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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - llvm.dbg.value uses out-of-scope local variable with -O1 or higher"
href="https://bugs.llvm.org/show_bug.cgi?id=51155">51155</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>llvm.dbg.value uses out-of-scope local variable with -O1 or higher
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>10.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>ryan.gl.scott@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=25043" name="attach_25043" title="The original C code.">attachment 25043</a> <a href="attachment.cgi?id=25043&action=edit" title="The original C code.">[details]</a></span>
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.</pre>
</div>
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