Add docs file on low-level changes
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## JGR's Patchpack: Low level changes
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This document describes low-level changes to the codebase which are not generally visible when actually running/playing the game, this is a non-exhaustive list.
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This document does not describe the player-visible changes/additions described in the main readme.
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### Crash logger and diagnostics
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* Additional logged items: current company ID, map size, configure invocation, detailed OS version (Unix), thread name (Unix, Win32), recently executed commands.
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* Better handling of crashes which occur in a non-main thread (ask the main thread to do the crash screenshot and savegame).
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* Support logging register values on Unix.
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* Support using libbfd for symbol lookup and line numbers (gcc/clang).
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* Support using gdb if available to add further detail to the crashlog (Unix).
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* Demangle C++ symbols (Unix).
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* Handle segfaults which occur within the crashlog handler (Unix).
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#### Assertions
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Various assertions are extended to log further information on failure.
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Various assertions which check the state of a tile are extended to dump the tile state (m1 - m8, etc.) on failure.
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#### Scope annotations
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Scopes (in the main thread) can be annotated with a functor/lambda which is called in the event of a crash to provide further information to add to the crash log.
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#### NewGRF debug window
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Add various supplementary non-GRF information, e.g. vehicle variable and flags.
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#### Logging
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Add yapfdesync, linkgraph and sound log levels.
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Extend desync and random logging.
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### Map
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Store tunnel start/end pairs in a pool, indexed in the start/end tiles.
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Set bit in map if level crossing is possibly occupied by a road vehicle.
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De-virtualise calls to AnimateTile().
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### Viewport
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Cache bridge/tunnel start and ends.
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Cache station sign bounds.
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### Data structures
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Various data structures have been replaced with B-tree maps/sets (cpp-btree library).
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Various lists have been replaced with vectors or deques, etc.
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Remove mutexes from SmallStack, only used from the main thread.
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Use std::string in CommandContainer instead of a giant static buffer.
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### Vehicles
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Cache the sprite_seq bounds.
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Index the order list in vector.
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Observe the operation of the NewGRF when getting the vehicle image/sprite, and elide further calls to the NewGRF if it can be determined that the result will be the same.
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Add consist flag for case where no vehicles in consist are on a slope.
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Add vehicle flag to mark the last vehicle in a consist with a visual effect.
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Index the vehicle list in per type arrays for use by CallVehicleTicks.
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### Network/multiplayer
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Add supplementary information to find server UDP packets and reply in an extended format with more info/wider fields if detected.
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Paginate UDP packets longer than the MTU across multiple packets.
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### Sprites/blitter
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Add a fast path to Blitter_32bppAnim::Draw.
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Replace sprite cache implementation.
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### Link graph
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Completely change link graph job scheduling to make the duration of a job and the number of jobs per thread instance variable according to the estimated size of the job.
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Various use of custom allocators, etc.
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Early abort link graph threads if abandoning/quitting the game.
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Various forms of caching and incremental updates to the link graph overlay.
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### Save and load
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Feature versioning, see readme and code.
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Extend gamelog to not truncate version strings.
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Save/load the map in a single chunk, such that it can be saved/loaded in one pass.
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Various other changes to savegame format and settings handling, see readme and code for details.
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Replace read/write accessors and buffering.
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### Command line
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Add switch: -J, quite after N days.
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Add savegame feature versions to output of -q.
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### Configure/build
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Changes to gcc/clang detection and flags
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Changes to version detection and the format of the version string.
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### Misc
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Use of __builtin_expect, byte-swap builtins, and various bitmath builtins.
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Add various debug console commands.
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Increase the number of file slots.
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Cache font heights.
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Change inheritance model of class Window to keep UndefinedBehaviorSanitizer happy.
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