David Kanter (dkanter.delete@this.realworldtech.com) on November 22, 2020 11:01 pm wrote:
> > > I think you need to look at what folks were saying at that time.
> > >
> > > For example, many people claim ARM has intrinsic advantages over x86. I was staking
> > > out positions many didn't and don't agree with. Specifically, that ARM has no real
> > > advantages over x86 for high performance cores (but does for small ones).
> >
> > Actually David if you look at the high performance Apple A13 vs Zen 2 (3950X) they have similar scores
> > on GB5 and are on similar processes yet the A13 is much more power efficient, retaining the ARM advantage.
> > Even with Zen 3 getting 23% higher scores on GB5 it is still in the same power efficiency ballpark as Zen
> > 2. x86 just cannot approach the power efficiency of ARM at any processor core performance level.
>
> As others pointed out, comparing a desktop-focused chiplet to a
> smartphone SoC makes for unrealistic power comparisons at best.
>
> Also, Geekbench isn't a good benchmark.
>
> Last, you should tell that to all the architects who have designed x86 and ARM processors.
> They will tell you - for a high-performance core, there is very little difference. That includes
> folks working (or previously working) at Apple, ARM, and many other companies.
>
> David

https://tspace.library.utoronto.ca/bitstream/1807/80713/1/Wong_Henry_T_201711_PhD_thesis.pdf is a good read.

Surprisingly, modern ARM instruction sets are not much simpler than x86, despite being called “RISC”. The ARM instruction set also needs decoding into micro-ops. The ARM memory system is similar to x86, supporting unaligned data accesses and hardware TLB miss handling. Unlike x86, ARMv7 also allows predication of most instructions and an implicit barrel shift of one operand for most arithmetic instructions, which are features that are difficult to implement in out-of-order processors. On the other hand, one particularly complex-to-implement feature of x86 is variable instruction length where the length is non-trivial to compute (1 to 15 bytes), while ARM Thumb’s variable-length instructions only have two possible lengths (2 or 4 bytes).
...
While x86 is more complex, the extra complexity due specifically to x86 is modest — much of the complexity of supporting an operating system (supporting exceptions, paging, privilege levels, etc.) is common to most instruction sets

So yeah, X86 is more complex. But once you're designing a complex, OOO core that supports everything a modern OS demands, X86's extra complexity is a drop in the bucket.