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Re: RFC: Rewrite x86-64 IFUNC selector in C



On 25/05/2017 18:38, H.J. Lu wrote:
> On Thu, May 25, 2017 at 2:25 PM, Erich Elsen <eriche@google.com> wrote:
>> Ok, I'll get started then.
>>
>> Are there any general comments about the attached conversion for
>> memcpy?  Just so I don't repeat the same wrong thing many times.
> 
> You missed:
> 
> /* Define multiple versions only for the definition in lib and for
>    DSO.  In static binaries we need memcpy before the initialization
>    happened.  */
> #if defined SHARED && IS_IN (libc)
> 
> +typedef void * (*memcpy_fn)(void *, const void *, size_t);
> +
> +extern void * __memcpy_erms(void *dest, const void *src, size_t n);
> +extern void * __memcpy_sse2_unaligned(void *dest, const void *src, size_t n);
> +extern void * __memcpy_sse2_unaligned_erms(void *dest, const void
> *src, size_t n);
> +extern void * __memcpy_ssse3(void *dest, const void *src, size_t n);
> +extern void * __memcpy_ssse3_back(void *dest, const void *src, size_t n);
> +extern void * __memcpy_avx_unaligned(void *dest, const void *src, size_t n);
> +extern void * __memcpy_avx_unaligned_erms(void *dest, const void
> *src, size_t n);
> +extern void * __memcpy_avx512_unaligned(void *dest, const void *src, size_t n);
> +extern void * __memcpy_avx512_unaligned_erms(void *dest, const void
> *src, size_t n);
> 
> Please use something similar to multiarch/strstr.c:
> 
> /* Redefine strstr so that the compiler won't complain about the type
>    mismatch with the IFUNC selector in strong_alias, below.  */
> #undef  strstr
> #define strstr __redirect_strstr
> #include <string.h>
> #undef  strstr
> ...
> extern __typeof (__redirect_strstr) __strstr_sse2 attribute_hidden;
> 
> +/* Defined in cacheinfo.c */
> +extern long int __x86_shared_cache_size attribute_hidden;
> +extern long int __x86_shared_cache_size_half attribute_hidden;
> +extern long int __x86_data_cache_size attribute_hidden;
> +extern long int __x86_data_cache_size_half attribute_hidden;
> +extern long int __x86_shared_non_temporal_threshold attribute_hidden;

It seems it will be used not only for memcpy, so I would suggest to add
on a common header on multiarch.

> 
> Remove them.
> static void * select_memcpy_impl(void) {
> +  const struct cpu_features* cpu_features_struct_p = __get_cpu_features ();
> +
> +  if (CPU_FEATURES_ARCH_P(cpu_features_struct_p, Prefer_ERMS)) {
> +    return __memcpy_erms;
> +  }
> +
> +  if (CPU_FEATURES_ARCH_P(cpu_features_struct_p, AVX512F_Usable)) {
> +    if (CPU_FEATURES_ARCH_P(cpu_features_struct_p, Prefer_No_VZEROUPPER))
> +      return __memcpy_avx512_unaligned_erms;
> +    return __memcpy_avx512_unaligned;
> +  }
> +
> +  if (CPU_FEATURES_ARCH_P(cpu_features_struct_p, AVX_Fast_Unaligned_Load)) {
> +    if (CPU_FEATURES_CPU_P(cpu_features_struct_p, ERMS)) {
> +      return __memcpy_avx_unaligned_erms;
> +
> +    }
> +    return __memcpy_avx_unaligned;
> +  }
> +  else {
> +    if (CPU_FEATURES_ARCH_P(cpu_features_struct_p, Fast_Unaligned_Copy)) {
> +      if (CPU_FEATURES_CPU_P(cpu_features_struct_p, ERMS)) {
> +        return __memcpy_sse2_unaligned_erms;
> +
> +      }
> +      return __memcpy_sse2_unaligned;
> +    }
> +    else {
> +      if (!CPU_FEATURES_CPU_P(cpu_features_struct_p, SSSE3)) {
> +        return __memcpy_sse2_unaligned;
> +
> +      }
> +      if (CPU_FEATURES_ARCH_P(cpu_features_struct_p, Fast_Copy_Backward)) {
> +        return __memcpy_ssse3_back;
> +
> +      }
> +      return __memcpy_ssse3;
> +    }
> +  }
> +}
> 
> Please
> 
> 1. Fix formatting.
> 2. Remove unnecessary {}.
> 3. Don't use "else".
> 
> +void *__new_memcpy(void *dest, const void *src, size_t n)
> +  __attribute__ ((ifunc ("select_memcpy_impl")));
> 
> Use "typeof" here.

We have the libc_ifunc{_redirect} to handle the __attribute__ ((ifunc)) support
from compiler.  I think you can use:

# include <string.h>

// extern __typeof (memcpy) __memcpy_<each supported one> attribute_hidden;

static void *memcpy_selector (void)
{
  // fill me.
}

libc_ifunc_hidden (memcpy, memcpy, memcpy_selector);
libc_hidden_def (memcpy)


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