cmd/compile, runtime: optimize comparison of large values with zero

[josharian]

package p

type T [20]byte

func f(x *T) bool {
	return *x == T{}
}

Currently this sets up and zeros twenty bytes and then calls memequal. Comparing a large type to zero should be easy; we can then dispatch instead to an optimized runtime routine that doesn't need to actually construct a zero value. (Should handle != as well.)

cc @randall77 @martisch

[martisch]

I would suggest to do small compares inline (up to x bytes, like compares, not sure where the cutoff is currently) and when bigger we can call a special runtime routine isZero(*p, len) or alike as suggested.

[cznic]

Why aren't non-scalar required zero values ready made in the text segment?

[josharian]

And for that matter, why not use runtime.zeroVal?

isZero will still be more efficient, though.

[martisch]

zeroVal can be used up to 1024 bytes.
I think after that if used in maps we allocate bigger zero values.

However flushing multiple cache lines and loading them with zeros for a zero compare seems wasteful and slow if the zero value is not already loaded.

[josharian]

Filed #23948 for using runtime.zeroVal.

[randall77]

See #24416 for large struct optimization.