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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 - warning: found local symbol in global part of symbol table with Swift compiled code"
href="https://bugs.llvm.org/show_bug.cgi?id=36771">36771</a>
</td>
</tr>
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<th>Summary</th>
<td>warning: found local symbol in global part of symbol table with Swift compiled code
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>All
</td>
</tr>
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<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>alblue@apple.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>When compiling Swift on Linux with 4.1-DEVELOPMENT-SNAPSHOT-2018-03-11-a and
LLD 6.0, I see a warning generated at link time complaining that a found local
symbol in global part of symbol table:
/tmp/cross-toolchain/swift.xctoolchain/usr/bin/ld.lld: warning: found local
symbol '__stop_swift2_protocol_conformances' in global part of symbol table in
file
/private/tmp/cross-toolchain/swift.xctoolchain/usr/lib/swift/linux/libFoundation.so
/tmp/cross-toolchain/swift.xctoolchain/usr/bin/ld.lld: warning: found local
symbol '__start_swift3_typeref' in global part of symbol table in file
/private/tmp/cross-toolchain/swift.xctoolchain/usr/lib/swift/linux/libFoundation.so
/tmp/cross-toolchain/swift.xctoolchain/usr/bin/ld.lld: warning: found local
symbol '__start_swift3_reflstr' in global part of symbol table in file
/private/tmp/cross-toolchain/swift.xctoolchain/usr/lib/swift/linux/libFoundation.so
This apparently is a new feature added in LLD 6.0 (see
<a href="https://reviews.llvm.org/D41257">https://reviews.llvm.org/D41257</a>). The question is, why is Swift (which uses
LLVM as the back end) writing local symbols in a global symbol table if this is
not allowed as per the ELF spec?
According to the author of the warning:
"
Look at Figure 1-13 on this PDF page 19 of
<a href="http://www.skyfree.org/linux/references/ELF_Format.pdf">http://www.skyfree.org/linux/references/ELF_Format.pdf</a> to see what it is. It
says that sh_info member in a section header for the symbol type (of type
SHT_SYMTAB and SHT_DYNSYM) must have an index one greater than the symbol table
index of the last local symbol (binding STB_LOCAL).
So, in a symbol table, all symbols after sh_info index must be non-local. That
warning message is emitted when lld found a violation of the spec.
If you want to fix it, you probably need to sort a symbol table so that all
local symbols are at beginning of a table, and set its last index + 1 to
sh_info member of the symbol table section header.
"
The question is why isn't LLVM's ELF backend performing this step, and
generating a potentially invalid ELF file?
For cross-reference purposes, the bug at Swift.org is
<a href="https://bugs.swift.org/browse/SR-7189">https://bugs.swift.org/browse/SR-7189</a> and the corresponding location where the
Swift compiler emits the symbols is
<a href="https://github.com/apple/swift/blob/master/stdlib/public/runtime/SwiftRT-ELF.cpp#L21-L37">https://github.com/apple/swift/blob/master/stdlib/public/runtime/SwiftRT-ELF.cpp#L21-L37</a>
- the symbols are marked with an independent section and hidden visibility
symbols:
__asm__("\t.section " #name ",\"a\"\n"); \
__attribute__((__visibility__("hidden"))) extern const char __start_##name;
\
__attribute__((__visibility__("hidden"))) extern const char __stop_##name;
FWIW when LLD 5 was being used to link the project, these same symbols weren't
being added to the dynamic exports automatically (which they were when linked
with Gold). I suspect the same issue exists for both.
The generated so that was causing the complaints is downloadable from the
libFoundation.so in the Swift download, which for the above is at
<a href="https://swift.org/builds/swift-4.1-branch/ubuntu1604/swift-4.1-DEVELOPMENT-SNAPSHOT-2018-03-11-a/swift-4.1-DEVELOPMENT-SNAPSHOT-2018-03-11-a-ubuntu16.04.tar.gz">https://swift.org/builds/swift-4.1-branch/ubuntu1604/swift-4.1-DEVELOPMENT-SNAPSHOT-2018-03-11-a/swift-4.1-DEVELOPMENT-SNAPSHOT-2018-03-11-a-ubuntu16.04.tar.gz</a>
and the file at
swift-4.1-DEVELOPMENT-SNAPSHOT-2018-03-11-a-ubuntu16.04/usr/lib/swift/linux/libFoundation.so</pre>
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