Has anybody debugged LLVM IR with GDB? I'm using dragonegg to transform C
into IR, then applying my optimizations. Passing "-g" to dragonegg doesn't
seem to work since it generates debug info for the C code, not the IR. I
really need GDB (lli doesn't solve my problems) in order to debug
multi-threaded and multi-process MPI code.
More clearly, if I have a file hello.ll and I execute:
$> llc hello.ll -march=x86-64 -o hello.s
$> mpicc -O0 hello.s -o hello
Can I debug "hello" with GDB at the IR level? If so, how? If not, would it
be possible to add debug info generation at the IR level (and how much
effort required)? I read something about DwarfDebug but it's not clear to me
how to use it or even if it does what I need. I hope I didn't miss any
obvious solution.
With the recent MCJIT work, C code compiled with LLVM can be debugged in GDB
(Debugging JIT-ed Code — LLVM 18.0.0git documentation). Alas, this doesn't mean IR
can be debugged as easily 
The problem is the following: the debug information attached to each IR
instruction by Clang refers to the original C code. When this debug info
passes through the various layers of optimizations, MC, JITting, it remains
pointing to C code. To debug IR, one would have to add debug info to each
instruction, actually pointing to the IR itself. Then I guess GDB would know
how to debug it, at least with a few tweaks, because GDB generally doesn't
terribly care which language it debugs, as long as it can follow the debug
info and the commands make sense (and this can be handled by adding
appropriate Python plugins).
Eli
Hi all,
For my own purposes, I wrote a pass that does exactly what you all are
describing: add debug metadata to LLVM IR.
As a pass, it had to tackle the problem of "This file needs to exist
on disk somewhere so gdb can find it", which I solved my dumping it
onto /tmp/ somewhere. Not a great solution (who deletes these?) but
worked well enough.
Another interesting issue is how to coexist with any existing debug
metadata, which can be useful for simultaneously debugging an IR
transform inline with the C source for instrumentation-style passes
like SAFECode, ASan/TSan.
Quick Example:
(gdb) break main
Breakpoint 1 at 0x4010b1: file
/home/wdietz2/magic/test/unit/test_loop.c, line 9.
(gdb) r
Starting program:
/home/wdietz2/llvm/32-obj-make/projects/magic/test/Output/test_loop
Breakpoint 1, main (argc=<value optimized out>, argv=<value optimized
) at /home/wdietz2/magic/test/unit/test_loop.c:9
9 unsigned k = 0;
Missing separate debuginfos, use: debuginfo-install
glibc-2.12-1.80.el6_3.5.x86_64 libgcc-4.4.6-4.el6.x86_64
libstdc++-4.4.6-4.el6.x86_64
(gdb) n
10 source(argc != 0, &k);
(gdb) n
14 %and.i.i.i.i104 = and i64 %4, 70368744177660
(gdb) n
15 %5 = load i8** @global, align 8
(gdb) n
18 store i32 16843009, i32* %6, align 1
(gdb) n
19 store i8 1, i8* getelementptr inbounds ([1 x i8]* @array,
i64 0, i64 0), align 1
(gdb) n
20 call coldcc void @runtime_func() nounwind
(gdb) n
11 while(i-- > argc)
(gdb) n
23 %and.i.i.i.i85 = and i64 %7, 70368744177660
(gdb) n
14 while(j++ < i) k += j;
(gdb) n
11 while(i-- > argc)
(gdb) n
14 while(j++ < i) k += j;
(gdb) n
102 %77 = load i8** @global, align 8
(gdb) n
105 %79 = load i32* %78, align 4
(gdb) n
106 %cmp7.i.i.i = icmp ne i32 %79, 0
(gdb) n
108 call void @llvm.memset.p0i8.i64(i8* %add.ptr.i.i.i.i86, i8
%conv8.i.i.i, i64 4, i32 1, i1 false) nounwind
(gdb) n
14 while(j++ < i) k += j;
(gdb) n
15 while(j-- > 0) k *= k + j;
(gdb) n
95 %69 = load i8** @global, align 8
(gdb) n
98 %71 = load i32* %70, align 4
(gdb)
The pass itself is rather simple--the hard problem it solves is
emitting the IR to disk and reasoning about what Instruction* is on
what line, which really shouldn't be a problem if done properly in
LLVM. If desired I can certainly make the code available on request.
In short, it seemed to work well for me and having it done properly in
LLVM itself would be great!
~Will
