From 991170bbab3e6afc74666d124f3f1dc7be942cd0 Mon Sep 17 00:00:00 2001
From: Wan-Teh Chang <wtc@google.com>
Date: Fri, 1 Aug 2025 16:20:52 -0700
Subject: [PATCH] Implement WEBP_DSP_INIT with SRWLOCK for Windows

A slim reader/writer (SRW) lock can be initialized statically with the
constant SRWLOCK_INIT. It is the only Windows synchronization object I
can find with this property.

Note: On old Windows versions that don't have SRWLOCK, use the fallback,
thread-unsafe implementation.

Change a NOLINT comment to a NOLINTNEXTLINE comment to prevent
clang-format from aligning the #else and #endif comments in undesired
way.

Bug: 435213378
Change-Id: Iecff615a14a1905aedd2c05ad9444889f711cc17
(cherry picked from commit 54f23b049e06b2397722c2b3ee8e0e7bc4e6bf9b)
---
 src/dsp/cpu.h | 41 +++++++++++++++++++++++++++++++++++++----
 1 file changed, 37 insertions(+), 4 deletions(-)

diff --git a/src/dsp/cpu.h b/src/dsp/cpu.h
index 60a1be51..780ac8d4 100644
--- a/src/dsp/cpu.h
+++ b/src/dsp/cpu.h
@@ -189,8 +189,40 @@
 #endif
 #endif
 
-#if defined(WEBP_USE_THREAD) && !defined(_WIN32)
-#include <pthread.h>  // NOLINT
+#if defined(WEBP_USE_THREAD)
+#if defined(_WIN32)
+#include <windows.h>
+
+#if _WIN32_WINNT >= 0x0600
+// clang-format off
+#define WEBP_DSP_INIT_VARS(func)               \
+  static VP8CPUInfo func##_last_cpuinfo_used = \
+      (VP8CPUInfo)&func##_last_cpuinfo_used;   \
+  static SRWLOCK func##_lock = SRWLOCK_INIT
+#define WEBP_DSP_INIT(func)                                \
+  do {                                                     \
+    AcquireSRWLockExclusive(&func##_lock);                 \
+    if (func##_last_cpuinfo_used != VP8GetCPUInfo) func(); \
+    func##_last_cpuinfo_used = VP8GetCPUInfo;              \
+    ReleaseSRWLockExclusive(&func##_lock);                 \
+  } while (0)
+// clang-format on
+#else   // _WIN32_WINNT < 0x0600
+// clang-format off
+#define WEBP_DSP_INIT_VARS(func)                        \
+  static volatile VP8CPUInfo func##_last_cpuinfo_used = \
+      (VP8CPUInfo)&func##_last_cpuinfo_used
+#define WEBP_DSP_INIT(func)                               \
+  do {                                                    \
+    if (func##_last_cpuinfo_used == VP8GetCPUInfo) break; \
+    func();                                               \
+    func##_last_cpuinfo_used = VP8GetCPUInfo;             \
+  } while (0)
+// clang-format on
+#endif  // _WIN32_WINNT >= 0x0600
+#else   // !defined(_WIN32)
+// NOLINTNEXTLINE
+#include <pthread.h>
 
 #define WEBP_DSP_INIT_VARS(func)               \
   static VP8CPUInfo func##_last_cpuinfo_used = \
@@ -203,7 +235,8 @@
     func##_last_cpuinfo_used = VP8GetCPUInfo;              \
     (void)pthread_mutex_unlock(&func##_lock);              \
   } while (0)
-#else   // !(defined(WEBP_USE_THREAD) && !defined(_WIN32))
+#endif  // defined(_WIN32)
+#else   // !defined(WEBP_USE_THREAD)
 #define WEBP_DSP_INIT_VARS(func)                        \
   static volatile VP8CPUInfo func##_last_cpuinfo_used = \
       (VP8CPUInfo)&func##_last_cpuinfo_used
@@ -213,7 +246,7 @@
     func();                                               \
     func##_last_cpuinfo_used = VP8GetCPUInfo;             \
   } while (0)
-#endif  // defined(WEBP_USE_THREAD) && !defined(_WIN32)
+#endif  // defined(WEBP_USE_THREAD)
 
 // Defines an Init + helper function that control multiple initialization of
 // function pointers / tables.