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<head>
<base href="http://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Compiler misses struct field alignment optimization opportunities."
href="http://llvm.org/bugs/show_bug.cgi?id=19882">19882</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Compiler misses struct field alignment optimization opportunities.
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>3.3
</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>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>C++
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>jujjyl@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Consider the following example:
#include <cstdint>
#include <stdio.h>
#include <cassert>
struct __attribute__((packed)) foo // packed attribute: we expect that the
compiler tightly lays out double after int, so total struct size 12 bytes.
{
__attribute__((aligned(4))) int x; // aligned attribute: we expect that
in any instance of this struct, the int is always aligned to 4 bytes.
double d; // Since this struct is packed, and the int before this is
aligned to 4 bytes, we can deduce that this double is also aligned to four
bytes!
};
char data[256];
__attribute__((noinline)) foo *getfoo() // Make compiler not optimize out the
whole thing we're doing at compiletime.
{
return (foo*)data;
}
int main()
{
assert(sizeof(foo) == 12);
foo *f = getfoo();
f->x = 42;
f->d = 1.0; // We expect this to perform a two 4-byte stores to write
the double.
printf("addr: %p, value: %f\n", &f->d, f->d); // We expect this to
perform load of the double as two 4-byte loads.
}
One would expect that in the generated bit code, the store and load of f->d
would be marked as aligned to 4 bytes, since it could be deduced from the
alignment of the previous field f->x, but this does not seem to be the case,
and f->d is treated as unaligned (aligned to 1 byte). Is it possible to improve
this analysis so that one doesn't need to annotate every field of a struct with
the __attribute__((aligned(4)))?
Tested in Emscripten fork of Clang 3.3. See also
<a href="https://github.com/kripken/emscripten/issues/2378">https://github.com/kripken/emscripten/issues/2378</a> for discussion of this in
Emscripten repo.</pre>
</div>
</p>
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