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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 - Variable's value late availability after Instruction Selection at O3"
href="https://bugs.llvm.org/show_bug.cgi?id=51941">51941</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Variable's value late availability after Instruction Selection at O3
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</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>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>DebugInfo
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>cristianassaiante@outlook.com
</td>
</tr>
<tr>
<th>CC</th>
<td>jdevlieghere@apple.com, keith.walker@arm.com, llvm-bugs@lists.llvm.org, paul_robinson@playstation.sony.com
</td>
</tr></table>
<p>
<div>
<pre>A variable (l_33) value is visile after its declarations, then its not visible
anymore after a next command and comes back in after another next command.
>From opt-bisect, it seems that the pass after which this behaviour occurs is:
X86 DAG->DAG Instruction Selection on function (main)
$ cat a.c
short a;
int b ;
int main()
{
{int l_33 = 0;
int *c = &l_33;
*c = a
;
*c ^= 2
;
b = l_33 ;
}
}
$ clang -v
clang version 13.0.0
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.5.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.5.0
Candidate multilib: .;@m64
Selected multilib: .;@m64
$ lldb -v
lldb version 13.0.0
$ clang -g -O3 -o opt a.c
$ lldb opt
(lldb) target create "opt"
Current executable set to
'/home/stepping/debuginfo-analysis/issues/issue14/opt' (x86_64).
(lldb) b main
Breakpoint 1: where = opt`main at a.c:7:12, address = 0x0000000000400480
(lldb) run
Process 28 launched: '/home/stepping/debuginfo-analysis/issues/issue14/opt'
(x86_64)
Process 28 stopped
* thread #1, name = 'opt', stop reason = breakpoint 1.1
frame #0: 0x0000000000400480 opt`main at a.c:7:12
4 {
5 {int l_33 = 0;
6 int *c = &l_33;
-> 7 *c = a
8 ;
9 *c ^= 2
10 ;
(lldb) frame var
(int) l_33 = 0
(int *) c = <no location, value may have been optimized out>
(lldb) n
Process 28 stopped
* thread #1, name = 'opt', stop reason = step over
frame #0: 0x0000000000400487 opt`main at a.c:9:12
6 int *c = &l_33;
7 *c = a
8 ;
-> 9 *c ^= 2
10 ;
11 b = l_33 ;
12 }
(lldb) frame var
(int) l_33 = <variable not available>
(int *) c = <no location, value may have been optimized out>
(lldb) n
Process 28 stopped
* thread #1, name = 'opt', stop reason = step over
frame #0: 0x000000000040048a opt`main at a.c:11:11
8 ;
9 *c ^= 2
10 ;
-> 11 b = l_33 ;
12 }
13 }
(lldb) frame var
(int) l_33 = 2
(int *) c = <no location, value may have been optimized out></pre>
</div>
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