[LLVMdev] Quirk in switch lowering

[Rafael Espíndola]

Would you mind reporting a bug in llvm.org/bugs?

Thanks

On 24 December 2013 17:42, Jasper Neumann <jasper.neumann at web.de> wrote:
> Hello all,
>
> in SelectionDAGBuilder::handleBTSplitSwitchCase
> (lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp) seems to be a quirk which
> can effectively create an else-if chain from a very sparse switch statement
> instead of a balanced binary tree.
>
> A possible solution is to weight an isolated single switch label with 0. To
> do this one can replace the assignments to LDensity und RDensity by:
>     volatile double LDensity =
>        (double)(LSize-1).roundToDouble() /
>                            (LEnd - First + 1ULL).roundToDouble();
>     volatile double RDensity =
>       (double)(RSize-1).roundToDouble() /
>                            (Last - RBegin + 1ULL).roundToDouble();
>
> I replaced ?Size by (?Size-1).
>
> Here is a more or less stupid test program (translate with "clang -O3 -S
> test.cpp" and then look at test.s):
>
> int main(int argc, char **argv) {
>   switch (argc) {
>     case 100: return 1;
>     case 200: return 2;
>     case 300: return 3;
>     case 400: return 4;
>     case 500: return 5;
>     case 600: return 6;
>     case 700: return 7;
>     case 800: return 8;
>     case 900: return 9;
>     default: return 0;
>     }
>   }
>
> Best regards
> Jasper Neumann
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