Add sk_careful_memcpy to catch undefined behavior in memcpy.

include/core/SkTypes.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkTypes_DEFINED
 #define SkTypes_DEFINED
 
 // IWYU pragma: begin_exports
 #include "SkPreConfig.h"
 #include "SkUserConfig.h"
 #include "SkPostConfig.h"
 #include <stddef.h>
 #include <stdint.h>
 
 #if defined(SK_ARM_HAS_NEON)
     #include <arm_neon.h>
 #elif SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSE2
     #include <immintrin.h>
 #endif
 // IWYU pragma: end_exports
 
 #include <string.h>
 
+/**
+ *  sk_careful_memcpy() is just like memcpy(), but guards against undefined behavior.
+ *
+ * It is undefined behavior to call memcpy() with null dst or src, even if len is 0.
+ * If an optimizer is "smart" enough, it can exploit this to do unexpected things.
+ *     memcpy(dst, src, 0);
+ *     if (src) {
+ *         printf("%x\n", *src);
+ *     }
+ * In this code the compiler can assume src is not null and omit the if (src) {...} check,
+ * unconditionally running the printf, crashing the program if src really is null.
+ * Of the compilers we pay attention to only GCC performs this optimization in practice.
+ */
+static inline void* sk_careful_memcpy(void* dst, const void* src, size_t len) {
+    // When we pass >0 len we had better already be passing valid pointers.
+    // So we just need to skip calling memcpy when len == 0.
+    if (len) {
+        memcpy(dst,src,len);
+    }
+    return dst;
+}
+
 /** \file SkTypes.h
 */
 
 /** See SkGraphics::GetVersion() to retrieve these at runtime
  */
 #define SKIA_VERSION_MAJOR  1
 #define SKIA_VERSION_MINOR  0
 #define SKIA_VERSION_PATCH  0
 
 /*
@@ skipping 631 matching lines @@
 private:
     void*       fPtr;
     size_t      fSize;  // can be larger than the requested size (see kReuse)
     uint32_t    fStorage[(kSize + 3) >> 2];
 };
 // Can't guard the constructor because it's a template class.
 
 #endif /* C++ */
 
 #endif