Different Dwarf

Clang doesn't give me the same source line information I get from Gcc. In V8, I'm working on a project where I embed source line information in code that is generated then embedded in a binary with inline assembly code. It works just great with GCC but not with Clang. How come?

How to build

You need both gcc and clang. Edit their locations in the Makefile if you need to then run make.

How to run

File foo.s is a little function with a debug break in it, and embedded source information which points (whimsically) to a file of quotes by C. S. Lewis. bar.s is an identical function. I compile foo.s like this:

gcc -g -c -o foo.o foo.s

The -g enables debugging information.

Whereas bar.s is compiled with clang like so:

clang -g -c bar.s

In gdb you can see the source integration working great for function foo():

mvstanton@yesod:~/src/different-dwarf$ gdb --args ./different-dwarf
GNU gdb (GDB) 8.2-gg2
Copyright (C) 2018 Free Software Foundation, Inc.
...
(gdb) r
Starting program: different-dwarf

Program received signal SIGTRAP, Trace/breakpoint trap.
0x0000000000400534 in foo () at quotes.txt:7
7       "Are the gods not just?"
(gdb)

If you print the call stack you'll see source line information in the output:

(gdb) bt
#0  0x0000000000400534 in foo () at quotes.txt:7
#1  0x0000000000400518 in main (argc=1, argv=0x7ffc138e2e48) at main.cc:8
(gdb)

Continue the run and you'll get a breakpoint in function bar(), compiled with clang:

(gdb) c
Continuing.
Hello there foo

Program received signal SIGTRAP, Trace/breakpoint trap.
0x000000000040055d in bar ()
(gdb) bt
#0  0x000000000040055d in bar ()
#1  0x0000000000400527 in main (argc=1, argv=0x7ffc138e2e48) at main.cc:9
(gdb)

As you see, no source information here. This despite the fact that the bar.o file does have a DWARF source section in it (.debug_line).

The problem seems to be that other necessary DWARF segments aren't present in the resulting object files. Specifically, objdump output reveals that foo.o has the following extra sections that aren't in bar.o:

• .debug_info (0x3d bytes)
• .debug_abbrev (0x14 bytes)
• .debug_aranges (0x30 bytes)
• .debug_str (0x52 bytes)

I'd really like to have this information on the clang side, as we build with the clang toolchain. Any ideas?