How to read variables optimized out in GDB? • Hacker News Board

Re: How to read variables optimized out in GDB?

#21

Post by khuey » Wed, Oct 12, 2022, 5:18 AM UTC

stefan_ wrote: post #6

userbinator wrote: post #5

This isn't something limited to GDB but I've always wondered why no one seems to have bothered to come up with a solution after all these years so that it can actually know that the variable is in a register at that point.

Having spent some time looking at DWARF data closely, I get the impression that there is an insane amount of information generated and the format can handle way way more complex scenarios than a simple "X is in register Y" - they got a full state machine and all in there. But the tools available.. simply do not consume it? I'm not entirely sure what the disconnect is.

The tools are perfectly capable of consuming it. Optimizers just don't emit it.

Re: How to read variables optimized out in GDB?

#22

Post by anyfoo » Wed, Oct 12, 2022, 9:33 PM UTC

noam_k wrote: post #20

anyfoo wrote: post #10

This is "How to read variables optimized out in GDB sometimes ", lest you might be misled to think that the value you are looking for is always hanging around somewhere. Sometimes, it's just gone, literally optimized out completely. Consider this piece of code: bool foo(void) { int v = some_system_call(); bool b = v > 500; some_function(b); ... /\* rest of code not using v \*/ } There is no reason for the original va…

The article actually suggests using the record command in that case.

Won't help with coredumps, or generally if you can't reproduce.

Re: How to read variables optimized out in GDB?

#23

Post by rurban » Thu, Oct 13, 2022, 2:30 AM UTC

When this made it into gdb some years ago, I wanted to print the optimized away values nevertheless with a ? prefix. This was easy. But then gdb got carried away and reworked everything in C++, and I could not print those values anymore.

https://github.com/rurban/binutils-gdb/commits/users/rurban/...

Re: How to read variables optimized out in GDB?

#24

Post by kingluo » Thu, Oct 13, 2022, 11:30 AM UTC

jmorse2 wrote: post #9

userbinator wrote: post #5

This isn't something limited to GDB but I've always wondered why no one seems to have bothered to come up with a solution after all these years so that it can actually know that the variable is in a register at that point.

Assuming you're referring to the articles first example, I suspect optimised-out is the correct outcome in that situation. The program is stopped at the start of one function, and the author is looking one stack frame up at this code: 0x0000564ab5d178c4 : callq *%rax => 0x0000564ab5d178c6 : mov %rbp,%rdi %rsi indeed hasn't been clobbered when the call executes, but it becomes liable to be clobbered during the executi…

Yes, if there is a call before the instruction, you should check if it changes the register you want. Particular case is particular analysis.