The B extension was finally ratified in May 2024, encompassing:
- Zba (addresses),
- Zbb (basics) and
- Zbs (single bits).
It does not include Zbc (base-2 polynomials).
If __riscv_hwprobe() fails, then the kernel version is presumably too
old. There is not much point falling back to the auxillary vector.
- The Linux kernel requires I, so the flag is always set on Linux, and
run-time detection is unnecessary. Our RISC-V assembler does anyway not
support targets without I.
- Linux can compile with or without F and D, but it cannot perform
run-time detection for them (a kernel with F support will not boot a
processor without F). The run-time detection is thus useless in that
case. Besides F and D extensions are used throughout the C code, so
their run-time detection would not be practical.
- Support for V was added in a later kernel version than riscv_hwprobe(),
so the system call will always be available if the kernel supports V.
The only exception would be vendor kernel forks, but those are known to
haphasardly pretend to support V on systems without actual V support, or
with only pre-ratification binary-incompatible version. Furthermore, a
large chunk of our optimisations require Zba and/or Zbb which cannot be
detected with HWCAP in those kernels.
For what it is worth, OpenJDK already took a similar action. Note that this
keeps AT_HWCAP usage for platforms with neither C run-time <sys/hwprobe.h>
nor kernel <asm/hwprobe.h>, notably kernels other than Linux.
The B Bit manipulation extension was not defined to this day, and
probably never will. Instead it was broken down into Zba, Zbb, Zbc and
Zbs with no particular blessed set to make up B.
This removes the bogus field test. Linux never set this bit, nor
(AFAICT) did FreeBSD or any other OS. We can always add it back in the
unlikely event that it gets taken into use.
Not all C run-times support this, and even then, it will be a while
before distributions provide recent enough versions thereof.
Since this is a trivial system call wrapper, we might just as well call
the corresponding kernel system call directly where the C run-time lacks
support but the kernel headers are new enough (as is the case on Debian
Unstable at the time of writing). In doing so, we need to add a few more
guards as the first suitable kernel (headers) release did not expose the
V, Zba and Zbb extensions.
This adds the Linux-specific function call to detect CPU features. Unlike
the more portable auxillary vector, this supports extensions other than
single lettered ones. At this point, FFmpeg already needs this to detect
Zba and Zbb at run-time, and probably will need it for Zvbb in the near
future.
Support will be available in glibc 2.40 onward.
The code was blindly assuming that Zbb or V implied Zba. While the
earlier is practically always true, the later broke some QEMU setups,
as V was introduced earlier than Zba.
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.
RVV defines a total of 12 different extensions, including:
- 5 different instruction subsets:
- Zve32x: 8-, 16- and 32-bit integers,
- Zve32f: Zve32x plus single precision floats,
- Zve64x: Zve32x plus 64-bit integers,
- Zve64f: Zve32f plus Zve64x,
- Zve64d: Zve64f plus double precision floats.
- 6 different vector lengths:
- Zvl32b (embedded only),
- Zvl64b (embedded only),
- Zvl128b,
- Zvl256b,
- Zvl512b,
- Zvl1024b,
- and the V extension proper: equivalent to Zve64f and Zvl128b.
In total, there are 6 different possible sets of supported instructions
(including the empty set), but for convenience we allocate one bit for
each type sets: up-to-32-bit ints (RVV_I32), floats (RVV_F32),
64-bit ints (RVV_I64) and doubles (RVV_F64).
Whence the vector size is needed, it can be retrieved by reading the
unprivileged read-only vlenb CSR. This should probably be a separate
helper macro if needed at a later point.
This introduces compile-time and run-time CPU detection on RISC-V. In
practice, I doubt that FFmpeg will ever see a RISC-V CPU without all of
I, F and D extensions, and if it does, it probably won't have run-time
detection. So the flags are essentially always set.
But as things stand, checkasm wants them that way. Compare the ARMV8
flag on AArch64. We are nowhere near running short on CPU flag bits.
