lavu/riscv: CPU flag for the Zbb extension

Unfortunately, it is common, and will remain so, that the Bit
manipulations are not enabled at compilation time. This is an official
policy for Debian ports in general (though they do not support RISC-V
officially as of yet) to stick to the minimal target baseline, which
does not include the B extension or even its Zbb subset.

For inline helpers (CPOP, REV8), compiler builtins (CTZ, CLZ) or
even plain C code (MIN, MAX, MINU, MAXU), run-time detection seems
impractical. But at least it can work for the byte-swap DSP functions.

libavutil/cpu.c

@@ -188,6 +188,7 @@ int av_parse_cpu_caps(unsigned *flags, const char *s)
         { "rvv-f32",  NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AV_CPU_FLAG_RVV_F32 },     .unit = "flags" },
         { "rvv-i64",  NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AV_CPU_FLAG_RVV_I64 },     .unit = "flags" },
         { "rvv",      NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AV_CPU_FLAG_RVV_F64 },     .unit = "flags" },
+        { "rvb-basic",NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AV_CPU_FLAG_RVB_BASIC },   .unit = "flags" },
 #endif
         { NULL },
     };

libavutil/cpu.h

@@ -86,6 +86,7 @@
 #define AV_CPU_FLAG_RVV_F32      (1 << 4) ///< Vectors of float's */
 #define AV_CPU_FLAG_RVV_I64      (1 << 5) ///< Vectors of 64-bit int's */
 #define AV_CPU_FLAG_RVV_F64      (1 << 6) ///< Vectors of double's
+#define AV_CPU_FLAG_RVB_BASIC    (1 << 7) ///< Basic bit-manipulations
 
 /**
  * Return the flags which specify extensions supported by the CPU.

libavutil/riscv/cpu.c

@@ -40,6 +40,8 @@ int ff_get_cpu_flags_riscv(void)
         ret |= AV_CPU_FLAG_RVF;
     if (hwcap & HWCAP_RV('D'))
         ret |= AV_CPU_FLAG_RVD;
+    if (hwcap & HWCAP_RV('B'))
+        ret |= AV_CPU_FLAG_RVB_BASIC;
 
     /* The V extension implies all Zve* functional subsets */
     if (hwcap & HWCAP_RV('V'))
@@ -57,6 +59,10 @@ int ff_get_cpu_flags_riscv(void)
 #endif
 #endif
 
+#ifdef __riscv_zbb
+    ret |= AV_CPU_FLAG_RVB_BASIC;
+#endif
+
     /* If RV-V is enabled statically at compile-time, check the details. */
 #ifdef __riscv_vectors
     ret |= AV_CPU_FLAG_RVV_I32;

tests/checkasm/checkasm.c

@@ -240,6 +240,7 @@ static const struct {
     { "RVVf32",   "rvv_f32",  AV_CPU_FLAG_RVV_F32 },
     { "RVVi64",   "rvv_i64",  AV_CPU_FLAG_RVV_I64 },
     { "RVVf64",   "rvv_f64",  AV_CPU_FLAG_RVV_F64 },
+    { "RVBbasic", "rvb_b",    AV_CPU_FLAG_RVB_BASIC },
 #elif ARCH_MIPS
     { "MMI",      "mmi",      AV_CPU_FLAG_MMI },
     { "MSA",      "msa",      AV_CPU_FLAG_MSA },