`b My::Function` in gdb breaks at last line of previous function

[llvmbot]

Bugzilla Link	13554
Resolution	FIXED
Resolved on	Dec 05, 2012 22:06
Version	trunk
OS	Linux
Blocks	llvm/llvm-bugzilla-archive#14330
Attachments	Command line used to build the .o, Preprocessed file, Object file
Reporter	LLVM Bugzilla Contributor
CC	@dwblaikie,@echristo

Extended Description

This is in Chrome, so the full binary is too big to upload, but I'll attach the relevant .ii, .o, and command line.

$ third_party/llvm-build/Release+Asserts/bin/clang++ --version
clang version 3.2 (trunk 159409)

[llvmbot]

assigned to @dwblaikie

[llvmbot]

GNU gdb (GDB) 7.4-gg5
...
Reading symbols from /usr/local/google/jyasskin/chromium/src/out/Debug/chrome...done.
(gdb) b LocationBarViewGtk::HandlePageActionHboxExpose
Breakpoint 1 at 0xc9aafe: file ../../chrome/browser/ui/gtk/location_bar_view_gtk.cc, line 1117.
(gdb) run
Starting program: /usr/local/google/jyasskin/chromium/src/out/Debug/chrome --enable-script-badges
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/libthread_db.so.1".
warning: the debug information found in "/usr/lib/debug//usr/lib/libxcb.so.1.1.0" does not match "/usr/lib/libxcb.so.1" (CRC mismatch).

warning: the debug information found in "/usr/lib/debug/usr/lib/libxcb.so.1.1.0" does not match "/usr/lib/libxcb.so.1" (CRC mismatch).

...

Breakpoint 1, LocationBarViewGtk::HandleExpose (this=0x7fffd6a408c0,
widget=0x7fffd566c4d0, event=0x7fffd4112490)
at ../../chrome/browser/ui/gtk/location_bar_view_gtk.cc:1117
1117 return FALSE; // Continue propagating the expose.
(gdb)

HandlePageActionHboxExpose starts on line 1123, and gdb seems to know that once it actually hits the breakpoint.

[echristo]

I think you said this was fixed in newer clangs Jeffrey?

[llvmbot]

Yep, seems to be working now.

[llvmbot]

And broken again as of r163674.

[dwblaikie]

*** Bug llvm/llvm-bugzilla-archive#14397 has been marked as a duplicate of this bug. ***

[echristo]

Trivial testcase:

void foo() {
return;
}

void bar() {
return;
}

int main (void) {
foo();
bar();
return 0;
}

athyra:~/tmp> /builds/build-llvm/Debug+Asserts/bin/clang -g foo.c
athyra:/tmp> gdb a.out
GNU gdb (GDB) 7.5-gg4
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later http://gnu.org/licenses/gpl.html
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux".

<http://wiki/Main/GnuDebugger FAQ: http://go/gdb Email: gdb-team IRC: gdb>
Reading symbols from /usr/local/google/home/echristo/tmp/a.out...done.
(gdb) b main
Breakpoint 1 at 0x4004ef: main. (2 locations)
(gdb) b foo
Breakpoint 2 at 0x4004c0: file foo.c, line 2.
(gdb) b bar
Note: breakpoint 1 also set at pc 0x4004d0.
Breakpoint 3 at 0x4004d0: file foo.c, line 6.
(gdb) q

[echristo]

FWIW this doesn't duplicate under darwin, though the line tables are slightly different:

(Darwin)
Address Line Column File ISA Flags

---

0x0000000000000000 1 0 1 0
0x0000000000000004 2 0 1 0 is_stmt prologue_end
0x0000000000000010 5 0 1 0
0x0000000000000014 6 0 1 0 is_stmt prologue_end
0x0000000000000020 9 0 1 0
0x000000000000002f 10 0 1 0 is_stmt prologue_end
0x0000000000000034 11 0 1 0 is_stmt
0x000000000000003e 12 0 1 0 is_stmt
0x0000000000000044 12 0 1 0 is_stmt end_sequence

executable on linux:

Address Line Column File ISA Flags

---

0x00000000004004c0 2 0 1 0 is_stmt prologue_end
0x00000000004004d0 6 0 1 0 is_stmt prologue_end
0x00000000004004e0 9 0 1 0
0x00000000004004ef 10 0 1 0 is_stmt prologue_end
0x00000000004004f4 11 0 1 0 is_stmt
0x00000000004004fe 12 0 1 0 is_stmt
0x0000000000400504 12 0 1 0 is_stmt end_sequence

[echristo]

Ah. This is -momit-leaf-frame-pointer which is turned off by default on darwin, but on for linux (for some reason).

That said changing them doesn't break anything or fix anything, though the line tables then match.

[llvmbot]

gdb needs a line number entry for the start of the function (e.g. first instruction of prologue), and gdb discards line number entries with !is_stmt.
[gdb could be smarter - this is just to explain what's happening]
This is so that when gdb goes to set a breakpoint on the symbol obtained from the ELF symbol table, a lookup of the line number given the pc doesn't search backwards beyond the start of the function.

[echristo]

The heuristic is a bit odd there, we should chat about it.

[dwblaikie]

Reapplied fix in 5a773bb
un-xfailed tests from llvm/llvm-bugzilla-archive#14397 in r169302

[dwblaikie]

mentioned in issue llvm/llvm-bugzilla-archive#14330

[dwblaikie]

mentioned in issue llvm/llvm-bugzilla-archive#14397