[LLVMdev] Debug metadata after simplifications

[Tim Besard]

Hi all,

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.

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?

Sincerely,
-- 
Tim Besard
Computer Systems Lab
Department of Electronics & Information Systems
Ghent University