructions to better match the source.  This is
> nice for disassembling an entire function, but it doesn't work for printing
> a single instruction.

Even for an entire function "source centric" disassembly can be a pain.
If some find it useful, great, but I never do.

> Change record instruction-history /m to use its own simple source interleaving
> algorithm.  The most important part is that instructions are printed in
> the order in which they were executed.  Before each instruction, we print
> a range of source lines that are attributed to the instruction's PC.  If the
> source line range has already been printed (or skipped) for the previous
> instruction, we skip it.
>
> The algorithm would fill in the missing lines if a PC is attributed to more
> than one source line.  I have not seen this.  There always seems to be at
> most one source line per PC.

One can imagine such a think happening more often with an LIW ISA.
[assuming the compiler cares to provide such info]

> I don't think that there is an MI consumer of "record instruction-history"
> and I have no idea if and how this might affect MI.

Good question. Dunno either.

> Alternatively, we could extend Doug Evan's new algorithm
> https://sourceware.org/ml/gdb-patches/2015-08/msg00335.html to take a vector
> of PCs.
>
> 2015-08-14  Markus Metzger <markus.t.metzger@intel.com>
>
> gdb/
>         * record-btrace.c (struct btrace_line_range): New.
>         (btrace_mk_line_range, btrace_line_range_add)
>         (btrace_line_range_is_empty, btrace_line_range_contains_range)
>         (btrace_find_line_range, btrace_print_lines): New.
>         (btrace_insn_history): Add source interleaving algorithm.

Yeah, I'd like to avoid having multiple copies of code
doing basically the same thing.
Plus, no one is going to look in record-btrace.c for
"smart disassembly support".
disasm.c should be providing the necessary API
that the rest of gdb an use.