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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 - potential missing shrink wrapping on x86_64"
href="https://bugs.llvm.org/show_bug.cgi?id=34786">34786</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>potential missing shrink wrapping on x86_64
</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>All
</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>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>compnerd@compnerd.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>```
void (*e)(void *);
extern void g(void *);
void f(void *p) {
if (__builtin_expect(e != (void *)0, 0))
e(p);
g(p);
}
```
%clang_cc1 -mrelocation-model static -triple i386-apple-macosx10.4.0 -S -Os -o
- %s
Generates:
```
_f:
movl _e, %eax
testl %eax, %eax
jne LBB0_1
jmp _g
LBB0_1:
subl $12, %esp
movl 16(%esp), %ecx
movl %ecx, (%esp)
calll *%eax
addl $12, %esp
jmp _g
```
However, note the following on x86_64:
%clang_cc1 -mrelocation-model static -triple x86_64-apple-macosx10.4.0 -S -Os
-o - %s
```
_f:
pushq %rbx
movq %rdi, %rbx
movq _e@GOTPCREL(%rip), %rax
movq (%rax), %rax
testq %rax, %rax
jne LBB0_1
LBB0_2:
movq %rbx, %rdi
popq %rbx
jmp _g
LBB0_1:
movq %rbx, %rdi
callq *%rax
jmp LBB0_2
```
On the case of x86_64, we do not generate a tail call to `g`, opting to
coalesce the BBs across the tail call. However, the call to `g` is explicitly
being marked as cold via the `__builtin_expect` with the false parameter.
Is there something that I'm overlooking or is this a limitation on our shrink
wrapping? Seems that this may be related to PR33868.</pre>
</div>
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