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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Dead store to aliasing bytes not removed on x86-68"
href="https://bugs.llvm.org/show_bug.cgi?id=40631">40631</a>
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<th>Summary</th>
<td>Dead store to aliasing bytes not removed on x86-68
</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>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>dvyukov@google.com, kcc@google.com, llvm-bugs@lists.llvm.org
</td>
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<pre>Created <span class=""><a href="attachment.cgi?id=21440" name="attach_21440" title="ip_tables.i">attachment 21440</a> <a href="attachment.cgi?id=21440&action=edit" title="ip_tables.i">[details]</a></span>
ip_tables.i
Compiling the attached repro (a simplified version of
ipv4/netfilter/ip_tables.c from the Linux kernel) as follows:
$ clang -fno-common -O2 -ftrivial-auto-var-init=pattern
-enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang -c
-o ip_tables.i.s ip_tables.i -S -mllvm -print-after-all -O2
produces redundant stores to acpar.hotdrop, acpar.thoff and acpar.fragoff in
the LLVM IR that end up in the object code:
ipt_do_table: # @ipt_do_table
.cfi_startproc
# %bb.0:
subq $40, %rsp
.cfi_def_cfa_offset 48
movq (%rdi), %rax
xorps %xmm0, %xmm0
movaps %xmm0, (%rsp)
movaps %xmm0, 16(%rsp)
movq $0, 32(%rsp)
movaps .LCPI0_0(%rip), %xmm0 # xmm0 =
[12297829382473034410,12297829382473034410]
movaps %xmm0, (%rsp)
movabsq $-6148914691236517206, %rcx # imm = 0xAAAAAAAAAAAAAAAA
movq %rcx, 16(%rsp)
movq %rcx, 24(%rsp)
movb $-86, 32(%rsp)
movzwl 2(%rax), %ecx
andl $8191, %ecx # imm = 0x1FFF
movl %ecx, 24(%rsp)
movzbl (%rax), %eax
andl $15, %eax
movl %eax, 28(%rsp)
movb $0, 32(%rsp)
(see the stores to 32(%rsp), 28(%rsp) and 24(%rsp))</pre>
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