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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [X86] LLVM 7.0.x optimises out variable init at -O1"
href="https://bugs.llvm.org/show_bug.cgi?id=40681">40681</a>
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<th>Summary</th>
<td>[X86] LLVM 7.0.x optimises out variable init at -O1
</td>
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<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>7.0
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Backend: X86
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>vit9696@avp.su
</td>
</tr>
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<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<pre>Created <span class=""><a href="attachment.cgi?id=21463" name="attach_21463" title="Test C file">attachment 21463</a> <a href="attachment.cgi?id=21463&action=edit" title="Test C file">[details]</a></span>
Test C file
LLVM 7.0 generates invalid code optimises out variable zeroing for 32-bit X86
at -O1 or higher when sanitizers are enabled. I was able to reproduce the issue
with AddressSanitizer or UndefinedBehaviorSanitizer enabled, yet I believe they
are just the trigger point. The IR looks fine, so most likely the issue lies in
LLVM itself.
The bug is not reproducible on LLVM 8.0 or trunk. If LLVM 7.1 release is
abandoned, it should be closed, otherwise I believe it is to be release
blocker.
Test example is provided in the attachment. Both C file and generated .S file.
clang -S -c -target i386-gnu-linux -march=pentium2 -pipe -nostdinc
-fno-asynchronous-unwind-tables -O1 -fno-builtin -I. -fno-omit-frame-pointer
-m32 -fno-stack-protector -fsanitize=address -c d.c -o d.S
Relevant comments for generated asm:
pushl %esi
...
# implicit-def: $esi ; allocates r temporary in %esi, which is filled with
random data
...
movl %esi, -16(%ebp)
...
calll func1
testl %eax, %eax
movl -16(%ebp), %ecx ; writes random data to %ecx
cmovsl %eax, %ecx ; if (%eax < 0) %ecx = %eax
movl %ecx, -16(%ebp) ; %ecx is returned back to stack
...
jns .LBB0_11 → if (%eax < 0) goto 11
jmp .LBB0_19
...
.LBB0_19:
...
movl -16(%ebp), %eax ; function returns random data when func1 returns >= 0
...
ret</pre>
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