[llvm-bugs] [Bug 51155] New: llvm.dbg.value uses out-of-scope local variable with -O1 or higher

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51155

            Bug ID: 51155
           Summary: llvm.dbg.value uses out-of-scope local variable with
                    -O1 or higher
           Product: clang
           Version: 10.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: ryan.gl.scott at gmail.com
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

Created attachment 25043
  --> https://bugs.llvm.org/attachment.cgi?id=25043&action=edit
The original C code.

The attached C file, when compiled with -O1 using Clang 10 or higher:

```
$ clang --version
clang version 10.0.0-4ubuntu1 
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

$ clang -S -g -O1 -emit-llvm -fno-discard-value-names test.c
```

Will produce the following bitcode for the addflt function:

```
; Function Attrs: norecurse nounwind readnone uwtable
define dso_local i32 @addflt(i32 %a, i32 %b) local_unnamed_addr #0 !dbg !10 {
entry:
  call void @llvm.dbg.value(metadata i32 %a, metadata !14, metadata
!DIExpression()), !dbg !21
  call void @llvm.dbg.value(metadata i32 %b, metadata !15, metadata
!DIExpression()), !dbg !21
  call void @llvm.dbg.value(metadata i32 %a, metadata !16, metadata
!DIExpression()), !dbg !21
  call void @llvm.dbg.value(metadata i32 %b, metadata !17, metadata
!DIExpression()), !dbg !21
  %shr = lshr i32 %a, 24, !dbg !22
  %sub = add nsw i32 %shr, -128, !dbg !23
  call void @llvm.dbg.value(metadata i32 %sub, metadata !18, metadata
!DIExpression()), !dbg !21
  %shr1 = lshr i32 %b, 24, !dbg !24
  call void @llvm.dbg.value(metadata i32 %sub2.neg, metadata !19, metadata
!DIExpression()), !dbg !21
  %sub2.neg = sub nsw i32 128, %shr1, !dbg !25
  %sub3 = add nsw i32 %sub2.neg, %sub, !dbg !26
  call void @llvm.dbg.value(metadata i32 %sub3, metadata !20, metadata
!DIExpression()), !dbg !21
  %shr4 = lshr i32 %b, %sub3, !dbg !27
  call void @llvm.dbg.value(metadata i32 %shr4, metadata !17, metadata
!DIExpression()), !dbg !21
  call void @llvm.dbg.value(metadata i32 undef, metadata !16, metadata
!DIExpression()), !dbg !21
  %add = sub i32 0, %a, !dbg !28
  %tobool = icmp eq i32 %shr4, %add, !dbg !28
  %add5 = add nsw i32 %shr, -127, !dbg !30
  %spec.select = select i1 %tobool, i32 %sub, i32 %add5, !dbg !32
  call void @llvm.dbg.value(metadata i32 %spec.select, metadata !18, metadata
!DIExpression()), !dbg !21
  ret i32 %spec.select, !dbg !33
}
```

The unusual part about this bitcode are these lines:

```
  call void @llvm.dbg.value(metadata i32 %sub2.neg, metadata !19, metadata
!DIExpression()), !dbg !21
  %sub2.neg = sub nsw i32 128, %shr1, !dbg !25
```

The call to @llvm.dbg.value mentions the local variable %sub2.neg before it is
declared. This does not appear to be a problem for LLVM itself, but I am using
an LLVM-based tool which becomes confused by out-of-scope local variables such
as %sub2.neg. As a user, I would expect %sub2.neg to be declared before being
used as a function argument, so I'm reporting this as a bug. If this is
expected behavior, however, it would be helpful to document this more clearly,
as I couldn't find any written documentation on the particular semantics of how
@llvm.dbg.value's arguments should be scoped.

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