In the Name of Performance

Sam Coleman

Who am I?

Who am I?

What is this not?• Not about overall architecture

What is this not?• Not about rescuing an entire codebase

What is this, then?• Let’s look at something gross• Let’s understand it• Let’s never do it unless it needs to be done• …“Performance”

Duff’s Device• Live coding• Please excuse the mess

Duff’s Device• Why?• Don’t do this.• …unless you have to.• memcpy available almost everywhere.

Bit Counting• aka “population count”, aka “Hamming Weight”

But First, Bit Operations• Bitwise: &, >>• Masking: “covering up” bits we don’t care about

h g f e d c b a

& 0 0 1 1 1 1 0 0

0 0 f e d c 0 0

Bit Counting• Live coding

Bit Countingh g f e d c b a

- h g f e d c b\ \ \

= h (g ^ h) f (e ^ f) d (c ^ d) b (a ^ b)

• The left column retains its original value.• The right column is “on” only if either d or c were on.• So across both columns, possible results are “10”, “01”, or “00”.• …you may recognize these as “2”, “1”, or “0”.

Bit Counting

+

h g f e d c b a

h g d c

Bit Counting• Sum of two 2-bit numbers can require 3 bits• Need to clean (mask) the input so we know which

bits of output are relevant• Result is clean, 4-bit integers

+

h g f e d c b a

i h g e d c

Bit Counting• Our “4-bit” inputs will only have up to 3 bits in use

(to store “4”) so the sum will take at most 4 bits• Only need to mask the output to get a clean, 8-bit

result – saves an operation vs. masking both inputs

+

p o n m l k j i h g f e d c b a

p o n m h g f e

Bit Counting- - - - p o n m - - - - l k j i - - - - h g f e - - - - d c b a0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1

- - - - p o n m - - - - l k j i - - - - h g f e - - - - d c b a

- - - - l k j i - - - - h g f e - - - - d c b a

- - - - h g f e - - - - d c b a

- - - - d c b a

… >> 24

Bit Counting• Why?• Don’t do this.• …unless you have to.• __builtin_popcount available almost everywhere.

Concluding Remarks• Old code can be a pain• Embedded code can be a pain• Progress can also be pain• Boyscout rule: leave the campground cleaner than

you found it.• Caveat: Don’t clean the campground without

understanding it!

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