https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97688
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> --- Hmm, it doesn't work on a i7-8565U either ... Doesn't work means check_vect () does exit (0). Those do not end up UNSUPPORTED (not sure if using a magic exit code to communicate this would be possible). Looks like $rcx is not cleared before 'cpuid' and we get garbage?! If I set $rcx to zero it works. The comment in cpuid.h says /* At least one cpu (Winchip 2) does not set %ebx and %ecx for cpuid leaf 1. Forcibly zero the two registers before calling cpuid as a precaution. */ but it does the clearing only for constant level (doesn't work at -O0 which is where I debugged check_vect). At -O2 I see Dump of assembler code for function check_vect: 0x0000000000400680 <+0>: push %rbx 0x0000000000400681 <+1>: mov $0x4006c0,%esi 0x0000000000400686 <+6>: mov $0x4,%edi 0x000000000040068b <+11>: callq 0x400490 <signal@plt> => 0x0000000000400690 <+16>: xor %eax,%eax 0x0000000000400692 <+18>: cpuid 0x0000000000400694 <+20>: cmp $0x6,%eax 0x0000000000400697 <+23>: jbe 0x4006a5 <check_vect+37> 0x0000000000400699 <+25>: mov $0x7,%eax 0x000000000040069e <+30>: cpuid 0x00000000004006a0 <+32>: and $0x20,%ebx 0x00000000004006a3 <+35>: jne 0x4006ac <check_vect+44> 0x00000000004006a5 <+37>: xor %edi,%edi 0x00000000004006a7 <+39>: callq 0x4004a0 <exit@plt> 0x00000000004006ac <+44>: xor %esi,%esi 0x00000000004006ae <+46>: mov $0x4,%edi 0x00000000004006b3 <+51>: pop %rbx 0x00000000004006b4 <+52>: jmpq 0x400490 <signal@plt> so no zeroing of the regs of the result of the __maxlevel call and we call cpuid with rax 0x7 7 rbx 0x68747541 1752462657 rcx 0x444d4163 1145913699 rdx 0x69746e65 1769238117 rsi 0x7fffffffd8b0 140737488345264 rdi 0x0 0 and on zen2 get rax 0x0 0 rbx 0x0 0 rcx 0x0 0 rdx 0x0 0 on i7-8565U the garbage is similar: rax 0x7 rbx 0x756e6547 rcx 0x6c65746e rdx 0x49656e69 with same all-zero result.
