[PATCH] Don't optimize out GDB JIT registrar

Project Infrastructure LLVM Dev List Archives
#1

Hi,

We switched from compiling LLVM with gcc to clang (3.3) and it appears that clang (correctly I think) optimizes away the GDBRegistrar’s __jit_debug_register_code() function that’s used to trigger reading debug info from JIT-ted code, breaking GDB support.

This patch forces it to leave the call using the method described here in the ‘noinline’ section:

http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html

Cheers,
Andrew

GDBRegistrar.patch (553 Bytes)

#2

Use asm volatile("":::"memory") to make sure that it doesn't leave
trackes. The noinline can likely go in that case...

Joerg

#3

Thanks Joerg.

I’ve made the change you suggested and verified that it still works. I think the noinline is still required though as this function can be called from a couple of places and gdb will want to set its breakpoint on the single function address. Let me know if you think otherwise though.

Cheers,
Andrew

GDBRegistrar.patch (573 Bytes)

#4

Can we get this applied?

#5

LGTM with a comment explaining the issue.

#6

Hi Rafael, comment explanation added now. Thanks.

GDBRegistrar-2.patch (735 Bytes)

#7

LGTM. Do you have commit access?

#8

I don’t have commit access, I wasn’t sure if this was the right protocol to get a patch accepted but just let me know if not and I’ll get it right the next time.

Cheers,
Andrew

#9

Wouldn’t LLVM_ATTRIBUTE_USED be the correct thing here to prevent dead stripping, if that’s the problem?

#10

My understanding is that they want the calls to not be removed, not
just to keep the symbol. Andrew, is my understanding correct?

Cheers,
Rafael

#11

Yes, that is correct.

#12

I tweaked the comment a bit and committed as r199350.