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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 - Failure to optimize tail call to jump when a pointer to a variable that dies before the tail call is passed to an external function"
href="https://bugs.llvm.org/show_bug.cgi?id=46126">46126</a>
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<th>Summary</th>
<td>Failure to optimize tail call to jump when a pointer to a variable that dies before the tail call is passed to an external function
</td>
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<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>gabravier@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>void external_function();
void takes_ptr(int *);
void f()
{
{
int x;
takes_ptr(&x);
}
external_function();
}
With -O3, LLVM produces this :
f(): # @f()
push rax
lea rdi, [rsp + 4]
call takes_ptr(int*)
call external_function()
pop rax
ret
GCC produces this :
f():
sub rsp, 24
lea rdi, [rsp+12]
call takes_ptr(int*)
add rsp, 24
jmp external_function()
Since `x` dies before `external_function` is called, there is no possibility
that `external_function` may depend on `x` still being alive (which I presume
is why GCC does not do this optimization when `x` is still alive by the time
`external_function` is called)</pre>
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