libavutil: Add saturating subtraction functions

Add av_sat_sub32 and av_sat_dsub32 as the subtraction analogues to
av_sat_add32/av_sat_dadd32.

Also clarify the formulas for dadd32/dsub32.

Signed-off-by: Andrew D'Addesio <modchipv12@gmail.com>

libavutil/arm/intmath.h

@@ -94,6 +94,22 @@ static av_always_inline int av_sat_dadd32_arm(int a, int b)
     return r;
 }
 
+#define av_sat_sub32 av_sat_sub32_arm
+static av_always_inline int av_sat_sub32_arm(int a, int b)
+{
+    int r;
+    __asm__ ("qsub %0, %1, %2" : "=r"(r) : "r"(a), "r"(b));
+    return r;
+}
+
+#define av_sat_dsub32 av_sat_dsub32_arm
+static av_always_inline int av_sat_dsub32_arm(int a, int b)
+{
+    int r;
+    __asm__ ("qdsub %0, %1, %2" : "=r"(r) : "r"(a), "r"(b));
+    return r;
+}
+
 #endif /* HAVE_ARMV6_INLINE */
 
 #if HAVE_ASM_MOD_Q

libavutil/common.h

@@ -260,13 +260,37 @@ static av_always_inline int av_sat_add32_c(int a, int b)
  *
  * @param  a first value
  * @param  b value doubled and added to a
- * @return sum with signed saturation
+ * @return sum sat(a + sat(2*b)) with signed saturation
  */
 static av_always_inline int av_sat_dadd32_c(int a, int b)
 {
     return av_sat_add32(a, av_sat_add32(b, b));
 }
 
+/**
+ * Subtract two signed 32-bit values with saturation.
+ *
+ * @param  a one value
+ * @param  b another value
+ * @return difference with signed saturation
+ */
+static av_always_inline int av_sat_sub32_c(int a, int b)
+{
+    return av_clipl_int32((int64_t)a - b);
+}
+
+/**
+ * Subtract a doubled value from another value with saturation at both stages.
+ *
+ * @param  a first value
+ * @param  b value doubled and subtracted from a
+ * @return difference sat(a - sat(2*b)) with signed saturation
+ */
+static av_always_inline int av_sat_dsub32_c(int a, int b)
+{
+    return av_sat_sub32(a, av_sat_add32(b, b));
+}
+
 /**
  * Clip a float value into the amin-amax range.
  * @param a value to clip
@@ -513,6 +537,12 @@ static av_always_inline av_const int av_parity_c(uint32_t v)
 #ifndef av_sat_dadd32
 #   define av_sat_dadd32    av_sat_dadd32_c
 #endif
+#ifndef av_sat_sub32
+#   define av_sat_sub32     av_sat_sub32_c
+#endif
+#ifndef av_sat_dsub32
+#   define av_sat_dsub32    av_sat_dsub32_c
+#endif
 #ifndef av_clipf
 #   define av_clipf         av_clipf_c
 #endif

libavutil/version.h

@@ -79,7 +79,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR  56
-#define LIBAVUTIL_VERSION_MINOR   4
+#define LIBAVUTIL_VERSION_MINOR   5
 #define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \