[LLVMdev] Debug metadata after simplifications

[Eric Christopher]

In order to identify loops I'm using the DILexicalScope metadata
> attached to the loop latch, but with some combinations of optimisations
> that metadata seems to disappear.
>
>
Looks like line number information yes?


> For example, when -simplifycfg removes a block because it only contains
> a branch, and -loop-simplify recreates that block because it turned out
> to be a back-edge of some loop, the metadata gets removed.
>
> Specifically, using the following testcase:
>   while (while_test()) {
>     if (if_test()) {
>       foo();
>     } else {
>       bar();
>     }
>   }
>
> ... Clang emits* the following IR:
> *after slightly modifying the code generator so that it also emits
> metadata for unconditional branches
>   while.cond:
>     %call = call zeroext i1 (...)* @while_test(), !dbg !8
>     br i1 %call, label %while.body, label %while.end, !dbg !8
>   while.body:
>     %call1 = call zeroext i1 (...)* @if_test(), !dbg !9
>     br i1 %call1, label %if.then, label %if.else, !dbg !9
>   if.then:
>     call void (...)* @foo(), !dbg !11
>     br label %if.end, !dbg !13
>   if.else:
>     call void (...)* @bar(), !dbg !14
>     br label %if.end, !dbg !16
>   if.end:
>     br label %while.cond, !dbg !17
>   while.end:
>     ret i32 0, !dbg !18
>
> If I run -simplifycfg, the if.then, if.else and if.end blocks get
> transformed as follows:
>   if.then:
>     call void (...)* @foo(), !dbg !11
>     br label %while.cond, !dbg !13
>   if.else:
>     call void (...)* @bar(), !dbg !14
>     br label %while.cond, !dbg !16
>
> Removal of the back-edge (and its metadata) is problematic for other
> loop passes, which is why -loop-simplify creates an artificial back-edge
> like this:
>   if.then:
>     call void (...)* @foo(), !dbg !11
>     br label %while.cond.backedge, !dbg !13
>   if.else:
>     call void (...)* @bar(), !dbg !14
>     br label %while.cond.backedge, !dbg !16
>   while.cond.backedge:
>     br label %while.cond
>
> Ultimately I end up with a back-edge with no metadata whatsoever,
> breaking my loop identification. What is the best way to work around
> this? Modify -simplifycfg so that it does not simplify loop back-edges?
> Or somehow add the metadata to the new back-edges?
>
>
You're identifying loops in IR based on line number information?

That said, we could probably figure out a way to add line information to
the back edge if we're getting something wrong in our optimized line tables
-
not a high priority though.

-eric
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20130326/03bc5a3d/attachment.html>