CODEWITHSUNDEEP
c++
tofutim
tofutim

Reputation: 23374

What does adding 15 and then applying &~0xf do?

I have some code I am studying where it is written:

(basenameOffset + (basenameTotal+15)) &~0xf

why would someone do this? What does it do? I can see that ~0xf is 0xfffffff0. Why would you block the last bit?

Upvotes: 1

Views: 357

Answers (2)

Cory Nelson
Cory Nelson

Reputation: 30001

This is a common algorithm to round up to a multiple of a power of two:

x = x + (pow2 - 1) & ~(pow2 - 1)

It is almost certainly being used to ensure proper alignment. Typically to make SIMD-optimal (16-byte for SSE, 32-byte for AVX, etc.) base addresses and/or to optimize cache usage.

Upvotes: 1

C. K. Young
C. K. Young

Reputation: 223083

It rounds up to the nearest multiple of 16. Presumably, this is to fix a size for allocating the buffer for the basename, whatever it is. :-)

However, if that were what it's for, i.e., deciding how big of a buffer to allocate, then this is not a good strategy. Ideally, you want to expand by a factor of 2 or at least 1.5 each time.

Upvotes: 4

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