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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Improve alias analysis by checking for known callsites"
href="https://bugs.llvm.org/show_bug.cgi?id=39546">39546</a>
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<th>Summary</th>
<td>Improve alias analysis by checking for known callsites
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
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<pre>#include <string.h>
void alias(char * s, char * p) {
strcpy(s,p); // s,p cannot alias since it would be UB
p[3] = *s;
p[4] = *s;
}
GCC:
alias:
push rbx
mov rbx, rsi
call strcpy
movzx eax, BYTE PTR [rax]
mov BYTE PTR [rbx+3], al
mov BYTE PTR [rbx+4], al
pop rbx
ret
Clang -O3:
alias: # @alias
push r14
push rbx
push rax
mov r14, rsi // btw, this is a weird issue
mov rbx, rdi
call strcpy
mov al, byte ptr [rbx]
mov byte ptr [r14 + 3], al
mov al, byte ptr [rbx] // useless reload
mov byte ptr [r14 + 4], al
add rsp, 8
pop rbx
pop r14
ret
So idea:
We have ptr P and Q. If P and Q are used as in a call site (*) where aliasing
would be UB, alias analysis can just bail out with noalias answer.
* strcpy, strncpy, memcpy, bcopy.. maybe even s(n)printf</pre>
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