Properly detect CRC32 APIs on aarch64 from configure.

[oerdnj]

The CRC32 APIs are optional for armv8-a. They became mandatory since
armv8.1-a.

[oerdnj]

The hard requirement for crc32 APIs on arm64 breaks compilation on Debian Jessie (I know, I know, this is going away) and on any compilation where CRC32 APIs are not available, f.e. the Renesas board doesn't have them implemented according to OSSystems/meta-browser@527ff15

[oerdnj]

@remicollet the old(er) CentOS probably don't have arm64 architecture, so this is not a problem for you, right?

[nikic]

Just to make sure, you did check that it continues to work if the API is available?

This seems somewhat weird in that actual availability of the ISA extension usually doesn't matter when it comes to compiling code (as the decision whether to use the code will be made at runtime). I guess this doesn't hold up if the specified target doesn't support it in any configuration...

[oerdnj]

Ok, this needs slight update. The __crc32d functions are header only, so it needs AC_LINK_IFELSE() macro. With that I get:

1. CFLAGS="-march=armv8-a -Og -g" ./configure --disable-all produces:

checking for aarch64 CRC32 API... no

2. CFLAGS="-march=armv8.1-a -Og -g" ./configure --disable-all produces:

checking for aarch64 CRC32 API... yes

And by manually adding #error macros to ext/standard/crc32.c branch, I get the correct results with both -march settings.

(I am running Debian buster arm64 in schroot using qemu-user-static FWIW...)

[oerdnj]

This seems somewhat weird in that actual availability of the ISA extension usually doesn't matter when it comes to compiling code (as the decision whether to use the code will be made at runtime). I guess this doesn't hold up if the specified target doesn't support it in any configuration...

It is weird, but from distro perspective it's the decision that's being made by the distros (e.g. what is the minimal denominator). This is already causing big problem on Raspbian because somebody made a decision that armhf on Raspbian is something else than armhf on stock Debian virtually making the packages not compatible on older arm architectures (RPi Zero, RPi 1/2a)...

But there's not much you can do apart from having an emulator that would kick in everytime invalid instruction is encountered... well and properly detecting the features based on -march.

[nikic]

Looks like an ltmain.sh update snuck in here.

[oerdnj]

Looks like an ltmain.sh update snuck in here.

Ooops, sorry, removed.

[nikic]

LGTM, CI failures are unrelated.

[carusogabriel]

Should we apply these changes to

php-src/Zend/zend_multiply.h

Line 69 in 623911f

#elif defined(__aarch64__) && defined(__GNUC__)

as well?

[nikic]

@carusogabriel As zend_multiply.h does not use crc32 instructions: No.