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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 - [Pwr8] legalization of bitcast between i16 to half"
href="https://bugs.llvm.org/show_bug.cgi?id=49092">49092</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>[Pwr8] legalization of bitcast between i16 to half
</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>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Backend: PowerPC
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>v.churavy@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, nemanja.i.ibm@gmail.com
</td>
</tr></table>
<p>
<div>
<pre>```
define half @convert(i16 %in) {
%f = bitcast i16 %in to half
ret half %f
}
```
llc -mcpu=pwr8
```
clrldi 3, 3, 32
bl __gnu_h2f_ieee
```
as far as I can tell this clears the upper 32 bits of `r3` and not the upper 48
bits.
`__gnu_h2f_ieee` expects a call with i16 and having non-zero higher bits can
trip up some implementations. Seen in the wild as
<a href="https://github.com/JuliaLang/julia/issues/39330">https://github.com/JuliaLang/julia/issues/39330</a> with
<a href="https://gist.github.com/vchuravy/7f7f21dadc93fb4f0aea3b5f236e360c">https://gist.github.com/vchuravy/7f7f21dadc93fb4f0aea3b5f236e360c</a> as a
reproducer:
```
[vchuravy@service0001 jl-pwr8]$ gcc -O2 -g2 -mcpu=power8 -c -fpic half.c
[vchuravy@service0001 jl-pwr8]$ gcc -shared -o libhalf.so half.o
[vchuravy@service0001 jl-pwr8]$ ../toolchain/tools/clang -mcpu=pwr8 main.ll
-L`pwd` -Wl,-rpath=`pwd` -lhalf
[vchuravy@service0001 jl-pwr8]$ ./a.out; echo $?
0
```
The expected answer for this reproducer is `1`.
Looking at that reproducer with GDB and I got the following result:
Run till exit from #0 __gnu_h2f_ieee (param=31744) at
/nobackup/users/vchuravy/dev/julia/src/intrinsics.cpp:1488
0x00000000100008f0 in julia_g_219 ()
Value returned is $1 = -inf
Run till exit from #0 __gnu_h2f_ieee (param=31744) at
/nobackup/users/vchuravy/dev/julia/src/intrinsics.cpp:1488
0x0000000010000944 in julia_g_219 ()
Value returned is $5 = inf
GDB thinks we are calling it with the same `param`, but we are passing the
argument in r3, which on the call that yields the wrong result is:
r3 0xffff7c00 4294933504
and on a call where it is the right result:
r3 0x7c00 31744
so the caller didn't clear the upper bits.
Indeed just before the call we do:
0x00000000100008e4 <+228>: mr r4,r3
0x00000000100008e8 <+232>: clrldi r3,r4,32
=> 0x00000000100008ec <+236>: bl 0x100005e0
<00000017.plt_call.__gnu_h2f_ieee>
Now a different question could be: Why are there spurious bit where there
should be none.</pre>
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