r213265 - Using a std::string instead of a StringRef because the Default case synthesizes a temporary std::string from a Twine. Assigning that into a StringRef causes the StringRef to refer to a temporary, and bad things happen.

[Aaron Ballman]

On Thu, Jul 17, 2014 at 10:03 AM, Alp Toker <alp at nuanti.com> wrote:
>
> On 17/07/2014 16:28, Aaron Ballman wrote:
>>
>> Author: aaronballman
>> Date: Thu Jul 17 08:28:50 2014
>> New Revision: 213265
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=213265&view=rev
>> Log:
>> Using a std::string instead of a StringRef because the Default case
>> synthesizes a temporary std::string from a Twine. Assigning that into a
>> StringRef causes the StringRef to refer to a temporary, and bad things
>> happen.
>>
>> This fixes a failing test case on Windows.
>>
>> Modified:
>>      cfe/trunk/lib/Driver/Tools.cpp
>>
>> Modified: cfe/trunk/lib/Driver/Tools.cpp
>> URL:
>> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Driver/Tools.cpp?rev=213265&r1=213264&r2=213265&view=diff
>>
>> ==============================================================================
>> --- cfe/trunk/lib/Driver/Tools.cpp (original)
>> +++ cfe/trunk/lib/Driver/Tools.cpp Thu Jul 17 08:28:50 2014
>> @@ -1037,12 +1037,12 @@ static void getMIPSTargetFeatures(const
>>     ABIName = getGnuCompatibleMipsABIName(ABIName);
>>       // Always override the backend's default ABI.
>> -  StringRef ABIFeature = llvm::StringSwitch<StringRef>(ABIName)
>> -                              .Case("32", "+o32")
>> -                              .Case("n32", "+n32")
>> -                              .Case("64", "+n64")
>> -                              .Case("eabi", "+eabi")
>> -                              .Default(("+" + ABIName).str());
>> +  std::string ABIFeature = llvm::StringSwitch<StringRef>(ABIName)
>> +                               .Case("32", "+o32")
>> +                               .Case("n32", "+n32")
>> +                               .Case("64", "+n64")
>> +                               .Case("eabi", "+eabi")
>> +                               .Default(("+" + ABIName).str());
>
>
> Is the optimizer smart enough to eliminate the evaluation of that str() call
> or will the string be built and discarded every time the StringSwitch is
> run?

I'm not certain (I've not tested it), but I would guess the str() call
is evaluated each time because it is used as an argument in a function
call which is evaluated.

~Aaron