initial commit, 4.5 stable

thirdparty/libjpeg-turbo/src/jpeg_nbits.h

@@ -0,0 +1,43 @@
+/*
+ * Copyright (C) 2014, 2021, 2024, D. R. Commander.
+ * Copyright (C) 2014, Olle Liljenzin.
+ * Copyright (C) 2020, Arm Limited.
+ *
+ * For conditions of distribution and use, see the accompanying README.ijg
+ * file.
+ */
+
+/*
+ * NOTE: If USE_CLZ_INTRINSIC is defined, then clz/bsr instructions will be
+ * used for bit counting rather than the lookup table.  This will reduce the
+ * memory footprint by 64k, which is important for some mobile applications
+ * that create many isolated instances of libjpeg-turbo (web browsers, for
+ * instance.)  This may improve performance on some mobile platforms as well.
+ * This feature is enabled by default only on Arm processors, because some x86
+ * chips have a slow implementation of bsr, and the use of clz/bsr cannot be
+ * shown to have a significant performance impact even on the x86 chips that
+ * have a fast implementation of it.  When building for Armv6, you can
+ * explicitly disable the use of clz/bsr by adding -mthumb to the compiler
+ * flags (this defines __thumb__).
+ */
+
+/* NOTE: Both GCC and Clang define __GNUC__ */
+#if (defined(__GNUC__) && (defined(__arm__) || defined(__aarch64__))) || \
+    defined(_M_ARM) || defined(_M_ARM64)
+#if !defined(__thumb__) || defined(__thumb2__)
+#define USE_CLZ_INTRINSIC
+#endif
+#endif
+
+#ifdef USE_CLZ_INTRINSIC
+#if defined(_MSC_VER) && !defined(__clang__)
+#define JPEG_NBITS_NONZERO(x)  (32 - _CountLeadingZeros(x))
+#else
+#define JPEG_NBITS_NONZERO(x)  (32 - __builtin_clz(x))
+#endif
+#define JPEG_NBITS(x)          (x ? JPEG_NBITS_NONZERO(x) : 0)
+#else
+extern const unsigned char jpeg_nbits_table[65536];
+#define JPEG_NBITS(x)          (jpeg_nbits_table[x])
+#define JPEG_NBITS_NONZERO(x)  JPEG_NBITS(x)
+#endif