cmd/compile: constant indexing to private array should be folded

[egonelbre]

Lookup tables are quite common, as in https://golang.org/src/math/bits/bits_tables.go. However none of functions that rely on those tables can be constant folded.

Simplest example is https://go.godbolt.org/z/MhMTqb which compiles to:

movblzx math/bits.rev8tab+12(SB), AX
movb    AL, "".~r0+8(SP)

The rev8tab only contains indexing to that table, so it should be possible to prove that the array is not being modified. Of course that only holds, when assignment, slicing or addressing operator is not used.

If such table is replaced with a string, then the constant folding does work. However, that seems like an ugly hack to have.

Related Issues:

• #10848
• #21872

[josharian]

If anyone wants to work on this, one way to approach it would be to mark the array symbol as readonly in the pre-SSA phase of the compiler (if you can prove it is in fact readonly). Then SSA optimizations for loading from readonly memory should kick in.

[egonelbre]

@mdempsky mentioned that two other concerns are assembly code and //go:linkname.

[egonelbre]

I also realized there's a way to declare an array or slice, such that it's guaranteed to be read-only:

func lut() [256]byte { return [256]byte{1,2,3,4} }

[gopherbot]

Change https://golang.org/cl/280645 mentions this issue: math/bits: folded reverse tables by using const string