RFC: Rewrite x86-64 IFUNC selector in C

[H.J. Lu]

On Thu, May 25, 2017 at 2:55 PM, Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
>
> 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)

Here is my take.  It only covers memcpy and mempcpy.   Please
extend it to memmove as well as *_chk functions.

-- 
H.J.
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