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Legacy isn’t a good explanation because the instruction decoder that turns x86 instructions into microcode is basically a fixed cost and makes up a tiny portion

Legacy isn’t a good explanation because the instruction decoder that turns x86 instructions into microcode is basically a fixed cost and makes up a tiny portion of the silicon. Clearly there is room for innovation in the x86 sphere as well because the AMD Zen architecture was created brand new from scratch and has become very competitive. The more likely explanation is that there is an exponential difficulty curve in CPU optimization and performance that Apple and their admittedly talented engineers are climbing very quickly. When an Ax chip hits true desktop performance sooner than later, the limit of physics is going to slow down Apple just like it has Intel and AMD. There isn’t any magic they are going to be able to do that suddenly gives them a 2x IPC gain or huge frequency advantage.

umanwizard 8y ago
Well my Intel chip needs a fan and my iPhone doesn’t. So it’s not an apples-to-apples comparison (no pun intended, please nobody reply with “I see what you did there”). The fact that someone can come up with a chip of similar performance to Intel’s isn’t that amazing. The fact that they can do so without needing a fan is.
- oliveshell 8y ago
  I agree— though it’s perhaps worth mentioning that a fan on the phones was used for the SPEC2006 benchmarks: > The performance measurement was run in a synthetic environment (read: bench fan cooling the phones) where we assured thermals wouldn’t be an issue for the 1-2 hours it takes to complete a full suite run. Still darn impressive, as in the real world you won’t be doing anything with your phone that maxes out the SoC for 2 hours straight.
- hesdeadjim 8y ago
  The performance comparisons are not remotely apples to apples to begin with. All that's been established is that an Ax chip is entering the realm of desktop performance. Under all the workloads you'd benchmark a regular desktop chip on (FPU, AVX, etc etc) the Ax chips are still going to be far behind at the TDP and frequency they run at. If you could pump up the TDP and frequency of the Ax chips to actual desktop class and throw a heatsink on so you don't get thermally throttled, I am sure Apple's chips would be competitive. It's an inevitability that we'll see a desktop-class Ax powered Macbook at some point soon-ish. When that happens we can start making legitimate comparisons.
tinus_hn 8y ago
Due to all that legacy the instruction decoder has a lot less opportunity for optimizations. There may be a physical limit to the size of transistors but the limit of computing performance is not ‘what Intel is doing’. There’s more to performance than instructions per second or clock frequencies. In fact these don’t lead to the same performance at all as ARM is RISC and x64 is CISC.
- z0d 8y ago
  That RISC and CISC, nobody mentioned until yours. Why are people only caring about Perf/watt or the SoC beating out the ST perf of the Skylake uarch. RISC based processor with that level of cache is always having an advantage due to the Load and Store at register level (esp in this A12 case of having HUGE L$) vs the CISC's more transistor level perf + DRAM to exec instructions, causing higher TDP requirements PLUS the multi instruction execution parallelism capabilities. And entirely different code of AArch64 and x86_64. Add the GPU and Direct X, and Encode/Decode functionality I know this is not the subject, the ARM processors simply cannot keep up. The author did also mention the limitations of the comparison also as mentioned above the Skylake Xeon is capable of running Hex channel ECC Memory with huge capacity that alone will outpace everything on ARM it's not a fair share of comparison at all. GB is an OS level optimized bench. There's simply no comparison point that exists. Like I'd say to the author do a 4D render bench like Cinebench then we talk on this. He should at-least have mentioned the uarch part. Also on a side note, Apple underselling the chips ? That's really first, since Apple always over promises. Their latest meltdown on the MacBooks with low quality/specced VRM throttling the CFL machines is a joke.
astrange 8y ago
That x86 support isn't free. Intel's SIMD implementations and new instructions only ever seem to target the high end; I don't think you could use AVX512 at low power. They also guarantee various things about memory ordering that ARM doesn't, and of course the page table, 16-bit boot modes, and everything are super complicated.

umanwizard 8y ago

Well my Intel chip needs a fan and my iPhone doesn’t. So it’s not an apples-to-apples comparison (no pun intended, please nobody reply with “I see what you did there”). The fact that someone can come up with a chip of similar performance to Intel’s isn’t that amazing. The fact that they can do so without needing a fan is.

oliveshell 8y ago
I agree— though it’s perhaps worth mentioning that a fan on the phones was used for the SPEC2006 benchmarks: > The performance measurement was run in a synthetic environment (read: bench fan cooling the phones) where we assured thermals wouldn’t be an issue for the 1-2 hours it takes to complete a full suite run. Still darn impressive, as in the real world you won’t be doing anything with your phone that maxes out the SoC for 2 hours straight.
hesdeadjim 8y ago
The performance comparisons are not remotely apples to apples to begin with. All that's been established is that an Ax chip is entering the realm of desktop performance. Under all the workloads you'd benchmark a regular desktop chip on (FPU, AVX, etc etc) the Ax chips are still going to be far behind at the TDP and frequency they run at. If you could pump up the TDP and frequency of the Ax chips to actual desktop class and throw a heatsink on so you don't get thermally throttled, I am sure Apple's chips would be competitive. It's an inevitability that we'll see a desktop-class Ax powered Macbook at some point soon-ish. When that happens we can start making legitimate comparisons.

tinus_hn 8y ago

Due to all that legacy the instruction decoder has a lot less opportunity for optimizations. There may be a physical limit to the size of transistors but the limit of computing performance is not ‘what Intel is doing’. There’s more to performance than instructions per second or clock frequencies. In fact these don’t lead to the same performance at all as ARM is RISC and x64 is CISC.

z0d 8y ago
That RISC and CISC, nobody mentioned until yours. Why are people only caring about Perf/watt or the SoC beating out the ST perf of the Skylake uarch. RISC based processor with that level of cache is always having an advantage due to the Load and Store at register level (esp in this A12 case of having HUGE L$) vs the CISC's more transistor level perf + DRAM to exec instructions, causing higher TDP requirements PLUS the multi instruction execution parallelism capabilities. And entirely different code of AArch64 and x86_64. Add the GPU and Direct X, and Encode/Decode functionality I know this is not the subject, the ARM processors simply cannot keep up. The author did also mention the limitations of the comparison also as mentioned above the Skylake Xeon is capable of running Hex channel ECC Memory with huge capacity that alone will outpace everything on ARM it's not a fair share of comparison at all. GB is an OS level optimized bench. There's simply no comparison point that exists. Like I'd say to the author do a 4D render bench like Cinebench then we talk on this. He should at-least have mentioned the uarch part. Also on a side note, Apple underselling the chips ? That's really first, since Apple always over promises. Their latest meltdown on the MacBooks with low quality/specced VRM throttling the CFL machines is a joke.

astrange 8y ago

That x86 support isn't free. Intel's SIMD implementations and new instructions only ever seem to target the high end; I don't think you could use AVX512 at low power. They also guarantee various things about memory ordering that ARM doesn't, and of course the page table, 16-bit boot modes, and everything are super complicated.