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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 - [missed optimization] failure to drop unused string if initialized after construction"
href="https://bugs.llvm.org/show_bug.cgi?id=45288">45288</a>
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<th>Summary</th>
<td>[missed optimization] failure to drop unused string if initialized after construction
</td>
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<th>Product</th>
<td>clang
</td>
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<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>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<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>C++
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>eyalroz@technion.ac.il
</td>
</tr>
<tr>
<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>(GodBolt example: <a href="https://godbolt.org/z/tYM3kS">https://godbolt.org/z/tYM3kS</a> )
Consider the following two functions:
void foo() {
std::string s { "This is not a small string" };
}
void bar() {
std::string s;
s = "This is not a small string";
}
clang++ is allowed, and should, optimize away the local string, and just have
these functions do nothing, compiling to :
foo():
ret
bar():
ret
but in fact (and with libc++), the string is dropped only for the first
function, not the second one:
foo(): # @foo()
ret
bar(): # @bar()
push rbx
sub rsp, 32
xorps xmm0, xmm0
movaps xmmword ptr [rsp], xmm0
mov qword ptr [rsp + 16], 0
mov rdi, rsp
mov esi, offset .L.str
call std::__1::basic_string<char, std::__1::char_traits<char>,
std::__1::allocator<char> >::assign(char const*)
test byte ptr [rsp], 1
je .LBB1_3
mov rdi, qword ptr [rsp + 16]
call operator delete(void*)
.LBB1_3:
add rsp, 32
pop rbx
ret
mov rbx, rax
test byte ptr [rsp], 1
je .LBB1_6
mov rdi, qword ptr [rsp + 16]
call operator delete(void*)
.LBB1_6:
mov rdi, rbx
call _Unwind_Resume
.L.str:
.asciz "This is not a small string"
This bug seems very much related to 45287, although here it's all libc++ code,
no libstdc++ involved. Still, I'm seeing exception code, which makes me assume
this also depends on 35052.</pre>
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