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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 - DSE fails to remove stores to a structure overwritten by asm."
href="https://bugs.llvm.org/show_bug.cgi?id=42672">42672</a>
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<th>Summary</th>
<td>DSE fails to remove stores to a structure overwritten by asm.
</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>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>glider@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>In the following example:
void _raw_spin_lock(void)
{
#if STRUCT
struct {
unsigned short owner, next;
} lockval;
lockval.owner = 1;
lockval.next = 2;
#else
int lockval;
lockval = 3;
#endif
asm ("nop" : "=r" (lockval));
}
|lockval| is declared as assembly output that is overwritten by the inline
assembly (as specified by constraints).
If |lockval| is a scalar type, its initialization is correctly omitted.
However if |lockval| is a struct, DSE is unable to figure out that the stores
to its fields are effectively dead:
$ clang -O2 -USTRUCT -c spinlock.c && objdump -d spinlock.o
spinlock.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <_raw_spin_lock>:
0: c3 retq
$ clang -O2 -DSTRUCT -c spinlock.c && objdump -d spinlock.o
spinlock.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <_raw_spin_lock>:
0: c7 44 24 f8 01 00 02 movl $0x20001,-0x8(%rsp)
7: 00
8: 90 nop
9: c3 retq</pre>
</div>
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