crypto/cipher: move xorBytes function to internal

[mengzhuo]

Some packages need an efficient xor function bytes, for example:

gogrep -x 'for $*_ { $m[i] = $k[i] ^ $l[i] }' std cmd
src/crypto/rand/rand_unix.go:149:3: for i := 0; i < aes.BlockSize; i++ { r.dst[i] = r.time[i] ^ r.seed[i]; }
src/crypto/rand/rand_unix.go:153:3: for i := 0; i < aes.BlockSize; i++ { r.seed[i] = r.time[i] ^ r.dst[i]; }

And about 18 code pieces in golang.org/x/crypto https://github.com/golang/crypto/search?l=Go&q=xor

If this CL( https://go-review.googlesource.com/c/go/+/125316 ) get merged in master we will get a SIMD xorBytes which will be a benefit for all other package that use xor function and reduce redundancy.

[agnivade]

Please see this #28113, which was already rejected.

[mengzhuo]

Oops Intially I want to use it at crypto/rand and math/big Maybe an SSA optimization like encoding/binary? Agniva De Sarker <notifications@github.com>：

…

Please see this #28113 <#28113>, which was already rejected. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#28465 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AA2DnuKmgQR1tP_iHEORAwIlfxReuO9Lks5upzBGgaJpZM4X_UcS> .

[randall77]

What did you want to use it for, exactly? math/big has an xor operation but I'd be very surprised if that function shows up in profiles of typical math/big operations.

If you want to use xor within the stdlib, we could move such a function to internal/bytealg and redirect current uses to there. That would not involve any new API.

[mengzhuo]

Moving xor into internal sounds great, I will change title to that.
But I found that math/big using xor for []Word(uint) instead of []byte.

replace target:
math/big

go/src/math/big/nat.go

Line 951 in d1836e6

func (z nat) xor(x, y nat) nat {

crypto/rand

go/src/crypto/rand/rand_unix.go

Line 152 in d1836e6

r.dst[i] = r.time[i] ^ r.seed[i]

[agnivade]

/cc @FiloSottile

[andybons]

This appears to be an internal implementation detail, not a user-facing change. @FiloSottile should still take a look, but removing the proposal tag.