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Intel actually provides better documentation on its CPU pipelines than ARM does, especially for anything more advanced than the Cortex-A9. Then with Cortex-A5 a
by brigade 12y ago
Intel actually provides better documentation on its CPU pipelines than ARM does, especially for anything more advanced than the Cortex-A9. Then with Cortex-A5 and A7 you don't even get instruction timings.
Cortex-A15 you get nothing official, even though there's quite a few hidden gotcha's that are critical for performance (Did you know that the NEON pipelines are 64-bit wide? Or that there's only 1 VFP pipeline whereas there's two symmetric NEON pipelines? That's some of the barest of basics, yet nowhere does ARM publicly document this info.)
Intel you get execution port categories (though you have to infer exactly which instructions fall into which categories), official timings (again inferring precise µop counts), and all sorts of details as to the inner workings of pipelines (random example: there's an entire section about how store forwarding works in Nehalem in the optimization manual, which is quite beyond the detail you'd see in any official ARM manual.)
And that's not counting Agner Fog and the details Intel feeds tech journalists.
- userbinator 12y agoI think one of the reasons ARM doesn't really want to provide instruction timings is because it sells IP cores, not hardware. Others who integrate ARM cores may make changes to the design that mean their instruction timings will be different, whereas Intel designs and manufacturers the CPUs themselves.
- brigade 12y agoIn software terms, that's more difficult than adding support for new HTML extensions given java byte code for a web browser. No one makes modifications that could affect instruction timings (which is a very fundamental aspect of a core), and I very much doubt such modifications are even allowed in the license you buy the cores under, even with a separate architectural license (which is for ground-up designs). Even NVIDIA who bragged about their A9 and A15 core revisions only gave feedback to ARM and had to have them actually implement the changes, and none of those changes affected instruction timing.
- wolf550e 12y agoDifferent ARMv7 designs have different timings, but I think Cortex A15 is always the same, though running at different frequencies and voltages and consuming different amounts of power.