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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_ASSIGNED "
title="ASSIGNED - Post RA scheduler incorrectly renames callee saved register"
href="https://bugs.llvm.org/show_bug.cgi?id=32608">32608</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Post RA scheduler incorrectly renames callee saved register
</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>ASSIGNED
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Common Code Generator Code
</td>
</tr>
<tr>
<th>Assignee</th>
<td>carrot@google.com
</td>
</tr>
<tr>
<th>Reporter</th>
<td>carrot@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Compile the following code with options
-fno-exceptions '-mcpu=power8' -O2 '-std=gnu++11'
class StringPiece {
public:
StringPiece(const char* data, int len): ptr_(data), length_(len) {}
int find(const char* s, int pos = 0);
const char* ptr_;
int length_;
};
bool foo(const char* hostname, int hostname_len) {
if (hostname_len <= 0)
return false;
const char* hostname_end = hostname + hostname_len;
if ((hostname_end[-1] == ' '))
return false;
if ((hostname[0] == '.') || (hostname_end[-2] == '.'))
return false;
if (hostname_len >= 4 && StringPiece(hostname, hostname_len).find("..", 0) !=
-1)
return false;
return true;
}
LLVM at revision r298955 generates following code for the later part of the
function
...
std 30, 48(1) # 8-byte Folded Spill
li 30, 0
addi 4, 3, .L.str@toc@l
addi 3, 1, 32
bl _ZN11StringPiece4findEPKci
nop
li 4, 1
cmpwi 0, 3, -1
ld 12, 48(1) # 8-byte Folded Reload
isel 3, 4, 30, 2
addi 1, 1, 64
ld 0, 16(1)
mtlr 0
blr
The instruction
ld 12, 48(1)
should be
ld 30, 48(1)
and moved after isel.
The problem can't be reproduced without patch r298955, but the bug is still
there.
The root cause is in post RA scheduler, llvm tries to break anti/output
dependence with register renaming. But for callee saved register restore
instruction, there is no explicit use of the register, simply rename the load
instruction can't restore the value of the register correctly.
We should not rename the callee saved register restore instruction.</pre>
</div>
</p>
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