The A7′s design improvements over the older A8 core are possible because ARM has had the past three years to carefully study how the Android OS uses existing ARM chips in the course of normal usage. Peter Greenhalgh, the chip architect behind the A7′s design, told me that his team did detailed profiling in order to learn exactly how different apps and parts of the Android OS stress the CPU, with the result that the team could design the A7 to fit the needs and characteristics of real-world smartphones. So in a sense, the A7 is the first CPU that’s quite literally tailor-made for Android, although those same microarchitectural optimizations will benefit for any other smartphone OS that uses the design.
The high-level block diagram for the A7 released at the event reveals an in-order design with an 8-stage integer pipeline. At the front of the pipeline, ARM has added three predecode stages, so that the instructions in the L1 are appropriately marked up before they go into the decode phase. Greenhalgh told me that A7 has extremely hefty branch prediction resources for a design this lean, so I’m guessing that the predecode phase involves tagging the branches and doing other work to cut down on mispredicts.
More at Cloudline.