Re: Detection of hadware feature => please do not use signal

I wrote:
> But that developer.arm.com page seems pretty definitive. Tomorrow
> I'll have a go at spinning up a 32-bit NetBSD installation and test
> it. Thanks for the research!

Conclusions:

1. Unsurprisingly, a 64-bit kernel exposes machdep.cpuN.cpu_id but
not machdep.id_isar; a 32-bit kernel the reverse.

2. 32-bit userland does not provide <aarch64/armreg.h>. (So I did
not try to use the ID_AA64ISAR0_EL1_CRC32 macro: we'd have had to
hard-code the field position anyway for 32-bit.)

3. It doesn't look to me like NetBSD really supports 32-bit userland
under a 64-bit kernel. Maybe it'd kind of work, but utilities like
cpuctl seem to be built for only one of the kernel APIs.

So I end up proposing the attached. If we do somehow find ourselves
running 32-bit with a 64-bit kernel, this will report "no CRC"
because the sysctl call will fail. I don't think it's worth
sweating over that case.

BTW, I found out that by default we'll build a software-CRC-only
implementation on 32-bit ARM NetBSD, at least in the "generic"
eabihf build:

checking for __crc32cb, __crc32ch, __crc32cw, and __crc32cd with CFLAGS=... no
checking for __crc32cb, __crc32ch, __crc32cw, and __crc32cd with CFLAGS=-march=armv8-a+crc... no
checking which CRC-32C implementation to use... slicing-by-8

It turns out that -march=armv8-a+crc would work, except:

configure:17624: checking for __crc32cb, __crc32ch, __crc32cw, and __crc32cd with CFLAGS=-march=armv8-a+crc
configure:17648: ccache gcc -o conftest -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wimplicit-fallthrough=3 -Wcast-function-type -Wshadow=compatible-local -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -Wno-format-truncation -Wno-stringop-truncation -g -O2 -march=armv8-a+crc -Wl,-z,now -Wl,-z,relro conftest.c -lz -ledit -lcurses -lpthread -lexecinfo -lrt -lm >&5
cc1: error: '-mfloat-abi=hard': selected processor lacks an FPU

If you force it with a compatible -mfpu switch (I used -mfpu=neon)
then you get a build with runtime CRC check, and all seems to work.
I did not try very hard to determine what the default -mfpu is here
or why -march=armv8-a+crc overrides whatever the default is. This
probably means that no end user is going to see any benefit from this
exercise, because they are not likely to try adding -mfpu by itself.
If they add "-march=armv8-a+crc -mfpu=whatever" to CFLAGS manually
then configure will deem that a runtime check is not required, so this
code still doesn't get exercised.

tl;dr: a lot of time spent on a probably-useless exercise. Still,
now that we've got the code we may as well commit it. The configure
issue was there all along.

regards, tom lane