[LLVMdev] Optimization passes and debug info

[Devang Patel]

On Jul 22, 2008, at 10:33 PM, Chris Lattner wrote:

> 1. I agree with Alexandre Oliva that the compiler should never "lie"
> in debug info.  If it has some information that is not accurate, it
> should respond with "I don't know" instead of making a wrong guess.
> This means that the debugger either says "your variable is certainly
> here" or it says "I don't know", it never says "your variable might be
> here".

I agree.

> 2. Based on #1, it is preferable for all optimizations to update debug
> info.  This improves the QoI of the debugging experience when the
> optimizations are enabled.  However, based on #1, if they can't or
> haven't been updated yet, they should just discard it.

What about possibility of skipping the optimization if it can not  
update debug info successfully ?


When, I presented three levels, I did not focus on size of debug info.  
In fact, I focused on #1 feature you mention above.

The levels, I presented intended to match choices the optimizer can  
take if debug information is available:

choice 1: I know how to update debug info correctly.
choice 2: I'm a vectorizer,  I do not know how to let the debugger  
single step in a vectorized loop, but I know how to update other debug  
information in this function.
choice 3: I do not know how to update debug info correctly. Should I  
do the optimization or skip ? Who wins, optimization level or debug- 
info level at the llvm-gcc command line ?

-
Devang