Commits on Apr 7, 2021

1. schema/gen/go: avoid Maybe pointers for small types …

   If we know that a schema type can be represented in Go with a small
   amount of bytes, using a pointer to store its "maybe" is rarely a good
   idea. For example, an optional string only weighs twice as much as a
   pointer, so a pointer adds overhead and will barely ever save any
   memory.
   
   Add a function to work out the byte size of a schema.TypeKind, relying
   on reflection and the basicnode package. Debug prints are also present
   if one wants to double-check the numbers. As of today, they are:
   
   	sizeOf(small): 32 (4x pointer size)
   	sizeOf(Bool): 1
   	sizeOf(Int): 8
   	sizeOf(Float): 8
   	sizeOf(String): 16
   	sizeOf(Bytes): 24
   	sizeOf(List): 24
   	sizeOf(Map): 32
   	sizeOf(Link): 16
   
   Below is the result on go-merkledag's BenchmarkRoundtrip after
   re-generating go-codec-dagpb with this change. Note that the dag-pb
   schema contains multiple optional fields, such as strings.
   
   	name         old time/op    new time/op    delta
   	Roundtrip-8    4.24µs ± 3%    3.78µs ± 0%  -10.87%  (p=0.004 n=6+5)
   
   	name         old alloc/op   new alloc/op   delta
   	Roundtrip-8    6.38kB ± 0%    6.24kB ± 0%   -2.26%  (p=0.002 n=6+6)
   
   	name         old allocs/op  new allocs/op  delta
   	Roundtrip-8       103 ± 0%        61 ± 0%  -40.78%  (p=0.002 n=6+6)
   
   Schema typekinds which don't directly map to basicnode prototypes, such
   as structs and unions, are left as a TODO for now.
   
   I did not do any measurements to arrive at the magic number of 4x, which
   is documented in the code. We might well increase it in the future, with
   more careful benchmarking. For now, it seems like a conservative starting
   point that should cover all basic types.
   
   Finally, re-generate within this repo.

   

   mvdan committed Apr 7, 2021
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