On AArch64 I think it's cheaper because losing one register doesn't hurt as much, since you've got twice as many.

nitpick: it's the number of physical registers that matter more here, not ISA registers

Why wouldn't the lower ISA register count lead to more spills, regardless of the physical count? Is store forwarding really that effective?

It isn't.

The ORC documentation [1] cites a 5-10% perf hit [2] on x86. Now, granted, we're talking about an architecture that has only 8 logical registers to begin with, so losing one is going to hurt a lot more than x86-64 (16 logical registers) or AARCH64 (32 logical registers). But, 5-10% is definitely a noticeable perf hit.

Phoronix [3] did a test of "-fno-omit-frame-pointer" during the discussion on whether to enable the flag for Fedora, this time on x86-64. They found an average 14% performance hit on a wide variety of benchmarks.

[1] https://www.kernel.org/doc/html/latest/x86/orc-unwinder.html [2] https://lore.kernel.org/all/20170602104048.jkkzssljsompjdwy@... [3] https://www.phoronix.com/review/fedora-frame-pointer/5

See also https://lists.fedoraproject.org/archives/list/[email protected]... - the botan phoronix results with frame pointers were probably measuring debug builds.