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<base href="http://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Bad code generation with cmpxchg16b and __uint128_t"
href="http://bugs.llvm.org/show_bug.cgi?id=32568">32568</a>
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<th>Summary</th>
<td>Bad code generation with cmpxchg16b and __uint128_t
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>4.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>LLVM Codegen
</td>
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<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>royger@freebsd.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=18249" name="attach_18249" title="Preprocessed file (-E)">attachment 18249</a> <a href="attachment.cgi?id=18249&action=edit" title="Preprocessed file (-E)">[details]</a></span>
Preprocessed file (-E)
Hello,
clang 3.8 and 4.0 at least seem to generate bad code around a cmpxchg16b asm
instruction that uses __uint128_t. The code is as follows:
ret = cmpxchg16b(entry, &old_ire, new_ire);
ASSERT(ret == old_ire.val);
This leads to clang generating the following asm:
mov %r15,%rdx
shr $0x20,%rdx
mov %r12,%rcx
mov %r15d,%eax
lock cmpxchg16b (%r14)
xor %eax,%eax
test %al,%al
I'm attaching the pre-processed output that causes this. The function where
this happens is update_irte.
The rune used to compile the file is:
clang -m64 -fno-strict-aliasing -std=gnu99 -Wall -Wstrict-prototypes
-Wdeclaration-after-statement -Wno-unused-local-typedefs -O1 -nostdinc
-fno-builtin -fno-common -Werror -Wredundant-decls -Wno-pointer-arith -pipe -g
-D__XEN__ -include /root/xen/xen/include/xen/config.h
'-D__OBJECT_FILE__="intremap.o"' -fno-omit-frame-pointer -MMD -MF
./.intremap.o.d -I/root/xen/xen/include
-I/root/xen/xen/include/asm-x86/mach-generic
-I/root/xen/xen/include/asm-x86/mach-default -DXEN_IMG_OFFSET=0x200000
'-D__OBJECT_LABEL__=drivers$passthrough$vtd$intremap.o' -msoft-float
-fno-stack-protector -fno-exceptions -Wnested-externs -DHAVE_GAS_VMX
-DHAVE_GAS_SSE4_2 -DHAVE_GAS_EPT -DHAVE_GAS_RDRAND -DHAVE_GAS_RDSEED
-mno-red-zone -mno-sse -fpic -fno-asynchronous-unwind-tables
-DGCC_HAS_VISIBILITY_ATTRIBUTE -c intremap.c -o intremap.o
Thanks, Roger.</pre>
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