riscv: probe for Zbb extension at load time

Due to hysterical raisins, most RISC-V Linux distributions target a
RV64GC baseline excluding the Bit-manipulation ISA extensions, most
notably:
- Zba: address generation extension and
- Zbb: basic bit manipulation extension.
Most CPUs that would make sense to run FFmpeg on support Zba and Zbb
(including the current FATE runner), so it makes sense to optimise for
them. In fact a large chunk of existing assembler optimisations relies
on Zba and/or Zbb.

Since we cannot patch shared library code, the next best thing is to
carry a flag initialised at load-time and check it on need basis.
This results in 3 instructions overhead on isolated use, e.g.:
1:  AUIPC rd, %pcrel_hi(ff_rv_zbb_supported)
    LBU   rd, %pcrel_lo(1b)(rd)
    BEQZ  rd, non_Zbb_fallback_code
    // Zbb code here

The C compiler will typically load the flag ahead of time to reducing
latency, and can also keep it around if Zbb is used multiple times in a
single optimisation scope. For this to work, the flag symbol must be
hidden; otherwise the optimisation degrades with a GOT look-up to
support interposition:
1:  AUIPC rd, GOT_OFFSET_HI
    LD    rd, GOT_OFFSET_LO(rd)
    LBU   rd, (rd)
    BEQZ  rd, non_Zbb_fallback_code
    // Zbb code here

This patch adds code to provision the flag in libraries using bit
manipulation functions from libavutil: byte-swap, bit-weight and
counting leading or trailing zeroes.

14 files changed, 67 insertions, 1 deletions

diff --git a/libavcodec/riscv/Makefile b/libavcodec/riscv/Makefile
index 590655f829..c180223141 100644
--- a/libavcodec/riscv/Makefile
+++ b/libavcodec/riscv/Makefile
@@ -77,3 +77,5 @@ RVV-OBJS-$(CONFIG_VP9_DECODER) += riscv/vp9_intra_rvv.o \
                                   riscv/vp9_mc_rvv.o
 OBJS-$(CONFIG_VORBIS_DECODER) += riscv/vorbisdsp_init.o
 RVV-OBJS-$(CONFIG_VORBIS_DECODER) += riscv/vorbisdsp_rvv.o
+
+SHLIBOBJS += riscv/cpu_common.o
diff --git a/libavcodec/riscv/cpu_common.c b/libavcodec/riscv/cpu_common.c
new file mode 100644
index 0000000000..17c9b392c9
--- /dev/null
+++ b/libavcodec/riscv/cpu_common.c
@@ -0,0 +1 @@
+#include "libavutil/riscv/cpu_common.c"
diff --git a/libavdevice/riscv/Makefile b/libavdevice/riscv/Makefile
new file mode 100644
index 0000000000..52857aacba
--- /dev/null
+++ b/libavdevice/riscv/Makefile
@@ -0,0 +1 @@
+SHLIBOBJS += riscv/cpu_common.o
diff --git a/libavdevice/riscv/cpu_common.c b/libavdevice/riscv/cpu_common.c
new file mode 100644
index 0000000000..17c9b392c9
--- /dev/null
+++ b/libavdevice/riscv/cpu_common.c
@@ -0,0 +1 @@
+#include "libavutil/riscv/cpu_common.c"
diff --git a/libavfilter/riscv/Makefile b/libavfilter/riscv/Makefile
index 277dde2aed..14a4470d96 100644
--- a/libavfilter/riscv/Makefile
+++ b/libavfilter/riscv/Makefile
@@ -1,2 +1,4 @@
 OBJS-$(CONFIG_AFIR_FILTER)                   += riscv/af_afir_init.o
 RVV-OBJS-$(CONFIG_AFIR_FILTER)               += riscv/af_afir_rvv.o
+
+SHLIBOBJS += riscv/cpu_common.o
diff --git a/libavfilter/riscv/cpu_common.c b/libavfilter/riscv/cpu_common.c
new file mode 100644
index 0000000000..17c9b392c9
--- /dev/null
+++ b/libavfilter/riscv/cpu_common.c
@@ -0,0 +1 @@
+#include "libavutil/riscv/cpu_common.c"
diff --git a/libavformat/riscv/Makefile b/libavformat/riscv/Makefile
new file mode 100644
index 0000000000..52857aacba
--- /dev/null
+++ b/libavformat/riscv/Makefile
@@ -0,0 +1 @@
+SHLIBOBJS += riscv/cpu_common.o
diff --git a/libavformat/riscv/cpu_common.c b/libavformat/riscv/cpu_common.c
new file mode 100644
index 0000000000..17c9b392c9
--- /dev/null
+++ b/libavformat/riscv/cpu_common.c
@@ -0,0 +1 @@
+#include "libavutil/riscv/cpu_common.c"
diff --git a/libavutil/riscv/Makefile b/libavutil/riscv/Makefile
index 7e9a51194b..5db4c432d9 100644
--- a/libavutil/riscv/Makefile
+++ b/libavutil/riscv/Makefile
@@ -1,7 +1,8 @@
 OBJS +=     riscv/float_dsp_init.o \
             riscv/fixed_dsp_init.o \
             riscv/lls_init.o \
-            riscv/cpu.o
+            riscv/cpu.o \
+            riscv/cpu_common.o
 RVV-OBJS += riscv/float_dsp_rvv.o \
             riscv/fixed_dsp_rvv.o \
             riscv/lls_rvv.o
diff --git a/libavutil/riscv/cpu.h b/libavutil/riscv/cpu.h
index af1440f626..bb8e08aa14 100644
--- a/libavutil/riscv/cpu.h
+++ b/libavutil/riscv/cpu.h
@@ -24,8 +24,22 @@
 #include "config.h"
 #include <stdbool.h>
 #include <stddef.h>
+#include "libavutil/attributes_internal.h"
 #include "libavutil/cpu.h"
 
+#ifndef __riscv_zbb
+extern attribute_visibility_hidden bool ff_rv_zbb_supported;
+#endif
+
+static inline av_const bool ff_rv_zbb_support(void)
+{
+#ifndef __riscv_zbb
+    return ff_rv_zbb_supported;
+#else
+    return true;
+#endif
+}
+
 #if HAVE_RVV
 /**
  * Returns the vector size in bytes (always a power of two and at least 4).
diff --git a/libavutil/riscv/cpu_common.c b/libavutil/riscv/cpu_common.c
new file mode 100644
index 0000000000..3ecf95809b
--- /dev/null
+++ b/libavutil/riscv/cpu_common.c
@@ -0,0 +1,33 @@
+/*
+ * Copyright © 2024 Rémi Denis-Courmont.
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "libavutil/cpu.h"
+
+#ifndef __riscv_zbb
+unsigned char ff_rv_zbb_supported = 0;
+
+#ifdef __ELF__
+__attribute__((constructor))
+static void probe_zbb(void)
+{
+    ff_rv_zbb_supported = (av_get_cpu_flags() & AV_CPU_FLAG_RVB_BASIC) != 0;
+}
+#endif
+#endif
diff --git a/libswscale/riscv/Makefile b/libswscale/riscv/Makefile
index 6038c8d873..7c8977d940 100644
--- a/libswscale/riscv/Makefile
+++ b/libswscale/riscv/Makefile
@@ -4,3 +4,5 @@ RV-OBJS += riscv/rgb2rgb_rvb.o
 RVV-OBJS += riscv/input_rvv.o \
             riscv/range_rvv.o \
             riscv/rgb2rgb_rvv.o
+
+SHLIBOBJS += riscv/cpu_common.o
diff --git a/libswscale/riscv/cpu_common.c b/libswscale/riscv/cpu_common.c
new file mode 100644
index 0000000000..17c9b392c9
--- /dev/null
+++ b/libswscale/riscv/cpu_common.c
@@ -0,0 +1 @@
+#include "libavutil/riscv/cpu_common.c"
diff --git a/tests/ref/fate/source b/tests/ref/fate/source
index d7b48a8b85..1703b36c02 100644
--- a/tests/ref/fate/source
+++ b/tests/ref/fate/source
@@ -3,16 +3,21 @@ libavcodec/file_open.c
 libavcodec/interplayacm.c
 libavcodec/log2_tab.c
 libavcodec/reverse.c
+libavcodec/riscv/cpu_common.c
 libavdevice/file_open.c
 libavdevice/reverse.c
+libavdevice/riscv/cpu_common.c
 libavfilter/file_open.c
 libavfilter/log2_tab.c
+libavfilter/riscv/cpu_common.c
 libavformat/file_open.c
 libavformat/golomb_tab.c
 libavformat/log2_tab.c
 libavformat/rangecoder_dec.c
+libavformat/riscv/cpu_common.c
 libswresample/log2_tab.c
 libswscale/log2_tab.c
+libswscale/riscv/cpu_common.c
 tools/uncoded_frame.c
 tools/yuvcmp.c
 Headers without standard inclusion guards: