Change inline frame breakpoint skipping logic (fix gdb.gdb/selftest.exp)

Change inline frame breakpoint skipping logic (fix gdb.gdb/selftest.exp)

Currently, gdb.gdb/selftest.exp fails if you build GDB with
optimization (-O2, etc.).

The reason is that after setting a breakpoint in captured_main, we
stop at:
 ...
 Breakpoint 1, captured_main_1 (context=<optimized out>) at src/gdb/main.c:492
 ...
while selftest_setup expects a stop at captured_main.

Here, captured_main_1 has been inlined into captured_main, and
captured_main has been inlined into gdb_main:

 ...
 $ nm ./build/gdb/gdb | egrep ' [tT] .*captured_main|gdb_main' | c++filt
 000000000061b950 T gdb_main(captured_main_args*)
 ...

Indeed, the two inlined functions show up in the backtrace:

 ...
 (gdb) bt
 #0  captured_main_1 (context=<optimized out>) at main.c:492
 #1  captured_main (data=<optimized out>) at main.c:1147
 #2  gdb_main (args=args@entry=0x7fffffffdb80) at main.c:1173
 #3  0x000000000040fea5 in main (argc=<optimized out>, argv=<optimized out>)
     at gdb.c:32
 ...

We're now stopping at captured_main_1 because commit ddfe970e6bec
("Don't elide all inlined frames") makes GDB present a stop at the
innermost inlined frame if the program stopped by a user breakpoint.

Now, the selftest.exp testcase explicitly asks to stop at
"captured_main", not "captured_main_1", so I'm thinking that it's
GDB'S behavior that should be improved.  That is what this commit
does, by only showing a stop at an inline frame if the user breakpoint
was set in that frame's block.

Before this commit:

 (top-gdb) b captured_main
 Breakpoint 1 at 0x792f99: file src/gdb/main.c, line 492.
 (top-gdb) r
 Starting program: build/gdb/gdb

 Breakpoint 1, captured_main_1 (context=<optimized out>) at src/gdb/main.c:492
 492       lim_at_start = (char *) sbrk (0);
 (top-gdb)

After this commit, we now instead get:

 (top-gdb) b captured_main
 Breakpoint 1 at 0x791339: file src/gdb/main.c, line 492.
 (top-gdb) r
 Starting program: build/gdb/gdb

 Breakpoint 1, captured_main (data=<optimized out>) at src/gdb/main.c:1147
 1147      captured_main_1 (context);
 (top-gdb)

and:

 (top-gdb) b captured_main_1
 Breakpoint 2 at 0x791339: file src/gdb/main.c, line 492.
 (top-gdb) r
 Starting program: build/gdb/gdb
 Breakpoint 2, captured_main_1 (context=<optimized out>) at src/gdb/main.c:492
 492       lim_at_start = (char *) sbrk (0);
 (top-gdb)

Note that both captured_main and captured_main_1 resolved to the same
address, 0x791339.  That is necessary to trigger the issue in
question.  The gdb.base/inline-break.exp testcase currently does not
exercise that, but the new test added by this commit does.  That new
test fails without the GDB fix and passes with the fix.  No
regressions on x86-64 GNU/Linux.

While at it, the THIS_PC comparison in stopped_by_user_bp_inline_frame
is basically a nop, so just remove it -- if a software or hardware
breakpoint explains the stop, then it must be that it was installed at
the current PC.

gdb/ChangeLog:
2018-06-19  Pedro Alves  <palves@redhat.com>

	* inline-frame.c (stopped_by_user_bp_inline_frame): Replace PC
	parameter with a block parameter.  Compare location's block symbol
	with the frame's block instead of addresses.
	(skip_inline_frames): Pass the current block instead of the
	frame's address.  Break out as soon as we determine the frame
	should not be skipped.

gdb/testsuite/ChangeLog:
2018-06-19  Pedro Alves  <palves@redhat.com>

	* gdb.opt/inline-break.c (func_inline_callee, func_inline_caller)
	(func_extern_caller): New.
	(main): Call func_extern_caller.
	* gdb.opt/inline-break.exp: Add tests for inline frame skipping
	logic change.