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diff --git a/m4/ax_gcc_x86_cpuid.m4 b/m4/ax_gcc_x86_cpuid.m4
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+dnl @synopsis AX_GCC_X86_CPUID(OP)
+dnl @summary run x86 cpuid instruction OP using gcc inline assembler
+dnl @category Misc
+dnl
+dnl On Pentium and later x86 processors, with gcc or a compiler that
+dnl has a compatible syntax for inline assembly instructions, run
+dnl a small program that executes the cpuid instruction with
+dnl input OP.  This can be used to detect the CPU type.
+dnl
+dnl On output, the values of the eax, ebx, ecx, and edx registers
+dnl are stored as hexadecimal strings as "eax:ebx:ecx:edx" in
+dnl the cache variable ax_cv_gcc_x86_cpuid_OP.
+dnl
+dnl If the cpuid instruction fails (because you are running a cross-compiler,
+dnl or because you are not using gcc, or because you are on a processor
+dnl that doesn't have this instruction), ax_cv_gcc_x86_cpuid_OP is set
+dnl to the string "unknown".
+dnl
+dnl This macro mainly exists to be used in AX_GCC_ARCHFLAG.
+dnl
+dnl @version 2008-12-06
+dnl @license GPLWithACException
+dnl @author Steven G. Johnson <stevenj@alum.mit.edu> and Matteo Frigo.
+AC_DEFUN([AX_GCC_X86_CPUID],
+[AC_REQUIRE([AC_PROG_CC])
+AC_LANG_PUSH([C])
+AC_CACHE_CHECK([for x86 cpuid $1 output], [ax_cv_gcc_x86_cpuid_$1],
+ [AC_RUN_IFELSE([AC_LANG_PROGRAM([#include <stdio.h>], [
+     int op = $1, eax, ebx, ecx, edx;
+     FILE *f;
+#if defined(__amd64__) || defined(__amd64) || defined(__x86_64__) || defined(__x86_64)
+     __asm__("push %%rbx\n\t"
+             "cpuid\n\t"
+             "pop %%rbx"
+             : "=a" (eax), "=c" (ecx), "=d" (edx)
+             : "a" (op));
+     __asm__("push %%rbx\n\t"
+             "cpuid\n\t"
+             "mov %%rbx, %%rax\n\t"
+             "pop %%rbx"
+             : "=a" (ebx), "=c" (ecx), "=d" (edx)
+             : "a" (op));
+#else
+     __asm__("push %%ebx\n\t"
+             "cpuid\n\t"
+             "pop %%ebx"
+             : "=a" (eax), "=c" (ecx), "=d" (edx)
+             : "a" (op));
+     __asm__("push %%ebx\n\t"
+             "cpuid\n\t"
+             "mov %%ebx, %%eax\n\t"
+             "pop %%ebx"
+             : "=a" (ebx), "=c" (ecx), "=d" (edx)
+             : "a" (op));
+#endif
+     f = fopen("conftest_cpuid", "w"); if (!f) return 1;
+     fprintf(f, "%x:%x:%x:%x\n", eax, ebx, ecx, edx);
+     fclose(f);
+     return 0;
+])], 
+     [ax_cv_gcc_x86_cpuid_$1=`cat conftest_cpuid`; rm -f conftest_cpuid],
+     [ax_cv_gcc_x86_cpuid_$1=unknown; rm -f conftest_cpuid],
+     [ax_cv_gcc_x86_cpuid_$1=unknown])])
+AC_LANG_POP([C])
+])