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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 - operator new with throw() omits NULL check at -O1 and higher"
href="https://bugs.llvm.org/show_bug.cgi?id=37109">37109</a>
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<tr>
<th>Summary</th>
<td>operator new with throw() omits NULL check at -O1 and higher
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>5.0
</td>
</tr>
<tr>
<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>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>C++
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>mattmiller1@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>I tested this example in various compilers on godbolt using "-fno-exceptions"
and
various -O levels.
<a href="https://godbolt.org/g/egWwC2">https://godbolt.org/g/egWwC2</a>
---------------------------------------
#include <stddef.h>
extern void * operator new(size_t sz) throw();
class Foo {
public:
Foo(int i) : _i(i) {}
private:
int _i;
};
void *
test_new() {
Foo *f = new Foo(5);
return f;
}
---------------------------------------
The expectation is if the operator new has a throw() keyword, there
will be a NULL check before initialization is done and no NULL check if the
throw() keyword is omitted.
This expectation is based on [expr.new]p13 that I find in this copy
of the standard:
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1905.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1905.pdf</a>
<quote>
[Note: unless an allocation function is declared with an empty
exception-specification (15.4), throw(), it indicates
failure to allocate storage by throwing a bad_alloc exception (clause 15,
18.4.2.1); it returns a non-null pointer otherwise.
If the allocation function is declared with an empty exception-specification,
throw(), it returns null to indicate failure
to allocate storage and a non-null pointer otherwise. — end note ] If the
allocation function returns null, initialization
shall not be done, the deallocation function shall not be called, and the value
of the new-expression shall be null.
</quote>
The results I got for various permutations of compiler this code in godbolt
are:
1. gcc (any version/any -O/new declaration w/ throw()): NULL check before
constructor init.
2. gcc (any version/any -O/new declaration w/o throw()): no NULL check before
constructor init.
3. clang (any version/any -O/new declaration w/o throw()): no NULL check before
constructor init.
4. clang (3.3 and prior/any -O/new declaration w/ throw()): NULL check before
constructor init.
5. clang (3.4.1 and later/-O0/new declaration w/ throw()): NULL check before
constructor init.
6. clang (3.4.1 and later/-O1 and higher/new declaration w/ throw()): no NULL
check before constructor init.
7. clang (3.4.1 and later/-O0/new declaration w/ throw()): NULL check before
constructor init.
E.g., below is the x86_64 assembly for clang 3.3 vs. 5.0.0 with -O3:
clang 3.3, -O3 -fno-exceptions
---------------------------------------
test_new(): # @test_new()
push RAX
mov EDI, 4
call operator new(unsigned long)
test RAX, RAX
je .LBB0_1
mov DWORD PTR [RAX], 5
pop RDX
ret
.LBB0_1:
xor EAX, EAX
pop RDX
ret
---------------------------------------
clang 5.0.0, -O3 -fno-exceptions
---------------------------------------
test_new(): # @test_new()
push rax
mov edi, 4
call operator new(unsigned long)
mov dword ptr [rax], 5
pop rcx
ret
---------------------------------------
We believe there may be a bug here where clang should be doing the NULL check
before initialization here regardless of the optimization level to be standard
compliant.</pre>
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