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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 - MSVC and Clang use different values for _MM_HINT constants; Windows SDK hardcodes MSVC values"
href="https://bugs.llvm.org/show_bug.cgi?id=37400">37400</a>
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<th>Summary</th>
<td>MSVC and Clang use different values for _MM_HINT constants; Windows SDK hardcodes MSVC values
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>6.0
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Headers
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>fabiang@radgametools.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>With clang-cl 6.0:
// ---- begin
#include <emmintrin.h>
#include <Windows.h> // <--- only with this present!
void f(const void *p)
{
_mm_prefetch((const char *)p, _MM_HINT_T0);
}
// ---- end
"clang-cl -c -O2 -FA prefetch.cpp" yields (only relevant parts):
# ---- begin
"?f@@YAXPEBX@Z": # @"\01?f@@YAXPEBX@Z"
# %bb.0:
prefetcht2 (%rcx)
retq
# ---- end
...huh? Some grepping later, it turns out that "um\winnt.h" in the Windows SDK
10.0.16299.0 (and presumably other versions as well, but I didn't check)
contains this:
C:\Program Files (x86)\Windows Kits\10\Include\10.0.16299.0>rg MM_HINT_T0
um\winnt.h
3266:#define _MM_HINT_T0 1
3296:#define PF_TEMPORAL_LEVEL_1 _MM_HINT_T0
7349:#define _MM_HINT_T0 1
7366:#define PF_TEMPORAL_LEVEL_1 _MM_HINT_T0
and indeed, the MSVC version of xmmintrin.h has _MM_HINT_T0 #defined to 1.
Long story short, for any translation unit that includes Windows.h,
_MM_HINT_* end up re-#defined to MSVC-specific values, which produce the wrong
instructions with clang-cl.
If the goal is to make clang-cl be able to compile apps using unmodified
Windows headers, then Clang needs to use the same values for _MM_HINT_* as MSVC
does. (Presumably with some remapping done in the frontend.) Sigh.</pre>
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