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<base href="http://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Clang loses alignment of union members"
href="http://llvm.org/bugs/show_bug.cgi?id=15424">15424</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Clang loses alignment of union members
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</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>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>jyasskin@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Clang seems to need an alignment to be repeated on fields of a union even
though they're guaranteed to have the highest alignment of any field.
$ cat test.c
#include <string.h>
typedef float vec __attribute__((vector_size(32)));
union U {
vec v;
char /*__attribute__((aligned(__alignof__(vec))))*/ c[sizeof(vec)];
};
void copy_vec(union U* u, const vec* v) { u->v = *v; }
void memcpy_char(union U* u, const vec* v) { memcpy(u->c, v, sizeof(vec)); }
void memcpy_union(union U* u, const vec* v) { memcpy(u, v, sizeof(vec)); }
Uncommenting the alignment above fixes the problem below.
Note the alignment of "16" in the memcpy call in memcpy_char below, as opposed
to the 32-byte alignment of the call in memcpy_union.
$ clang -Xclang -target-feature -Xclang +avx -S test.c -O2 -Wall -o -
-emit-llvm
...
define void @memcpy_char(%union.U* nocapture %u, <8 x float>* nocapture %v) #0
{
%1 = bitcast %union.U* %u to i8*
%2 = bitcast <8 x float>* %v to i8*
tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %1, i8* %2, i64 32, i32 16, i1
false)
ret void
}
define void @memcpy_union(%union.U* nocapture %u, <8 x float>* nocapture %v) #0
{
%1 = bitcast %union.U* %u to i8*
%2 = bitcast <8 x float>* %v to i8*
tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %1, i8* %2, i64 32, i32 32, i1
false)
ret void
}
...
This has the effect of pessimizing the generated code into not using AVX
instructions:
$ clang -Xclang -target-feature -Xclang +avx -S test.c -O2 -Wall -o -
copy_vec: # @copy_vec
.cfi_startproc
# BB#0:
vmovaps (%rsi), %ymm0
vmovaps %ymm0, (%rdi)
vzeroupper
ret
memcpy_char: # @memcpy_char
.cfi_startproc
# BB#0:
vmovups 16(%rsi), %xmm0
vmovups %xmm0, 16(%rdi)
vmovaps (%rsi), %xmm0
vmovaps %xmm0, (%rdi)
ret
memcpy_union: # @memcpy_union
.cfi_startproc
# BB#0:
vmovaps (%rsi), %ymm0
vmovaps %ymm0, (%rdi)
vzeroupper
ret
This is clang r175875.</pre>
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