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This article has several paragraphs discussing how the decode width of x86 front ends is limited by the need to discover instruction boundaries, which in turn l

by fanf2 2y ago

This article has several paragraphs discussing how the decode width of x86 front ends is limited by the need to discover instruction boundaries, which in turn limits the useful issue width. Big ARM cores have much larger decode width than x86 cores, so they don’t need SMT to keep their execution units busy. ISA doesn’t matter any more because all the CISCiest CISCs and the RISCiest RISCs have been discarded (except for RISC V…) so modern CPUs don’t have to cope with multiple memory addresses per instruction or indirect addressing.

kllrnohj 2y ago
> Big ARM cores have much larger decode width than x86 cores Not in general they don't. Apple's does, and Qualcomm's newest Snapdragon X Elite also does. But most big ARM cores don't have larger decode widths than x86 cores. The Cortex-A78 is a 4-wide decode, same as the majority of x86 CPUs on the market. ARM's latest & greatest Cortex X2 is only a 5-wide decode, it only just finally surpassed the original Zen design (4-wide). Also Zen 5 bumps to an 8-wide decode, the same as what Apple & Qualcomm's best ARM chips can do.
topspin 2y ago
> limited by the need to discover instruction boundaries Keller specifically addressed this. His view is that yes, the x86 instruction width problem is real, but it's not an important performance bottleneck because decoders are now sophisticated enough that other bottlenecks (predictors) dominate the net performance. Yes, this means more power usage and die area, but only a part of the market is sensitive to this: where other considerations dominate the cost premium of x86 can be ignored. He also believes that "RISC-V is the future of computing," (not surprising from the CEO of a RISC-V vendor,) so it isn't as if the legacy ISAs are somehow just fine. But ISA complexity isn't the key determinant in that future. The keys are the open ISA, commoditization of designs and high software portability. This makes sense to me. x86, despite it's inherent issues, has fought off alternatives before. But now, as per Keller, there is a Cambrian explosion of RISC-V designs appearing, and what emerges from that is going to prevail against any legacy ISA, not just x86. Ulitimately, therefore, the x86 "tax" doesn't actually matter.

by fanf2 2y ago

kllrnohj 2y ago

> Big ARM cores have much larger decode width than x86 cores Not in general they don't. Apple's does, and Qualcomm's newest Snapdragon X Elite also does. But most big ARM cores don't have larger decode widths than x86 cores. The Cortex-A78 is a 4-wide decode, same as the majority of x86 CPUs on the market. ARM's latest & greatest Cortex X2 is only a 5-wide decode, it only just finally surpassed the original Zen design (4-wide). Also Zen 5 bumps to an 8-wide decode, the same as what Apple & Qualcomm's best ARM chips can do.

topspin 2y ago

> limited by the need to discover instruction boundaries Keller specifically addressed this. His view is that yes, the x86 instruction width problem is real, but it's not an important performance bottleneck because decoders are now sophisticated enough that other bottlenecks (predictors) dominate the net performance. Yes, this means more power usage and die area, but only a part of the market is sensitive to this: where other considerations dominate the cost premium of x86 can be ignored. He also believes that "RISC-V is the future of computing," (not surprising from the CEO of a RISC-V vendor,) so it isn't as if the legacy ISAs are somehow just fine. But ISA complexity isn't the key determinant in that future. The keys are the open ISA, commoditization of designs and high software portability. This makes sense to me. x86, despite it's inherent issues, has fought off alternatives before. But now, as per Keller, there is a Cambrian explosion of RISC-V designs appearing, and what emerges from that is going to prevail against any legacy ISA, not just x86. Ulitimately, therefore, the x86 "tax" doesn't actually matter.