arm: add a cpu flag for the VFPv2 vector mode

The vector mode was deprecated in ARMv7-A/VFPv3 and various cpu
implementations do not support it in hardware. Vector mode code will
depending the OS either be emulated in software or result in an illegal
instruction on cpus which does not support it. This was not really
problem in practice since NEON implementations of the same functions are
preferred. It will however become a problem for checkasm which tests
every cpu flag separately.

Since this is a cpu feature newer cpu do not support anymore the
behaviour of this flag differs from the other flags. It can be only
activated by runtime cpu feature selection.

libavcodec/arm/dcadsp_init_arm.c

@@ -59,7 +59,7 @@ av_cold void ff_dcadsp_init_arm(DCADSPContext *s)
 {
     int cpu_flags = av_get_cpu_flags();
 
-    if (have_vfp(cpu_flags) && !have_vfpv3(cpu_flags)) {
+    if (have_vfp_vm(cpu_flags)) {
         s->lfe_fir[0]      = ff_dca_lfe_fir32_vfp;
         s->lfe_fir[1]      = ff_dca_lfe_fir64_vfp;
         s->qmf_32_subbands = ff_dca_qmf_32_subbands_vfp;
@@ -75,7 +75,7 @@ av_cold void ff_synth_filter_init_arm(SynthFilterContext *s)
 {
     int cpu_flags = av_get_cpu_flags();
 
-    if (have_vfp(cpu_flags) && !have_vfpv3(cpu_flags))
+    if (have_vfp_vm(cpu_flags))
         s->synth_filter_float = ff_synth_filter_float_vfp;
     if (have_neon(cpu_flags))
         s->synth_filter_float = ff_synth_filter_float_neon;

libavcodec/arm/fft_init_arm.c

@@ -40,7 +40,7 @@ av_cold void ff_fft_init_arm(FFTContext *s)
 {
     int cpu_flags = av_get_cpu_flags();
 
-    if (have_vfp(cpu_flags) && !have_vfpv3(cpu_flags)) {
+    if (have_vfp_vm(cpu_flags)) {
         s->fft_calc     = ff_fft_calc_vfp;
 #if CONFIG_MDCT
         s->imdct_half   = ff_imdct_half_vfp;

libavcodec/arm/fmtconvert_init_arm.c

@@ -38,7 +38,7 @@ av_cold void ff_fmt_convert_init_arm(FmtConvertContext *c, AVCodecContext *avctx
 {
     int cpu_flags = av_get_cpu_flags();
 
-    if (have_vfp(cpu_flags)) {
+    if (have_vfp_vm(cpu_flags)) {
         if (!have_vfpv3(cpu_flags)) {
             c->int32_to_float_fmul_scalar = ff_int32_to_float_fmul_scalar_vfp;
             c->int32_to_float_fmul_array8 = ff_int32_to_float_fmul_array8_vfp;

libavutil/arm/cpu.c

@@ -131,6 +131,10 @@ int ff_get_cpu_flags_arm(void)
     if (flags & AV_CPU_FLAG_ARMV6T2)
         flags |= AV_CPU_FLAG_ARMV6;
 
+    /* set the virtual VFPv2 vector mode flag */
+    if ((flags & AV_CPU_FLAG_VFP) && !(flags & (AV_CPU_FLAG_VFPV3 | AV_CPU_FLAG_NEON)))
+        flags |= AV_CPU_FLAG_VFP_VM;
+
     return flags;
 }
 

libavutil/arm/cpu.h

@@ -30,6 +30,11 @@
 #define have_vfpv3(flags)   CPUEXT(flags, VFPV3)
 #define have_neon(flags)    CPUEXT(flags, NEON)
 
+/* some functions use the VFPv2 vector mode which is deprecated in ARMv7-A
+ * and might trap on such CPU depending on the OS configuration */
+#define have_vfp_vm(flags)                                              \
+    (have_armv6(flags) && ((flags) & AV_CPU_FLAG_VFP_VM))
+
 /* Some functions use the 'setend' instruction which is deprecated on ARMv8
  * and serializing on some ARMv7 cores. This macro ensures such functions
  * are only enabled on ARMv6. */

libavutil/cpu.c

@@ -124,6 +124,7 @@ int av_parse_cpu_flags(const char *s)
         { "armv6",    NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AV_CPU_FLAG_ARMV6    },    .unit = "flags" },
         { "armv6t2",  NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AV_CPU_FLAG_ARMV6T2  },    .unit = "flags" },
         { "vfp",      NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AV_CPU_FLAG_VFP      },    .unit = "flags" },
+        { "vfp_vm",   NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AV_CPU_FLAG_VFP_VM   },    .unit = "flags" },
         { "vfpv3",    NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AV_CPU_FLAG_VFPV3    },    .unit = "flags" },
         { "neon",     NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AV_CPU_FLAG_NEON     },    .unit = "flags" },
 #elif ARCH_AARCH64
@@ -204,6 +205,7 @@ static const struct {
     { AV_CPU_FLAG_ARMV6,     "armv6"      },
     { AV_CPU_FLAG_ARMV6T2,   "armv6t2"    },
     { AV_CPU_FLAG_VFP,       "vfp"        },
+    { AV_CPU_FLAG_VFP_VM,    "vfp_vm"     },
     { AV_CPU_FLAG_VFPV3,     "vfpv3"      },
     { AV_CPU_FLAG_NEON,      "neon"       },
 #elif ARCH_PPC

libavutil/cpu.h

@@ -62,6 +62,7 @@
 #define AV_CPU_FLAG_VFPV3        (1 << 4)
 #define AV_CPU_FLAG_NEON         (1 << 5)
 #define AV_CPU_FLAG_ARMV8        (1 << 6)
+#define AV_CPU_FLAG_VFP_VM       (1 << 7) ///< VFPv2 vector mode, deprecated in ARMv7-A and unavailable in various CPUs implementations
 
 /**
  * Return the flags which specify extensions supported by the CPU.

libavutil/version.h

@@ -54,7 +54,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR 55
-#define LIBAVUTIL_VERSION_MINOR  3
+#define LIBAVUTIL_VERSION_MINOR  4
 #define LIBAVUTIL_VERSION_MICRO  0
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \

tests/checkasm/checkasm.c

@@ -89,6 +89,7 @@ static const struct {
     { "ARMV6",    "armv6",    AV_CPU_FLAG_ARMV6 },
     { "ARMV6T2",  "armv6t2",  AV_CPU_FLAG_ARMV6T2 },
     { "VFP",      "vfp",      AV_CPU_FLAG_VFP },
+    { "VFP_VM",   "vfp_vm",   AV_CPU_FLAG_VFP_VM },
     { "VFPV3",    "vfp3",     AV_CPU_FLAG_VFPV3 },
     { "NEON",     "neon",     AV_CPU_FLAG_NEON },
 #elif ARCH_PPC