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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - std::memcpy with incorrect align attribute on arguments introduces undefined behavior"
href="https://bugs.llvm.org/show_bug.cgi?id=49708">49708</a>
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<th>Summary</th>
<td>std::memcpy with incorrect align attribute on arguments introduces undefined behavior
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>All
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<th>OS</th>
<td>All
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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
</td>
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<th>Component</th>
<td>LLVM Codegen
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>gonzalo.gadeschi@gmail.com
</td>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>Example (<a href="https://clang.godbolt.org/z/7r8qndWzG">https://clang.godbolt.org/z/7r8qndWzG</a>):
#include <cstring>
void f(int* src, int* dst) {
std::memcpy(src, dst, sizeof(int));
}
void g(char* a, char *b) {
f((int*)a, (int*)b);
}
lowers to: call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 %6, i8* align 4 %8,
i64 4, i1 false)
The "align 4" in the pointer arguments of memcpy is incorrect.
No C++ code loads/stores from the src and dst pointers at type "int", and IIUC
std::memcpy(void*,void*,size) access its arguments at type "unsigned char*", so
TBAA does not allow inferring that the pointers are 4 byte aligned in the
memcpy.
As a consequence, the memcpy loads/stores with an incorrect alignment, and the
behavior is undefined.</pre>
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