[llvm-bugs] [Bug 33355] New: Loop Unswitch creates duplicated dbg.declare

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=33355

            Bug ID: 33355
           Summary: Loop Unswitch creates duplicated dbg.declare
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: mattias.v.eriksson at ericsson.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 18591
  --> https://bugs.llvm.org/attachment.cgi?id=18591&action=edit
Reproducer for duplicated dbg.declare

The function f2 has a byval argument that is dead ("dum"). f2 is
inlined into f3 and the dbg.declare is inlined to reflect
that "dum" is "p1" in the inlined code.

Input to 'Function Integration/Inlining':

; Function Attrs: nounwind readonly
define i16 @f2(%struct.S* byval nocapture readnone align 1 %dum)
local_unnamed_addr #0 !dbg !11 {
entry:
  call void @llvm.dbg.declare(metadata %struct.S* %dum, metadata !18, metadata
!19), !dbg !20
  %0 = load i16, i16* @a, align 1, !dbg !21, !tbaa !22
  ret i16 %0, !dbg !26
}


define void @f3(%struct.S* byval align 1 %p1) local_unnamed_addr #2 !dbg !27 {
entry:
  call void @llvm.dbg.declare(metadata %struct.S* %p1, metadata !31, metadata
!19), !dbg !32
  br label %lbl1, !dbg !33

lbl1:                                             ; preds = %lbl1, %entry
  %call = call i16 @f2(%struct.S* byval nonnull align 1 %p1), !dbg !34

After 'Function Integration/Inlining':

; Function Attrs: nounwind
define void @f3(%struct.S* byval align 1 %p1) local_unnamed_addr #2 !dbg !27 {
entry:
  call void @llvm.dbg.declare(metadata %struct.S* %p1, metadata !31, metadata
!19), !dbg !32
  br label %lbl1, !dbg !33

lbl1:                                             ; preds = %lbl1, %entry
  call void @llvm.dbg.declare(metadata %struct.S* %p1, metadata !18, metadata
!19), !dbg !34
  %0 = load i16, i16* @a, align 1, !dbg !36, !tbaa !22
  call void @f1(i16 %0), !dbg !37

Some relevant metadata:
!18 = !DILocalVariable(name: "dum", arg: 1, scope: !11, file: !3, line: 8,
type: !14)
!19 = !DIExpression()
!31 = !DILocalVariable(name: "p1", arg: 1, scope: !27, file: !3, line: 15,
type: !14)

A while later, 'Unswitch loops' duplicates dum's dbg.declare:

define void @f3(%struct.S* byval nocapture readonly align 1 %p1)
local_unnamed_addr #2 !dbg !27 {
...
lbl1.us:                                          ; preds = %lbl1.us,
%entry.split.us
  call void @llvm.dbg.declare(metadata %struct.S* %p1, metadata !18, metadata
!19), !dbg !34
  %1 = load i16, i16* @a, align 1, !dbg !36, !tbaa !22
  call void @f1(i16 %1), !dbg !37
...
lbl1:                                             ; preds = %lbl1, %entry.split
  call void @llvm.dbg.declare(metadata %struct.S* %p1, metadata !18, metadata
!19), !dbg !34
  %2 = load i16, i16* @a, align 1, !dbg !36, !tbaa !22
  call void @f1(i16 %2), !dbg !37

And then in the late stages of llc I get a crash in DwarfDebug
because it expects that there can't be duplicate dbg.declares.
(Same assertion as in bug 33157.)

I am a bit confused about how dbg.declare is supposed to work in
this case. The documentation says: "This intrinsic provides
information about a local element (e.g., variable).  The first
argument is metadata holding the alloca for the variable." But
there is no alloca associated with %p1 here. Is this an omission
in the documentation?

Are duplicated dbg.declares allowed? Then the bug is in
DwarfDebug for crashing, not in loop unswitch.

Reproduce like this:
opt -S -inline -functionattrs -licm -loop-unswitch -o - inline.ll | grep
dbg.declare | sort | uniq -c
      1   call void @llvm.dbg.declare(metadata %struct.S* %dum, metadata !18,
metadata !19), !dbg !20
      2   call void @llvm.dbg.declare(metadata %struct.S* %p1, metadata !18,
metadata !19), !dbg !34
      1   call void @llvm.dbg.declare(metadata %struct.S* %p1, metadata !31,
metadata !19), !dbg !32
      1 declare void @llvm.dbg.declare(metadata, metadata, metadata) #1

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