Use intrinsic for GetHighestSetBitIndex

[benaadams]

Changes

• Use BitOperations.LeadingZeroCount rather than a bunch of ifs

Types of changes

What types of changes does your code introduce?

• Optimization

Testing

Requires testing

• No

[smartprogrammer93]

Benchmarks?

[benaadams]

Benchmarks?

Is single instruction lzcnt vs 6 conditional branches

Before

G_M000_IG03:                ;; offset=0x0010
       movzx    rcx, bl
       test     cl, 128
       je       SHORT G_M000_IG06

G_M000_IG04:                ;; offset=0x0018
       mov      eax, 8

G_M000_IG05:                ;; offset=0x001D
       add      rsp, 32
       pop      rbx
       ret      

G_M000_IG06:                ;; offset=0x0023
       test     cl, 64
       je       SHORT G_M000_IG08
       mov      eax, 7

G_M000_IG07:                ;; offset=0x002D
       add      rsp, 32
       pop      rbx
       ret      

G_M000_IG08:                ;; offset=0x0033
       test     cl, 32
       je       SHORT G_M000_IG10
       mov      eax, 6

G_M000_IG09:                ;; offset=0x003D
       add      rsp, 32
       pop      rbx
       ret      

G_M000_IG10:                ;; offset=0x0043
       test     cl, 16
       je       SHORT G_M000_IG11
       mov      eax, 5
       jmp      SHORT G_M000_IG14

G_M000_IG11:                ;; offset=0x004F
       test     cl, 8
       je       SHORT G_M000_IG12
       mov      eax, 4
       jmp      SHORT G_M000_IG14

G_M000_IG12:                ;; offset=0x005B
       test     cl, 4
       je       SHORT G_M000_IG13
       mov      eax, 3
       jmp      SHORT G_M000_IG14

G_M000_IG13:                ;; offset=0x0067
       mov      eax, 2
       test     cl, 2
       cmove    eax, ecx

G_M000_IG14:                ;; offset=0x0072
       add      rsp, 32
       pop      rbx
       ret

After

G_M000_IG03:                ;; offset=0x0010
       movzx    rax, bl
       lzcnt    eax, eax
       neg      eax
       add      eax, 32

G_M000_IG04:                ;; offset=0x001C
       add      rsp, 32
       pop      rbx
       ret