In the process of developing a piece of middleware, I need to translate
from a bit-array into a bitmask. I am struggling to find a way to express
this in go that doesn't result in terrible performance.
The approaches I would try in most other languages were along the lines of:
```
mask = (bool1 << bitno1) | (bool2 << bitno2);
// or
mask = (bool1 ? value1 : 0) | (bool2 ? value2 : 0);
```
but instead, after reading several old (circa 1.5) posts, I'd landed at
```
func maskIfTrue(mask uint, predicate bool) uint {
if predicate {
return mask
}
return 0
}
mask = maskIfTrue(mask1, bool1) | maskIfTrue(mask2, bool2)
```
Here is a (boiled-down & reduced) comparison of the go implementation vs a
simple C implementation compiled with -O0 and -Os:
The go version is branch-crazy.
Is there some way I can write this that will produce simpler/efficient code
and also not be code salad? I don't have control over the relative ordering
of the bools or the bitfield values, and this is a hot path?
Go branchiness:
```
nop
cmpb 1(AX), $0
jeq featToMask_pc94
movl $2, DX
featToMask_pc19:
nop
cmpb 2(AX), $0
jeq featToMask_pc90
movl $4, BX
featToMask_pc30:
nop
```
The "FeatToMask" C transliteration when compiled with optimization
*disabled* (-O0) looks similar, but even -O1 fixes that:
```
FeatToMask:
mov eax, edi
movzx eax, ah
mov esi, edi
shr esi, 16
mov ecx, edi
shr ecx, 24
mov rdx, rdi
shr rdx, 32
shr rdi, 40
or eax, esi
or eax, ecx
or eax, edx
or eax, edi
movzx eax, al
ret
```
and with -Os you get down to something better than the
naive-C-implementation at the top of the source
```
FeatToMask:
mov QWORD PTR [rsp-8], rdi
mov al, BYTE PTR [rsp-7]
or al, BYTE PTR [rsp-6]
or al, BYTE PTR [rsp-5]
or eax, DWORD PTR [rsp-4]
or al, BYTE PTR [rsp-3]
movzx eax, al
ret
```
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