[PATCH] configure.ac: fix __get_cpuid_count check to catch link failure

[Carl Edquist]

(Was: Re: [PATCH] wc: Add AVX2 optimization when counting only lines)

So, it looks like the check for avx2 support is broken - the build broke 
for me after the update.  Seems it is doing a compile check when a link 
check is necessary.

I was seeing this in my build:

    src/wc.c: In function 'avx2_supported':
    src/wc.c:153:13: warning: implicit declaration of function 
'__get_cpuid_count' [-Wimplicit-function-declaration]
           if (! __get_cpuid_count (7, 0, &eax, &ebx, &ecx, &edx))
                 ^

    ...

    src/wc.o: In function `avx2_supported':
    /.../coreutils/src/wc.c:153: undefined reference to `__get_cpuid_count'
    collect2: error: ld returned 1 exit status
    Makefile:9415: recipe for target 'src/wc' failed


My config.h has:

    #define USE_AVX2_WC_LINECOUNT 1


My config.log has:

    configure:77113: checking if __get_cpuid_count exists
    configure:77129: gcc -c -g -O2  conftest.c >&5
    conftest.c: In function 'main':
    conftest.c:842:7: warning: implicit declaration of function 
'__get_cpuid_count' [-Wimplicit-function-declaration]
           __get_cpuid_count(7, 0, &eax, &ebx, &ecx, &edx);
           ^
    configure:77129: $? = 0
    configure:77131: result: yes


... So it looks like '__get_cpuid_count' not being declared is just 
treated as a warning, and so the compile test succeeds.  The actual 
failure happens at link time.  Would it be more appropriate to use 
AC_LINK_IFELSE instead of AC_COMPILE_IFELSE ?

I tried swapping this out (as in the attached patch), and now it correctly 
omits the #define for USE_AVX2_WC_LINECOUNT, and wc compiles & links 
properly.


Thanks,
Carl

0001-configure.ac-fix-__get_cpuid_count-check.patch
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