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

[David Blaikie]

On Thu, Nov 20, 2014 at 2:48 PM, Sean Silva <chisophugis at gmail.com> wrote:

>
>
> 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?)
>

Taking a pseudorandom document from the Internet:
http://herbsutter.com/2008/01/01/gotw-88-a-candidate-for-the-most-important-const/


>
> -- Sean Silva
>
>
>>
>>
>>>
>>> -- Sean Silva
>>>
>>>
>>>>
>>>>
>>>>>          [offsetInSect](const SectionOffsetAndAtom &ao,
>>>>>                         uint64_t targetAddr) -> bool {
>>>>>            // Each atom has a start offset of its slice of the
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> llvm-commits mailing list
>>>>> llvm-commits at cs.uiuc.edu
>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>>>>
>>>>
>>>>
>>>> _______________________________________________
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>>>>
>>>>
>>>
>>
>
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