[LLVMdev] Line number and merged calls

nicolas geoffray nicolas.geoffray at gmail.com
Sat Feb 13 10:09:51 PST 2010

Hi Jakob,

Thanks for the reply!

On Sat, Feb 13, 2010 at 7:02 PM, Jakob Stoklund Olesen <stoklund at 2pi.dk>wrote:

> On Feb 13, 2010, at 8:19 AM, nicolas geoffray wrote:
> > Is this a bug with respect to accurate line number information with
> optimizations?
> No. The way LLVM is designed, debug information is not allowed to interfere
> with code generation. In fact, if including debug information does change
> the generated code in any way, it is considered a bug.


> > If not, is there a way to disable this optimization?
> Yes.

Great! :)

> > I browsed the source code to find out where this optimization occurs
> during code generation, but couldn't find anything. Can someone point me
> where this optimization is performed?
> The transformation you describe is called tail merging or branch folding.
> The opposite transformation also occurs. It is called tail duplication.
> Try -disable-branch-fold and perhaps -disable-tail-duplicate.
> Note that debug information on optimized code is a best effort thing. If
> you depend on certain information to be present, you will almost certainly
> run into more problems like this.
So what I currently depend on, is that each call instruction must have an
accurate line number information. I don't care about the line number of
other instructions. Will I still run into trouble? If the debug info is
attached to the call, how can LLVM optimizers (codegen or IR) mess with it
(except with that special branch folding optimization you just mentioned)?


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