I don't understand the compile failure under Clang, since the
respective code is guarded by `defined(__GNUC__)`[1].

Anyhow, we should use feature detection (AC_TRY_LINK) instead of
inferring by compiler, in my opinion.

[1] <https://github.com/php/php-src/blob/php-7.3.0beta3/Zend/zend_cpuinfo.c#L31-L35>

clang defines __GNUC__. clang identifies itself as gcc 4.2.1. If you want to check if it's actually clang, you could check for __clang__.

The problem is still there in 7.3.0RC2.

The system does have cpuid.h, but this cpuid.h does not have __cpuid_count.

Here is a fresh build log from PHP 7.3.0RC2:

https://build.macports.org/builders/ports-10.7_x86_64_legacy-builder/builds/78703/steps/install-port/logs/stdio

Does it work for you if you remove the HAVE_CPUID_H part[1] (plus
the respective #endif[2])?

[1] <https://github.com/php/php-src/blob/php-7.3.0RC2/Zend/zend_cpuinfo.c#L32-L37>
[2] <https://github.com/php/php-src/blob/php-7.3.0RC2/Zend/zend_cpuinfo.c#L45>

Yes, if I edit Zend/zend_cpuinfo.c and change "# ifdef HAVE_CPUID_H" to "# if 0" then it builds and runs. So I think it just needs a more specific check for the existence of __cpuid_count and not just the existence of cpuid.h.

Please check the attached check-cpuid-count patch.

Sorry for the delay. The configure test says __cpuid_count
is available, though it's obviously not. Here's what
config.log says about it:


configure:5874: checking whether __cpuid_count is available
configure:5896: /usr/bin/clang -c -pipe -Os -arch x86_64 -fvisibility=hidden -I/opt/local/include -no-cpp-precomp conftest.c >&5
conftest.c:34:3: warning: implicit declaration of function '__cpuid_count' is invalid in C99 [-Wimplicit-function-declaration]
  __cpuid_count(0, 0, eax, ebx, ecx, edx);
  ^
1 warning generated.
configure:5896: $? = 0
configure:5907: result: yes


It looks like the problem is that the compiler is being
invoked using the "-c" flag, which skips the link phase
which is where the error occurs. I can reproduce this
with a small test program:


$ clang -c test.c; echo $?
test.c:5:1: warning: implicit declaration of function '__cpuid_count' is invalid in C99 [-Wimplicit-function-declaration]
__cpuid_count(0, 0, eax, ebx, ecx, edx);
^
1 warning generated.
0


$ clang test.c; echo $?
test.c:5:1: warning: implicit declaration of function '__cpuid_count' is invalid in C99 [-Wimplicit-function-declaration]
__cpuid_count(0, 0, eax, ebx, ecx, edx);
^
1 warning generated.
Undefined symbols for architecture x86_64:
  "___cpuid_count", referenced from:
      _main in test-eIiJ3P.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
1

> It looks like the problem is that the compiler is being invoked
> using the "-c" flag, which skips the link phase which is where
> the error occurs.

Then we'll have to replace the `AC_COMPILE_IFELSE` with
`AC_LINK_IFELSE`.  Can you please try that?

Yes, it works with AC_LINK_IFELSE. Thanks!