[lld] r222455 - [mach-o] use reference with "auto" to prevent copies

[Sean Silva]

On Thu, Nov 20, 2014 at 2:39 PM, David Blaikie <dblaikie at gmail.com> wrote:

>
>
> On Thu, Nov 20, 2014 at 2:39 PM, Sean Silva <chisophugis at gmail.com> wrote:
>
>>
>>
>> On Thu, Nov 20, 2014 at 1:46 PM, David Blaikie <dblaikie at gmail.com>
>> wrote:
>>
>>>
>>>
>>> On Thu, Nov 20, 2014 at 1:19 PM, Nick Kledzik <kledzik at apple.com> wrote:
>>>
>>>> Author: kledzik
>>>> Date: Thu Nov 20 15:19:58 2014
>>>> New Revision: 222455
>>>>
>>>> URL: http://llvm.org/viewvc/llvm-project?rev=222455&view=rev
>>>> Log:
>>>> [mach-o] use reference with "auto" to prevent copies
>>>>
>>>> Patch by Jean-Daniel Dupas
>>>>
>>>> Modified:
>>>>     lld/trunk/lib/ReaderWriter/MachO/File.h
>>>>
>>>> Modified: lld/trunk/lib/ReaderWriter/MachO/File.h
>>>> URL:
>>>> http://llvm.org/viewvc/llvm-project/lld/trunk/lib/ReaderWriter/MachO/File.h?rev=222455&r1=222454&r2=222455&view=diff
>>>>
>>>> ==============================================================================
>>>> --- lld/trunk/lib/ReaderWriter/MachO/File.h (original)
>>>> +++ lld/trunk/lib/ReaderWriter/MachO/File.h Thu Nov 20 15:19:58 2014
>>>> @@ -118,13 +118,13 @@ public:
>>>>    MachODefinedAtom *findAtomCoveringAddress(const Section &section,
>>>>                                              uint64_t offsetInSect,
>>>>                                              uint32_t
>>>> *foundOffsetAtom=nullptr) {
>>>> -    auto pos = _sectionAtoms.find(&section);
>>>> +    const auto& pos = _sectionAtoms.find(&section);
>>>>
>>>
>>> This one's a bit weird - 'find' (for any standard-like container, at
>>> least) returns an iterator by value. While taking a reference to that is
>>> valid (reference lifetime extension & all that) it's a bit surprising &
>>> unnecessary (has basically the same semantics as the non-reference code).
>>>
>>>
>>>>      if (pos == _sectionAtoms.end())
>>>>        return nullptr;
>>>> -    auto vec = pos->second;
>>>> +    const auto& vec = pos->second;
>>>>      assert(offsetInSect < section.content.size());
>>>>      // Vector of atoms for section are already sorted, so do binary
>>>> search.
>>>> -    auto atomPos = std::lower_bound(vec.begin(), vec.end(),
>>>> offsetInSect,
>>>> +    const auto& atomPos = std::lower_bound(vec.begin(), vec.end(),
>>>> offsetInSect,
>>>>
>>>
>>> Same here, lower_bound just returns an iterator by value, this doesn't
>>> look like it's avoiding any copies.
>>>
>>
>> Also, wouldn't this create a reference to a temporary?
>>
>
> Right - but reference extension semantics mean this isn't actually
> incorrect/unsafe...
>

"reference extension semantics" doesn't ring a bell. Do you have a link to
a (no pun intended) reference? (or could you briefly explain?)

-- Sean Silva


>
>
>>
>> -- Sean Silva
>>
>>
>>>
>>>
>>>>          [offsetInSect](const SectionOffsetAndAtom &ao,
>>>>                         uint64_t targetAddr) -> bool {
>>>>            // Each atom has a start offset of its slice of the
>>>>
>>>>
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>>>
>>>
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>>>
>>
>
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