[Bug target/83401] New: cpuid function for leaf 7 may not be accessed when sub-leaf (%ecx) is undefined

[warnerme at ptd dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83401

            Bug ID: 83401
           Summary: cpuid function for leaf 7 may not be accessed when
                    sub-leaf (%ecx) is undefined
           Product: gcc
           Version: 5.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: warnerme at ptd dot net
  Target Milestone: ---

In the file cpuid.h, there is a problem with the definition:

#define __cpuid(level, a, b, c, d)                      \
  __asm__ ("cpuid\n\t"                                  \
           : "=a" (a), "=b" (b), "=c" (c), "=d" (d)     \
           : "0" (level))

With the ecx undefined, the leaf 7 (Extended Features %eax == 7) can not be
accessed.  Leaf 7 (%eax) can only be accessed when sub-leaf (%ecx) is 0

I suggest instead:
#define __cpuid(level, a, b, c, d)                      \
  __asm__ ("cpuid\n\t"                                  \
           : "=a" (a), "=b" (b), "=c" (c), "=d" (d)     \
           : "0" (level), "2" (0))