compiler magic

Magic is here, it is at your fingertips.

Recently, I've come across a not so efficient implementation of a isEven function ```c++ bool isEven(int number) { int numberCompare = 0; bool even = true;

while (number != numberCompare)
{
    even = !even;
    numberCompare++;
}
return even;

} ``` ... Surprisingly, Clang/LLVM is able to optimize the iterative algorithm down to the constant time algorithm (GCC fails on this one). In Clang 10 with full optimizations, this code compiles down to:

llvm ; Function Attrs: norecurse nounwind readnone ssp uwtable define zeroext i1 @_Z6isEveni(i32 %0) local_unnamed_addr #0 { %2 = and i32 %0, 1 %3 = icmp eq i32 %2, 0 ret i1 %3 }

^ That, is magic

This is the list of optimization passes (the order matters) that LLVM applies with flag -O2. Note that some of them are run multiple times : -targetlibinfo -tti -targetpassconfig -tbaa -scoped-noalias -assumption-cache-tracker -profile-summary-info -forceattrs -inferattrs -ipsccp -called-value-propagation -attributor -globalopt -domtree -mem2reg -deadargelim -domtree -basicaa -aa -loops -lazy-branch-prob -lazy-block-freq -opt-remark-emitter -instcombine -simplifycfg -basiccg -globals-aa -prune-eh -inline -functionattrs -domtree -sroa -basicaa -aa -memoryssa -early-cse-memssa -speculative-execution -aa -lazy-value-info -jump-threading -correlated-propagation -simplifycfg -domtree -basicaa -aa -loops -lazy-branch-prob -lazy-block-freq -opt-remark-emitter -instcombine -libcalls-shrinkwrap -loops -branch-prob -block-freq -lazy-branch-prob -lazy-block-freq -opt-remark-emitter -pgo-memop-opt -basicaa -aa -loops -lazy-branch-prob -lazy-block-freq -opt-remark-emitter -tailcallelim -simplifycfg -reassociate -domtree -loops -loop-simplify -lcssa-verification -lcssa -basicaa -aa -scalar-evolution -loop-rotate -memoryssa -licm -loop-unswitch -simplifycfg -domtree -basicaa -aa -loops -lazy-branch-prob -lazy-block-freq -opt-remark-emitter -instcombine -loop-simplify -lcssa-verification -lcssa -scalar-evolution -indvars -loop-idiom -loop-deletion -loop-unroll -mldst-motion -phi-values -aa -memdep -lazy-branch-prob -lazy-block-freq -opt-remark-emitter -gvn -phi-values -basicaa -aa -memdep -memcpyopt -sccp -demanded-bits -bdce -aa -lazy-branch-prob -lazy-block-freq -opt-remark-emitter -instcombine -lazy-value-info -jump-threading -correlated-propagation -basicaa -aa -phi-values -memdep -dse -aa -memoryssa -loops -loop-simplify -lcssa-verification -lcssa -scalar-evolution -licm -postdomtree -adce -simplifycfg -domtree -basicaa -aa -loops -lazy-branch-prob -lazy-block-freq -opt-remark-emitter -instcombine -barrier -elim-avail-extern -basiccg -rpo-functionattrs -globalopt -globaldce -basiccg -globals-aa -domtree -float2int -lower-constant-intrinsics -domtree -loops -loop-simplify -lcssa-verification -lcssa -basicaa -aa -scalar-evolution -loop-rotate -loop-accesses -lazy-branch-prob -lazy-block-freq -opt-remark-emitter -loop-distribute -branch-prob -block-freq -scalar-evolution -basicaa -aa -loop-accesses -demanded-bits -lazy-branch-prob -lazy-block-freq -opt-remark-emitter -loop-vectorize -loop-simplify -scalar-evolution -aa -loop-accesses -lazy-branch-prob -lazy-block-freq -loop-load-elim -basicaa -aa -lazy-branch-prob -lazy-block-freq -opt-remark-emitter -instcombine -simplifycfg -domtree -loops -scalar-evolution -basicaa -aa -demanded-bits -lazy-branch-prob -lazy-block-freq -opt-remark-emitter -slp-vectorizer -opt-remark-emitter -instcombine -loop-simplify -lcssa-verification -lcssa -scalar-evolution -loop-unroll -lazy-branch-prob -lazy-block-freq -opt-remark-emitter -instcombine -memoryssa -loop-simplify -lcssa-verification -lcssa -scalar-evolution -licm -lazy-branch-prob -lazy-block-freq -opt-remark-emitter -transform-warning -alignment-from-assumptions -strip-dead-prototypes -globaldce -constmerge -domtree -loops -branch-prob -block-freq -loop-simplify -lcssa-verification -lcssa -basicaa -aa -scalar-evolution -block-freq -loop-sink -lazy-branch-prob -lazy-block-freq -opt-remark-emitter -instsimplify -div-rem-pairs -simplifycfg -domtree -basicaa -aa -memoryssa -loops -loop-simplify -lcssa-verification -lcssa -scalar-evolution -licm -verify -print-module ↩