[llvm-commits] Add option to emit ELF symbol files for debugging the LLVM JIT with GDB

[Reid Kleckner]

Ping?  Please review.  My internship is over and I'd like to finish
this up before I start classes.

Reid

On Fri, Aug 21, 2009 at 7:53 PM, <reid.kleckner at gmail.com> wrote:
> Hello LLVM devs,
>
> I've been working on a debugging interface between GDB and LLVM, and I'd
> like to get comments on the LLVM side of the interface now that the GDB
> side has been committed.  I don't plan to check this in until after the
> code freeze.  Please take a look on Rietveld.  I expect to iterate on
> this a couple more times.
>
> Here's the description of the change from Rietveld:
> ---------------------
>
> This patch implements the JIT side of the GDB JIT debugging interface
> that I added to GDB.  To enable this feature, either build the JIT in
> debug mode to enable it by default or pass -jit-emit-debug to lli.
>
> Right now, the only debug information that this communicates to GDB is
> call frame information, since it's already being generated to support
> exceptions in the JIT.  Eventually, when DWARF generation isn't tied so
> tightly to AsmPrinter, it will be easy to push that information to GDB
> through this interface.
>
> Here's a step-by-step breakdown of how the feature works:
> - The JIT generates the machine code and DWARF call frame info
> (.eh_frame/.debug_frame) for a function into memory.
> - The JIT copies that info into an in-memory ELF file with a symbol for
> the function.
> - The JIT creates a code entry pointing to the ELF buffer and adds it to
> a linked list hanging off of a global descriptor at a special symbol
> that GDB knows about.
> - The JIT calls a function marked noinline that GDB knows about and has
> put an internal breakpoint in.
> - GDB catches the breakpoint and reads the global descriptor to look for
> new code.
> - When sees there is new code, it reads the ELF from the inferior's
> memory and adds it to itself as an object file.
> - The JIT continues, and the next time we stop the program, we are able
> to produce a proper backtrace.
>
> Consider running the following program through the JIT:
>
> #include <stdio.h>
> void baz(short z) {
>  long w = z + 1;
>  printf("%d, %x\n", w, *((int*)NULL));  // SEGFAULT here
> }
> void bar(short y) {
>  int z = y + 1;
>  baz(z);
> }
> void foo(char x) {
>  short y = x + 1;
>  bar(y);
> }
> int main(int argc, char** argv) {
>  char x = 1;
>  foo(x);
> }
>
> Here is a backtrace before this patch:
> Program received signal SIGSEGV, Segmentation fault.
> [Switching to Thread 0x2aaaabdfbd10 (LWP 25476)]
> 0x00002aaaabe7d1a8 in ?? ()
> (gdb) bt
> #0  0x00002aaaabe7d1a8 in ?? ()
> #1  0x0000000000000003 in ?? ()
> #2  0x0000000000000004 in ?? ()
> #3  0x00032aaaabe7cfd0 in ?? ()
> #4  0x00002aaaabe7d12c in ?? ()
> #5  0x00022aaa00000003 in ?? ()
> #6  0x00002aaaabe7d0aa in ?? ()
> #7  0x01000002abe7cff0 in ?? ()
> #8  0x00002aaaabe7d02c in ?? ()
> #9  0x0100000000000001 in ?? ()
> #10 0x00000000014388e0 in ?? ()
> #11 0x00007fff00000001 in ?? ()
> #12 0x0000000000b870a2 in llvm::JIT::runFunction (this=0x1405b70,
> F=0x14024e0, ArgValues=@0x7fffffffe050)
>   at /home/rnk/llvm-gdb/lib/ExecutionEngine/JIT/JIT.cpp:395
> #13 0x0000000000baa4c5 in llvm::ExecutionEngine::runFunctionAsMain
> (this=0x1405b70, Fn=0x14024e0, argv=@0x13f06f8, envp=0x7fffffffe3b0)
>   at /home/rnk/llvm-gdb/lib/ExecutionEngine/ExecutionEngine.cpp:377
> #14 0x00000000007ebd52 in main (argc=2, argv=0x7fffffffe398,
> envp=0x7fffffffe3b0) at /home/rnk/llvm-gdb/tools/lli/lli.cpp:208
>
> And a backtrace after this patch:
> Program received signal SIGSEGV, Segmentation fault.
> 0x00002aaaabe7d1a8 in baz ()
> (gdb) bt
> #0  0x00002aaaabe7d1a8 in baz ()
> #1  0x00002aaaabe7d12c in bar ()
> #2  0x00002aaaabe7d0aa in foo ()
> #3  0x00002aaaabe7d02c in main ()
> #4  0x0000000000b870a2 in llvm::JIT::runFunction (this=0x1405b70,
> F=0x14024e0, ArgValues=...)
>   at /home/rnk/llvm-gdb/lib/ExecutionEngine/JIT/JIT.cpp:395
> #5  0x0000000000baa4c5 in llvm::ExecutionEngine::runFunctionAsMain
> (this=0x1405b70, Fn=0x14024e0, argv=..., envp=0x7fffffffe3c0)
>   at /home/rnk/llvm-gdb/lib/ExecutionEngine/ExecutionEngine.cpp:377
> #6  0x00000000007ebd52 in main (argc=2, argv=0x7fffffffe3a8,
> envp=0x7fffffffe3c0) at /home/rnk/llvm-gdb/tools/lli/lli.cpp:208
>
> ----------------
>
> Thanks!
> Reid
>
> http://codereview.appspot.com/91042
>