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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - static_cast<T*>(this) does not propagate non-null invariant of 'this'"
href="https://bugs.llvm.org/show_bug.cgi?id=35909">35909</a>
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<th>Summary</th>
<td>static_cast<T*>(this) does not propagate non-null invariant of 'this'
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>All
</td>
</tr>
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<th>OS</th>
<td>All
</td>
</tr>
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<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>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>dave@znu.io
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
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<pre>static_cast<T*>(this) does not propagate non-null invariant of 'this'.
Consider the following code (found during code gen analysis of a LLVM
subproject):
class A {
int a;
};
class B {
int b;
public:
A *getAsA();
};
class X : public A, public B {
int x;
};
A *B::getAsA() {
if (b == 42) {
auto temp = static_cast<X*>(this);
//__builtin_assume(temp != nullptr);
return temp;
}
return nullptr;
}
void helper(A *);
void test(B *b) {
auto temp = b->getAsA();
if (temp)
return helper(temp);
}
WITHOUT the __builtin_assume(), the following suboptimal x86 code is generated:
movq %rdi, %rax
addq $-4, %rdi
je .LBB1_2
cmpl $42, (%rax)
jne .LBB1_2
jmp _Z6helperP1A # TAILCALL
.LBB1_2:
retq
WITH the __builtin_assume(), the following optimal x86 code is generated:
cmpl $42, (%rdi)
jne .LBB1_1
addq $-4, %rdi
jmp _Z6helperP1A # TAILCALL
.LBB1_1:
retq</pre>
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