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

[Sean Silva]

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?

-- 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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