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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - llc generates wrong code for x86 with -march=atom"
href="https://llvm.org/bugs/show_bug.cgi?id=26876">26876</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>llc generates wrong code for x86 with -march=atom
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>3.7
</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>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>arne.hasselbring@googlemail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=16004" name="attach_16004" title="A minimal source code example that compiles to bad code">attachment 16004</a> <a href="attachment.cgi?id=16004&action=edit" title="A minimal source code example that compiles to bad code">[details]</a></span>
A minimal source code example that compiles to bad code
Compiling the attached file with the following command line gives a program
that crashes when returning from foo because of improper stack operations.
Compilation command line:
$ clang++ -std=c++14 -m32 -march=atom -o prog main.cpp
Executing prog results in
$ ./prog
Before foo()
Segmentation fault
This seems to happen if std::chrono::system_clock::now() is the last operation
in a function. It doesn't seem to matter whether the result is written into a
variable or not. Initially the problem occured when initializing the last
member variable of a class with now(). For this minimal example, I had to add
an __attribute__((noinline)) to foo() to get the error, but it is exactly the
same as in the original code.
I traced this down to an error in the generated assembly code. foo is compiled
to the following code:
8048780: 55 push %ebp
8048781: 89 e5 mov %esp,%ebp
8048783: 8d 64 24 e8 lea -0x18(%esp),%esp
8048787: 8d 45 f8 lea -0x8(%ebp),%eax
804878a: 89 04 24 mov %eax,(%esp)
804878d: e8 3e fe ff ff call 80485d0
<_ZNSt6chrono3_V212system_clock3nowEv@plt>
8048792: 8d 64 24 19 lea 0x19(%esp),%esp
8048796: 5d pop %ebp
8048797: c3 ret
You can see that in the beginning, 0x18 is subtracted from %esp and in the end,
0x19 is added. std::chrono::system_clock::now() adds another 4 bytes to %esp
(by returning with ret 0x04, I suspect this has to do with some ABI
callee-cleanup things), so in the end, the stack pointer is 5 bytes higher than
it should be. So it leads to a corrupted stack and the ret at 0x8048797 jumps
into some undefined place.
Compiling without -march=atom leads to a correct addition of 0x14 to %esp.
The clang/llvm is compiled by crosstool-ng, clang++ --version gives:
clang version 3.7.0 (<a href="https://github.com/llvm-mirror/clang.git">https://github.com/llvm-mirror/clang.git</a>
40b68b4c02b9d9e1e4138815747adf5589496240)
(<a href="https://github.com/llvm-mirror/llvm.git">https://github.com/llvm-mirror/llvm.git</a>
17611b180b77406671cf0819a452dd128ce7481c)</pre>
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