[llvm-dev] StringSwitch efficiency

[Paul C. Anagnostopoulos via llvm-dev]

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);
>}