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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 - Missed optimization: movups instruction is used even when the memory is known to be aligned"
href="https://bugs.llvm.org/show_bug.cgi?id=46479">46479</a>
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<th>Summary</th>
<td>Missed optimization: movups instruction is used even when the memory is known to be aligned
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>10.0
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: X86
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>josephcsible@gmail.com
</td>
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<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<p>
<div>
<pre>Consider this C code:
extern struct foo {
long x, y, z;
} s;
void f(struct foo);
void g(void) {
f(s);
}
At -O3, clang compiles it into this:
g:
subq $24, %rsp
movq s+16(%rip), %rax
movq %rax, 16(%rsp)
movups s(%rip), %xmm0
movups %xmm0, (%rsp)
callq f
addq $24, %rsp
retq
Because of ABI requirements, the stack pointer must be 16-byte aligned right
before "callq f", so the second "movups" can instead be a "movaps".</pre>
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