[X86] Simplify __cpuid_count

[MaskRay]

Summary

Use the x86-32 implementation for x86-64, i.e. replace =r plus xchgq with =b.
Using =r seems like a workaround when RBX as a base register did not
work well =b (2014-09 416efb5f909e45f67ee81d382a708fc41666c5c6), likely
no longer relevant today.

[craig.topper]

I think it's still relevant. We had the same issue on Windows as recently as https://github.com/llvm/llvm-project/issues/49477 see also https://github.com/llvm/llvm-project/issues/17204

[MaskRay]

Ah, OK. I am investigating a miscompile that : "=a"(__eax), "=r" (__ebx), "=c"(__ecx), "=d"(__edx) \ may assign "=r" to use RDX. The output then looks like

xchgq %rbx, %rdx
cpuid
xchgq %rbx, %rdx

and rbx is clobbered, since cpuid writes rax/rbx/rcx/rdx.

This patch can work around the bug.

clang/lib/Headers/cpuid.h

 #define bit_3DNOWP      0x40000000
 #define bit_3DNOW       0x80000000
 
 /* Features in %ebx for leaf 0x80000008 */
 #define bit_CLZERO      0x00000001
 #define bit_WBNOINVD    0x00000200
 
 
-#if __i386__
 #define __cpuid(__leaf, __eax, __ebx, __ecx, __edx) \
     __asm("cpuid" : "=a"(__eax), "=b" (__ebx), "=c"(__ecx), "=d"(__edx) \
                   : "0"(__leaf))
 
 #define __cpuid_count(__leaf, __count, __eax, __ebx, __ecx, __edx) \
     __asm("cpuid" : "=a"(__eax), "=b" (__ebx), "=c"(__ecx), "=d"(__edx) \
                   : "0"(__leaf), "2"(__count))
-#else
-/* x86-64 uses %rbx as the base register, so preserve it. */
-#define __cpuid(__leaf, __eax, __ebx, __ecx, __edx) \
-    __asm("  xchgq  %%rbx,%q1\n" \
-          "  cpuid\n" \
-          "  xchgq  %%rbx,%q1" \
-        : "=a"(__eax), "=r" (__ebx), "=c"(__ecx), "=d"(__edx) \
-        : "0"(__leaf))
-
-#define __cpuid_count(__leaf, __count, __eax, __ebx, __ecx, __edx) \
-    __asm("  xchgq  %%rbx,%q1\n" \
-          "  cpuid\n" \
-          "  xchgq  %%rbx,%q1" \
-        : "=a"(__eax), "=r" (__ebx), "=c"(__ecx), "=d"(__edx) \
-        : "0"(__leaf), "2"(__count))
-#endif
 
 static __inline int __get_cpuid_max (unsigned int __leaf, unsigned int *__sig)
 {
     unsigned int __eax, __ebx, __ecx, __edx;
 #if __i386__
     int __cpuid_supported;
 
     __asm("  pushfl\n"