feat: MAP_SHARED client windows + occluded blit — BWM CPU 70% → 34% - #253
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Three optimizations that halve BWM's CPU usage while improving FPS: 1. MAP_SHARED client windows (op=21): Maps client window physical pages read-only into BWM's address space, eliminating the kernel page-by-page copy in read_window_buffer (op=14). Each window's backing pages are mapped once at discovery time. 2. Lightweight dirty check (op=22): New check_window_dirty syscall compares generation counters without any pixel copy. Replaces the full read_window_buffer call for detecting changes. 3. Occluded blit with span subtraction: When blitting a dirty window, pixels covered by higher-z windows are skipped using per-row span subtraction. Eliminates z-order repair entirely (no re-blitting cached pixels for overlapping windows). Results on Parallels ARM64 (1920x1200, 4 windows): - CPU: 70% → 34% (51% reduction) - FPS: ~100 → ~130 (30% increase) - Iteration time: 6451us → 4449us - Heap savings: ~8MB (removed client_pixel_buf + per-window pixel_cache) - Visual quality: No glitching — occluded blit handles z-order correctly Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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Summary
Results (Parallels ARM64, 1920x1200, 4 windows)
Test plan
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