Re:Glibc sysdeps: dl_platform detection effectively performs...
by arghwhat on Tuesday September 10, 2019 @10:13AM
(#20928397)
> Do IBM engineers have responsibility towards Qualcomm engineers
Yes, in every way. However, with Qualcomm not having any PowerPC architectures, there isn't much harm that could be done. And for reference, Intel also tests AMD as they should, and even submits performance improvements for AMD as they should.
But this is quite bad: Instead of: `if (supports(generic_feature)) { do_with_generic_feature(); } else { slow_approach(); }`, they did `if (intel_haswell) { do_with_generic_feature(); } else { slow_approach(); }`.
The only time where that is acceptable from as big a contributor as Intel, is if they tested and concluded that other CPUs were actually slower using this feature.
> What happens if there's a new CPU company in two years that would also happen to work fine with these flags ?
That is exactly why the feature flags exist in this generic code! You can query what instruction sets are available on the CPU. Vendor-specific code should only be added to deal with product-specific defect.
Yes, in every way. However, with Qualcomm not having any PowerPC architectures, there isn't much harm that could be done. And for reference, Intel also tests AMD as they should, and even submits performance improvements for AMD as they should.
But this is quite bad: Instead of: `if (supports(generic_feature)) { do_with_generic_feature(); } else { slow_approach(); }`, they did `if (intel_haswell) { do_with_generic_feature(); } else { slow_approach(); }`.
The only time where that is acceptable from as big a contributor as Intel, is if they tested and concluded that other CPUs were actually slower using this feature.
> What happens if there's a new CPU company in two years that would also happen to work fine with these flags ?
That is exactly why the feature flags exist in this generic code! You can query what instruction sets are available on the CPU. Vendor-specific code should only be added to deal with product-specific defect.