Re: perf report: fix off-by-one for non-activation frames

From: Jan Kratochvil
Date: Fri Jun 16 2017 - 02:15:07 EST

On Mon, 15 May 2017 17:04:44 +0200, Milian Wolff wrote:

commit 1982ad48fc82c284a5cc55697a012d3357e84d01
Author: Milian Wolff <milian.wolff@xxxxxxxx>
Date: Wed May 24 15:21:25 2017 +0900

> --- a/tools/perf/util/unwind-libdw.c
> +++ b/tools/perf/util/unwind-libdw.c
> @@ -168,12 +168,16 @@ frame_callback(Dwfl_Frame *state, void *arg)
> + if (!isactivation)
> + --pc;
> +

FYI I find it as a regression a bit:

30c563 gdb_main (/usr/libexec/gdb)
fae48 main (/usr/libexec/gdb)
0x000055555564ee43 <+51>: callq 0x55555585f340 <gdb_main(captured_main_args*)>
0x000055555564ee48 <+56>: mov 0x18(%rsp),%rcx

39e32e gdb_main (/usr/libexec/gdb)
10b6fa main (/usr/libexec/gdb)
0x000055555565f6f6 <+54>: callq 0x5555558f17a0 <gdb_main(captured_main_args*)>
0x000055555565f6fb <+59>: mov 0x18(%rsp),%rcx

In backtraces it is correct to show the source line of the calling line - as
perf does now after your fix - but one still should report PC address of the
start of the next instruction. At least this is what debuggers are used to

#9 gdb_main (args=0x7fffffffe2e0) at ../../gdb/main.c:1257
#10 0x000055555565f6fb in main (argc=<optimized out>, argv=<optimized out>) at ../../gdb/gdb.c:40
0x000055555565f6f6 <+54>: callq 0x5555558f17a0 <gdb_main(captured_main_args*)>
=> 0x000055555565f6fb <+59>: mov 0x18(%rsp),%rcx
Line 40 of "../../gdb/gdb.c" starts at address 0x55555565f6f6 <main(int, char**)+54> and ends at 0x55555565f6fb <main(int, char**)+59>.
Line 41 of "../../gdb/gdb.c" starts at address 0x55555565f6fb <main(int, char**)+59> and ends at 0x55555565f715.

You see "gdb.c:40" and 0x000055555565f6fb in the backtrace despite
0x55555565f6fb is already line 41.

This is also why elfutils reports separately PC and 'isactivation' flag.
Instead of just reporting decreased PC.