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Re: [PATCH] x86: Correct index_cpu_LZCNT [BZ # 23456]
- From: Carlos O'Donell <carlos at redhat dot com>
- To: "H.J. Lu" <hjl dot tools at gmail dot com>, libc-alpha at sourceware dot org
- Date: Thu, 26 Jul 2018 14:05:42 -0400
- Subject: Re: [PATCH] x86: Correct index_cpu_LZCNT [BZ # 23456]
- References: <20180726164650.6543-1-hjl.tools@gmail.com>
On 07/26/2018 12:46 PM, H.J. Lu wrote:
> cpu-features.h has
>
> #define bit_cpu_LZCNT (1 << 5)
> #define index_cpu_LZCNT COMMON_CPUID_INDEX_1
> #define reg_LZCNT
>
> But the LZCNT feature bit is in COMMON_CPUID_INDEX_80000001:
>
> Initial EAX Value: 80000001H
> ECX Extended Processor Signature and Feature Bits:
> Bit 05: LZCNT available
>
> index_cpu_LZCNT should be COMMON_CPUID_INDEX_80000001, not
> COMMON_CPUID_INDEX_1. The VMX feature bit is in COMMON_CPUID_INDEX_1:
>
> Initial EAX Value: 01H
> Feature Information Returned in the ECX Register:
> 5 VMX
>
> [BZ # 23456]
> * sysdeps/x86/cpu-features.h (index_cpu_LZCNT): Set to
> COMMON_CPUID_INDEX_80000001.
OK for 2.28.
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
> ---
> sysdeps/x86/cpu-features.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/sysdeps/x86/cpu-features.h b/sysdeps/x86/cpu-features.h
> index 624e681e96..3807a9d611 100644
> --- a/sysdeps/x86/cpu-features.h
> +++ b/sysdeps/x86/cpu-features.h
> @@ -204,7 +204,7 @@ extern const struct cpu_features *__get_cpu_features (void)
> # define index_cpu_HTT COMMON_CPUID_INDEX_1
> # define index_cpu_BMI1 COMMON_CPUID_INDEX_7
> # define index_cpu_BMI2 COMMON_CPUID_INDEX_7
> -# define index_cpu_LZCNT COMMON_CPUID_INDEX_1
> +# define index_cpu_LZCNT COMMON_CPUID_INDEX_80000001
Confirmed.
CPUID.EAX=80000001H:ECX.LZCNT[bit 5]: if 1 indicates the processor supports the LZCNT instruction.
> # define index_cpu_MOVBE COMMON_CPUID_INDEX_1
> # define index_cpu_POPCNT COMMON_CPUID_INDEX_1
> # define index_cpu_IBT COMMON_CPUID_INDEX_7
>