https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103429
Bug ID: 103429 Summary: Optimization of Auto-generated condition chain is not giving good lookup tables. Product: gcc Version: 12.0 Status: UNCONFIRMED Keywords: missed-optimization Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: ed at edwardrosten dot com Target Milestone: --- I've got come generated condition chains (using recursive templates) and am getting some odd/suboptimal optimization results. Code is provided below and with a godbolt link. In the first case (without a force inline), the compiler inlines the functions but does not perform condition chain optimization. In the second case (identical code but with force inline), it will optimize condition chains but only with exactly 5 elements. Otherwise it will end up with an if-else structure indexing optimized 5 element condition chains, and an if-else chain for anything spare. It only attempts the optimization from gcc 11 onwards, I checked on trunk too. Example: https://godbolt.org/z/c9xbPqq7r Here's the code: template<int I> void f(); constexpr int N=5; template<int I=0> static inline void f_dispatch(int i){ if constexpr (I == N) return; else if(i == I) f<I>(); else f_dispatch<I+1>(i); } template<int I=0> __attribute__((always_inline)) static inline void f_dispatch_always_inline(int i){ if constexpr (I == N) return; else if(i == I) f<I>(); else f_dispatch_always_inline<I+1>(i); } void run(int i){ f_dispatch<>(i); } void run_inline(int i){ f_dispatch_always_inline<>(i); }