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Re: Support for Runtime CPU type detection via builtins (issue5754058)
On Mon, Apr 23, 2012 at 1:43 PM, Sriraman Tallam <firstname.lastname@example.org> wrote:
> On Mon, Apr 23, 2012 at 12:16 PM, Uros Bizjak <email@example.com> wrote:
>> On Mon, Apr 23, 2012 at 6:59 PM, Sriraman Tallam <firstname.lastname@example.org> wrote:
>>>>> i386 maintainers - ?Is this patch ok?
>>>> Has the community reached the consensus on how this kind of
>>>> functionality has to be implemented? I have followed the discussion a
>>>> bit, but IIRC, there was no clear decision. Without this decision, I
>>>> am not able to review the _implementation_ of agreed functionality for
>>>> x86 target.
>>>> (I apologize if I have missed the decision, please point me to the
>>>> discussion in this case.)
>>> The discussions are here:
>>> and follow-ups to this.
>>> I am not sure about consensus, but the important points raised were:
>>> 1) Constructor ordering: What if some constructors fire before
>>> cpu_indicator_init?, which determines the CPU. I addressed this
>>> problem by making the priority of cpu_indicator_init to be the highest
>>> possible. Still, IFUNC initializers will fire before and they have to
>>> explicitly call __builtin_cpu_init() before checking the CPU type.
>>> 2) Reducing the number of builtins : It is only two now.
>>> ? ? ? * config/i386/i386.c (build_processor_features_struct): New function.
>>> ? ? ? (build_processor_model_struct): New function.
>>> ? ? ? (make_var_decl): New function.
>>> ? ? ? (get_field_from_struct): New function.
>>> ? ? ? (fold_builtin_target): New function.
>>> ? ? ? (ix86_fold_builtin): New function.
>>> ? ? ? (ix86_expand_builtin): Expand new builtins by folding them.
>>> ? ? ? (make_cpu_type_builtin): New functions.
>>> ? ? ? (ix86_init_platform_type_builtins): Make the new builtins.
>>> ? ? ? (ix86_init_builtins): Make new builtins to detect CPU type.
>>> ? ? ? (TARGET_FOLD_BUILTIN): New macro.
>>> ? ? ? (IX86_BUILTIN_CPU_INIT): New enum value.
>>> ? ? ? (IX86_BUILTIN_CPU_IS): New enum value.
>>> ? ? ? (IX86_BUILTIN_CPU_SUPPORTS): New enum value.
>>> ? ? ? * config/i386/i386-builtin-types.def: New function type.
>>> ? ? ? * testsuite/gcc.target/builtin_target.c: New testcase.
>>> ? ? ? * doc/extend.texi: Document builtins.
>>> ? ? ? * libgcc/config/i386/i386-cpuinfo.c: New file.
>>> ? ? ? * libgcc/config/i386/t-cpuinfo: New file.
>>> ? ? ? * libgcc/config.host: Include t-cpuinfo.
>>> ? ? ? * libgcc/config/i386/libgcc-glibc.ver: Version symbols __cpu_model
>>> ? ? ? and __cpu_features.
>> The patch is OK.
>> I guess that AVX is left as an exercise for a x86 maintainer ;)
> I will add AVX, and make all the changes.
3 more comments:
1. Your implementation isn't thread-safe. With my suggestion of
you should change get_intel_cpu to return processor_vendor and
set _cpu_model.__cpu_vendor after setting up all other bits.
+ unsigned int __cpu_cmov : 1;
+ unsigned int __cpu_mmx : 1;
+ unsigned int __cpu_popcnt : 1;
+ unsigned int __cpu_sse : 1;
+ unsigned int __cpu_sse2 : 1;
+ unsigned int __cpu_sse3 : 1;
+ unsigned int __cpu_ssse3 : 1;
+ unsigned int __cpu_sse4_1 : 1;
+ unsigned int __cpu_sse4_2 : 1;
may not be thread-safe since only char/short/int/long long
stores are atomic and it isn't extensible. You can use
unsigned int __cpu_features;
to make it extensible and store atomic.
3. You may move
unsigned int __cpu_features;
into struct __processor_model to only export one symbol
instead of 2.