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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [X86] Call to __tls_get_addr makes the function non-leaf after it was determined to be a leaf"
href="http://llvm.org/bugs/show_bug.cgi?id=20243">20243</a>
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<th>Summary</th>
<td>[X86] Call to __tls_get_addr makes the function non-leaf after it was determined to be a leaf
</td>
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<th>Product</th>
<td>new-bugs
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>dario.domizioli@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=12754" name="attach_12754" title="Test case source code (see description for command lines)">attachment 12754</a> <a href="attachment.cgi?id=12754&action=edit" title="Test case source code (see description for command lines)">[details]</a></span>
Test case source code (see description for command lines)
Consider a simple example which accesses the address of a thread-local
variable:
// ===== ===== tls_non_leaf.cpp ===== =====
struct SomeStruct {
int x;
};
static thread_local SomeStruct ts = {0};
SomeStruct* foo(SomeStruct *t) {
return (t == 0) ? &ts : t;
}
// ===== ===== tls_non_leaf.cpp ===== =====
In PIC mode on X86, the access to &ts is lowered to a call to __tls_get_addr.
However, the lowering of such access to such call is performed under
X86AsmPrinter::EmitInstruction (which calls lowerTlsAddr), well after the
function was analysed to determine whether it contained any function calls
(i.e. whether it was a leaf or not).
This can have issues with features like -momit-leaf-frame-pointer, because the
function is not actually a leaf but it was incorrectly considered a leaf.
To reproduce, compile the source to assembly this way:
clang -O2 -fpic -fno-omit-frame-pointer -momit-leaf-frame-pointer -S -std=c++11
-o omit.s tls_non_leaf.cpp
And then this way:
clang -O2 -fpic -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -S
-std=c++11 -o no-omit.s tls_non_leaf.cpp
In theory, since the function is not really a leaf, the different options for
the leaf frame pointer should not make a difference. However, there is a
difference in the generated code because the function is mistakenly recognized
as a leaf.
My opinion is that the expansion to __tls_get_addr happens too late, it should
happen at instruction selection stage rather than in the AsmPrinter.
I have tested this on x86_64-unknown-linux with trunk at revision 212522.
However I believe this problem affects all x86_64 triples that support TLS.</pre>
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