libobs: Add SIMD-optimized audio pipeline with memory pooling

[marcusbooker77]

Summary

This PR introduces three major performance optimizations to the libobs audio and video pipelines:

1. SIMD-Vectorized Audio Mixing (obs-audio-optimized.c)

• SSE2 path: Processes 4 floats per iteration with _mm_add_ps — available on all x86-64 CPUs
• AVX2 path: Processes 8 floats per iteration with _mm256_add_ps — enabled via runtime CPUID detection, no recompile needed
• Cache prefetching: _mm_prefetch hints on both source and destination buffers reduce L1 cache misses
• Replaces the scalar *(mix++) += *(aud++) hot loop in mix_audio() that runs every audio tick (~21ms at 48kHz)

2. Video Frame Copy with Non-Temporal Stores (copy_video_plane_optimized)

• For large planes (>256 KB), uses _mm_stream_si128 non-temporal stores to bypass CPU cache, avoiding cache pollution from write-once frame data
• Processes 64 bytes per iteration (4×128-bit loads + streaming stores)
• Falls back to _mm_storeu_si128 vectorized copies for aligned-but-small planes, and plain memcpy for unaligned destinations
• Validates 16-byte alignment on both base pointer and stride before using streaming stores
• Contiguous-layout fast path when width == dst_stride == src_stride avoids per-line overhead

3. Pre-Allocated Audio Buffer Memory Pool (obs-audio-pool.c/h)

• Arena-based pool with 64-byte-aligned blocks (cache-line aligned, required for SIMD NT stores)
• Geometric growth (doubles arena size on exhaustion) — amortized O(1) allocation
• Eliminates per-frame malloc/free overhead in the audio pipeline hot path
• Thread-safe via pthread_mutex with minimal contention (lock held only during pointer swap)

4. Multi-Threaded Audio Processing (obs-audio-threaded.c/h)

• Fixed-size thread pool (defaults to logical_cores - 1, capped at 16)
• Power-of-2 ring buffer job queue with pthread_cond signaling
• Barrier-based completion wait for deterministic frame boundaries
• Graceful fallback: if the queue is full, the job runs synchronously on the caller thread with a LOG_WARNING
• Diagnostics: tracks peak queue depth for profiling

5. Lock-Free SPSC Queue (util/spsc-queue.h)

• Single-producer, single-consumer lock-free queue using atomic load/store with memory_order_acquire/release
• Power-of-2 capacity with mask-based indexing (no modulo)
• Header-only, zero dependencies beyond <stdatomic.h> / MSVC intrinsics

Supporting Changes

• obs-audio.c: Calls mix_audio_optimized() and zero_audio_buffer_optimized() instead of scalar loops
• obs-video.c: Calls copy_video_plane_optimized() for set_gpu_converted_plane() and copy_rgbx_frame(); adds MMCSS AvSetMmThreadCharacteristics("Pro Audio") on Windows for the graphics thread
• obs-video-gpu-encode.c: Adds MMCSS thread priority boost for the GPU encode thread
• obs-source.c: Integrates pool-allocated audio buffers for per-source audio data
• obs.c: Initializes and tears down the audio pool and thread pool during obs_startup/obs_shutdown
• obs-internal.h: Adds pool and thread pool pointers to obs_core_audio

Expected Performance Impact

Based on analysis and architectural properties (formal benchmarks in progress):

Optimization	Mechanism	Expected CPU Reduction
SIMD audio mixing	4-8× throughput on mix loop	15-25% on audio thread
Non-temporal video copy	Cache bypass for large frames	20-30% on frame copy
Memory pool	Eliminates malloc/free per frame	Reduced allocation overhead
Thread pool	Parallel source rendering	Better multi-core utilization
MMCSS priority	Windows thread scheduling	Fewer priority inversions

Target: ≥15% overall CPU reduction in typical streaming scenarios (1080p30, 4+ sources).

Compatibility

• Runtime CPU detection: AVX2 path only activates if CPUID confirms support; SSE2 path is the baseline (guaranteed on all x86-64)
• Fallback paths: Every SIMD function falls back to scalar for remaining elements and small buffers
• Cross-platform: Uses OBS's existing os_atomic_* wrappers and pthreads; compiles on Windows/Linux/macOS
• ABI compatible: No changes to public API; all new symbols are internal to libobs
• Backward compatible: Existing plugins are unaffected

Test Plan

• Build succeeds on Windows (MSVC), Linux (GCC/Clang), macOS (AppleClang)
• Audio output is bit-identical to baseline (verified via waveform comparison)
• No dropped frames in 1080p30 streaming with 4+ sources
• No crashes in 6-hour stress test
• SIMD path activates on AVX2-capable hardware (verify via LOG_INFO message)
• Scalar fallback works on non-AVX2 hardware
• Memory pool grows correctly under high source count (16+ audio sources)
• Thread pool shuts down cleanly (no hangs, no leaks)
• obs-perf-monitor.ps1 shows CPU reduction vs baseline build

Files Changed

File	Change
libobs/obs-audio-optimized.c	New — SSE2/AVX2 mixing, NT video copy, SIMD zero-fill
libobs/obs-audio-pool.c	New — Arena-based aligned memory pool
libobs/obs-audio-pool.h	New — Pool API header
libobs/obs-audio-threaded.c	New — Thread pool + job queue
libobs/obs-audio-threaded.h	New — Thread pool API header
libobs/util/spsc-queue.h	New — Lock-free SPSC queue
libobs/CMakeLists.txt	Add new source files to build
libobs/obs-audio.c	Call optimized mixing/zero functions
libobs/obs-video.c	Call optimized copy; MMCSS priority
libobs/obs-video-gpu-encode.c	MMCSS priority for GPU encode thread
libobs/obs-source.c	Pool-allocated audio buffers
libobs/obs.c	Init/teardown of pool and thread pool
libobs/obs-internal.h	Pool/threadpool pointers in core struct

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