CODEWITHSUNDEEP
iphonecastingfloating-pointinteger
Aran Mulholland
Aran Mulholland

Reputation: 23945

What is the fastest int to float conversion on the iPhone?

I am converting some Int16s and Int32s to float and then back again.

I'm just using a straight cast, but doing this 44100 times per second (any guesses what its for? :) )

Is a cast efficient? Can it be done any faster?

P.S Compile for thumb is turned off.

Upvotes: 0

Views: 1524

Answers (2)

cdespinosa
cdespinosa

Reputation: 20799

There are only two ways to know.

1) Read the code the compiler generates for promoting ints to floats in your case.

2) Measure the performance of the code the compiler generates vs. other options.

To do the former, set the SDK to Device and the Active Architecture to arm, and choose Build > Show Assembly Code. Then read the compiler-generated code.

If you are smarter than a compiler then you can write your own assembly code and use it instead. Odds are you aren't.

If you are doing an operation many, many times, Instruments will do a good job at showing you how many processor samples it's taking. But Jim's point is valid, and you shouldn't dismiss it as unhelpful: in an operation involving math on floating-point numbers, compiler type promotion is the least of your worries. Chips are built to do that in two or three cycles, and compilers usually manage to make that happen. But the effects processing you're doing will probably take thousands of cycles. The promotion will be lost in the noise.

Upvotes: 1

Jim Lewis
Jim Lewis

Reputation: 45115

Is a cast efficient? In your case, I'd guess it's efficient enough.

Can it be done faster? Maybe...but would it be worth the effort? Have you benchmarked it and discovered a performance problem due to the cast operations?

If you're doing anything mathematically nontrivial with the floating point sample data, I'd be really surprised if the casts turned out to be a significant bottleneck!

Upvotes: 1

Related Questions