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<base href="https://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 --- - Union store codegen is silly"
href="https://llvm.org/bugs/show_bug.cgi?id=31777">31777</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Union store codegen is silly
</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>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>Common Code Generator Code
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>llvm-bugzilla@jfbastien.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Consider this code:
#include <inttypes.h>
#include <string.h>
union U {
uint64_t i;
uint16_t s;
};
void foo(U* u) {
u->i = 0;
u->s = 42;
}
void bar(U* u) {
memset(u, 0, sizeof(U));
u->s = 42;
}
void baz(U* u) {
u->s = 42;
}
Let's ignore that foo could ignore the zero stores (because i isn't the active
member after the store to s): the codegen for this is pretty suboptimal
(<a href="https://godbolt.org/g/yS10Ki">https://godbolt.org/g/yS10Ki</a>). The user's intent was to zero out padding bits,
and in many architectures you only need one store to do so (with, in some
cases, one materialization of the value).
x86-64:
foo(U*): # @foo(U*)
movq $0, (%rdi)
movw $42, (%rdi)
retq
bar(U*): # @bar(U*)
movq $0, (%rdi)
movw $42, (%rdi)
retq
baz(U*): # @baz(U*)
movw $42, (%rdi)
retq
ARMv8:
foo(U*):
mov r1, #0
str r1, [r0]
str r1, [r0, #4]
mov r1, #42
strh r1, [r0]
bx lr
bar(U*):
mov r1, #0
str r1, [r0]
str r1, [r0, #4]
mov r1, #42
strh r1, [r0]
bx lr
baz(U*):
mov r1, #42
strh r1, [r0]
bx lr
I'd expect foo and bar to look like what GCC 7 trunk does:
foo(U*):
movq $42, (%rdi)
ret</pre>
</div>
</p>
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