[llvm-dev] StringSwitch efficiency

[Chris Lattner via llvm-dev]

Awesome, yay for composition and smart compilers :-)

-Chris

> On Nov 1, 2020, at 10:07 AM, Paul C. Anagnostopoulos via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> I used Compiler Explorer to look at the compiled code and, indeed, it is quite good. The person who wrote that particular StringSwitch put the keywords in frequency-of-use order, so that the sequence of same-length comparisons is checking for the more common cases first.
> 
> At 10/31/2020 03:52 PM, Fāng-ruì Sòng wrote:
>> If the argument is unknown, it seems that SROA can essentially
>> optimize a long chain of bcmp(string comparison) calls to dispatches
>> on the argument length.
>> Then SimplfyCFGOpt::SimplifyBranchOnICmpChain can generate a switch
>> instruction which will eventually lower to a jump table.
>> ExpandMemCmp.cpp can generate efficient comparisons for these strings.
>> There are cases where multiple memory loads are produced while a
>> memory load can be reused,
>> but overall the produced code looks pretty good.
>> 
>> int foo(llvm::StringRef s) {
>>   return llvm::StringSwitch<int>(s)
>>     .Case("int", 5)
>>     .Case("bit", 3)
>>     .Case("bits", 7)
>>     .Case("string", 8)
>>     .Case("list", 4)
>>     .Case("code", 2)
>>     .Case("dag", 1)
>>     .Case("class", 4)
>>     .Case("def", 3)
>>     .Case("foreach", 9)
>>     .Case("defm", 1)
>>     .Case("defset", 0)
>>     .Case("multiclass", 1)
>>     .Case("field", 4)
>>     .Case("let", 3)
>>     .Case("in", 9)
>>     .Case("defvar", 1)
>>     .Case("if", 0)
>>     .Case("then", 3)
>>     .Case("else", 7)
>>     .Default(5);
>> }
> 
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