https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119387
--- Comment #14 from Patrick Palka <ppalka at gcc dot gnu.org> --- Created attachment 60941 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=60941&action=edit reduced memory-hog testcase I managed to distill a large chunk of the front end memory-hog into the attached testcase (compile with -std=c++20 -O -fsyntax-only) which initializes a 512-tuple. The initialization is clearly non-constant, but after P2280 during speculative constexpr folding we no longer immediately give up evaluating the hefty recursive initialization, and in the meantime create a lot of garbage evaluating all the recursive constexpr calls in vain (and seemingly quadratically). The constexpr call cache doesn't help here because the constructor arguments are non-constant. r14-5978 (unoptimized checking): Time variable usr sys wall GGC phase setup : 0.04 ( 0%) 0.01 ( 8%) 0.09 ( 1%) 1773k ( 3%) phase parsing : 9.39 ( 99%) 0.12 ( 92%) 9.53 ( 98%) 66M ( 97%) phase lang. deferred : 0.08 ( 1%) 0.00 ( 0%) 0.09 ( 1%) 128 ( 0%) |name lookup : 7.75 ( 81%) 0.05 ( 38%) 7.85 ( 81%) 305k ( 0%) |overload resolution : 0.07 ( 1%) 0.01 ( 8%) 0.18 ( 2%) 3802k ( 5%) garbage collection : 0.08 ( 1%) 0.00 ( 0%) 0.09 ( 1%) 0 ( 0%) parser (global) : 0.00 ( 0%) 0.01 ( 8%) 0.01 ( 0%) 585k ( 1%) parser struct body : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 31k ( 0%) parser function body : 0.03 ( 0%) 0.00 ( 0%) 0.04 ( 0%) 2623k ( 4%) template instantiation : 9.08 ( 95%) 0.10 ( 77%) 9.22 ( 95%) 57M ( 84%) constant expression evaluation : 0.27 ( 3%) 0.01 ( 8%) 0.25 ( 3%) 6087k ( 9%) varconst : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 0 ( 0%) TOTAL : 9.51 0.13 9.71 67M r14-5979 (unoptimized checking): Time variable usr sys wall GGC phase setup : 0.01 ( 0%) 0.00 ( 0%) 0.02 ( 0%) 1773k ( 0%) phase parsing : 35.94 (100%) 0.52 (100%) 36.53 (100%) 457M (100%) phase lang. deferred : 0.15 ( 0%) 0.00 ( 0%) 0.15 ( 0%) 128 ( 0%) |name lookup : 7.59 ( 21%) 0.04 ( 8%) 7.86 ( 21%) 324k ( 0%) |overload resolution : 0.11 ( 0%) 0.00 ( 0%) 0.10 ( 0%) 3809k ( 1%) garbage collection : 0.15 ( 0%) 0.00 ( 0%) 0.15 ( 0%) 0 ( 0%) parser function body : 0.03 ( 0%) 0.01 ( 2%) 0.03 ( 0%) 2623k ( 1%) template instantiation : 9.10 ( 25%) 0.10 ( 19%) 9.18 ( 25%) 57M ( 12%) constant expression evaluation : 26.81 ( 74%) 0.41 ( 79%) 27.32 ( 74%) 397M ( 86%) TOTAL : 36.10 0.52 36.70 459M trunk (optimized release): Time variable wall GGC phase setup : 0.01 ( 1%) 1813k ( 1%) phase parsing : 1.42 ( 99%) 328M ( 99%) |name lookup : 0.01 ( 0%) 328k ( 0%) |overload resolution : 0.01 ( 1%) 3810k ( 1%) parser function body : 0.00 ( 0%) 2345k ( 1%) template instantiation : 0.13 ( 9%) 58M ( 18%) constant expression evaluation : 1.28 ( 90%) 267M ( 81%) TOTAL : 1.43 330M
