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How does this work long term? So say ten years down the line.. RVA34 comes out. But you will compile for RV22 and then have have spaghetti of 10+ platform speci
by geokon 2y ago
How does this work long term? So say ten years down the line.. RVA34 comes out. But you will compile for RV22 and then have have spaghetti of 10+ platform specific codepaths in the binary？
I feel at some point you're either going to have penalties from the all the platform check and jumps or you're going to only use these extension in critical hotpaths and they get underutilized (what you see with SSE in x64 and the whole mess on ARM)
- magicalhippo 2y agoTime will tell. As they note in the page I linked to, they've decided to tackle that one when they get there. I'll agree it sounds like it could get messy. That said, I'm sort of imagining the core set of extensions to not grow significantly larger. Perhaps they'll add some granularity to the various profile levels, like embedded application CPUs like in set-top boxes might not need all the features of a full-blow desktop or server CPU.
- brucehoult 2y agoIt's not going to be every year! And each one runs all software from all previous profiles. Right now there is RVA20 (almost all shipping hardware), RVA22 (only Canaan K230 and SpacemiT K1/M1 shipping now, more coming next year e.g. SG2380), and RVA23 just ratified, hardware in probably 2-3 years. Worrying about multiplication of RISC-V profiles is ignoring the existing reality of every other platform. There are currently 17 different releases of ARMv8-A and ARMv9-A. Have you seen the extensions list on any recent x86? My laptop reports: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf tsc_known_freq pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb ssbd ibrs ibpb stibp ibrs_enhanced tpr_shadow flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid rdseed adx smap clflushopt clwb intel_pt sha_ni xsaveopt xsavec xgetbv1 xsaves split_lock_detect user_shstk avx_vnni dtherm ida arat pln pts hwp hwp_notify hwp_act_window hwp_epp hwp_pkg_req hfi vnmi umip pku ospke waitpkg gfni vaes vpclmulqdq rdpid movdiri movdir64b fsrm md_clear serialize arch_lbr ibt flush_l1d arch_capabilities vnmi preemption_timer posted_intr invvpid ept_x_only ept_ad ept_1gb flexpriority apicv tsc_offset vtpr mtf vapic ept vpid unrestricted_guest vapic_reg vid ple shadow_vmcs ept_mode_based_exec tsc_scaling usr_wait_pause
- snvzz 2y agoComically, x86-64v4 requires AVX-512, and years after, Intel still releasing CPUs that do not support AVX-512. So the world is stuck with x86-64v3, more or less haswell/zen1 level.