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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 --- - 8-bit and 16-bit atomic operations return wrongly extended results"
href="https://llvm.org/bugs/show_bug.cgi?id=27599">27599</a>
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<th>Summary</th>
<td>8-bit and 16-bit atomic operations return wrongly extended results
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>Other
</td>
</tr>
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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>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Backend: SystemZ
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>koriakin@0x04.net
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<th>Classification</th>
<td>Unclassified
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</tr></table>
<p>
<div>
<pre>The following function:
char f(char *x, char y, char z) {
return __sync_val_compare_and_swap(x, y, z);
}
compiles to that:
define zeroext i8 @f(i8* nocapture, i8 zeroext, i8 zeroext) #0 {
%4 = cmpxchg i8* %0, i8 %1, i8 %2 seq_cst seq_cst
%5 = extractvalue { i8, i1 } %4, 0
ret i8 %5
}
which compiles to that:
f: # @f
# BB#0:
stmg %r13, %r15, 104(%r15)
risbg %r1, %r2, 0, 189, 0
sll %r2, 3
lcr %r5, %r2
l %r0, 0(%r1)
.LBB0_1: # =>This Inner Loop Header: Depth=1
rll %r14, %r0, 8(%r2)
risbg %r3, %r14, 32, 55, 0
crjlh %r14, %r3, .LBB0_3
# BB#2: # in Loop: Header=BB0_1 Depth=1
risbg %r4, %r14, 32, 55, 0
rll %r13, %r4, -8(%r5)
cs %r0, %r13, 0(%r1)
jl .LBB0_1
.LBB0_3:
llgfr %r2, %r14
lmg %r13, %r15, 104(%r15)
br %r14
The function ends with a llgfr, zero extending i32 to i64 - however, the high
24 bits of %r14 contain junk at this point, so the result will be wrong. This
breaks ASan tests when the ASan runtime is compiled by clang.
It seems CodeGen assumes that sub-word atomic operations return zero-extended
(or sign-extended if TLI says so) results, and SystemZ doesn't deliver.</pre>
</div>
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