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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Optimize away exception allocation and throws handled by catch"
href="https://bugs.llvm.org/show_bug.cgi?id=35052">35052</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Optimize away exception allocation and throws handled by catch
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>All
</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>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>antoshkka@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Following function
void foo() {
try {
throw 1;
} catch(...) {}
}
produces the following assembly at -O2:
foo(): # @foo()
push rax
mov edi, 4
call __cxa_allocate_exception
mov dword ptr [rax], 1
mov esi, offset typeinfo for int
xor edx, edx
mov rdi, rax
call __cxa_throw
mov rdi, rax
call __cxa_begin_catch
pop rax
jmp __cxa_end_catch
This is suboptimal as the exception is catched and ignored. Optimal assembly
would be:
foo():
ret
For exception classes with user provided constructors and destructors that have
side effect, leave the calls to constructor and destructor but do no actually
throw the exception:
foo():
call user_exception()
call ~user_exception()
rep ret
The `catch(...){}` expression used widely in C++ code, optimizing it would
improve binary size and performance a lot.
Optimization must happen after the inlining. Some realistic code examples were
the optimization could be very useful if triggered after inlining:
// Destructors
bar::~bar() {
try {
cleanup_some_dangerous_resources();
} catch(...) {}
}
// Logging
#define SafeLog(X) try { g_logger << X << std::endl; } catch (...){}
// Poor Try* functions
bool try_parse(const char* text) {
try {
parse_text(text); // throws
} catch (...) {
return false;
}
return true;
}</pre>
</div>
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