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<base href="https://bugs.llvm.org/">
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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 - Deduce memcpy copy range"
href="https://bugs.llvm.org/show_bug.cgi?id=41782">41782</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Deduce memcpy copy range
</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>Linux
</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>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>bool foo(int *a, int *b, unsigned n) {
memcpy(a, b, 8); // memcpy expanded
return a == b;
}
foo(int*, int*, unsigned int): # @foo(int*, int*,
unsigned int)
mov rax, qword ptr [rsi]
mov qword ptr [rdi], rax
cmp rdi, rsi
sete al
ret
bool foo1(int *a, int *b, unsigned n) {
n = (n % 9);
memcpy(a, b, n);
return a == b;
}
bool foo2(int *a, int *b, unsigned n) {
while (n > 8) {
--n;
}
memcpy(a, b, n);
return a == b;
}
Clang emits memcpy call in foo1 and foo2, Clang does not expand it as in the
"foo" case, but it should.
foo1(int*, int*, unsigned int): # @foo(int*, int*,
unsigned int)
push r14
push rbx
push rax
mov r14, rsi
mov rbx, rdi
mov eax, edx
imul rax, rax, 954437177
shr rax, 33
lea eax, [rax + 8*rax]
sub edx, eax
call memcpy
cmp rbx, r14
sete al
add rsp, 8
pop rbx
pop r14
ret
foo2(int*, int*, unsigned int): # @foo(int*, int*,
unsigned int)
push r14
push rbx
push rax
mov r14, rsi
mov rbx, rdi
cmp edx, 8
mov eax, 8
cmovae edx, eax
call memcpy
cmp rbx, r14
sete al
add rsp, 8
pop rbx
pop r14
ret</pre>
</div>
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