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

[reid.kleckner at gmail.com]

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