Assembly: convert floatingpointvalue to signed byte

By : genesys

I currently try to write a program for the VFP unit of the iPhone using ARM assembly code. The VFP can do floatingpoint calculations, but AFAIK no integer arithmetic. However, it can convert a float to a signed integer (4 bytes). Also, according to this quickreference: it seems it does not support any shifting operations

what i would like to do is to convert 4 floats of which i'm sure that each is larger than -127 and smaller than 127 into 4 signed bytes.

if i'd have shifting operations available, i could convert the float to signed integer, then shift the value by 12 bytes to the left (8 and 4 bytes for the next two values respectively) and bitwise OR all four together.

however, since shifting is not available, i need to find another way to do it. Also - i cannot use integer arithmetics (so i can't multiply the already converted integer by 2^n in order to shift but i have to work on the float instead).

Anyone knows how i could achieve that?

btw for those familar with the ARM architecture - i don't want to switch to Thumb instructions, because this is done in a loop operating on many elements and i don't want to switch between thumb and arm instructions inside this loop (since that's expensive)



additional question: how can I normalize a Vector with three elements?

By : genesys


You want the VFP ftosis instruction, which converts a single-precision FP value to a 4 byte integer. If you have four floats in s0-s3, then after doing:

ftosis s0, s0
ftosis s1, s1
ftosis s2, s2
ftosis s3, s3

you have four 4 byte integers in s0-s3, which can be stored contiguously to memory with a fstm.

On an ARM processor that supports NEON, you can use vcvt.s32.f32 q0, q0 to do four conversions with one instruction.

Edit to answer your follow-up question, here's a simple example function which takes as input a pointer to four floats in memory and returns the converted values packed into a single int32_t:

    fldmias   r0,  {s4-s7}
    ftosizs   s0,   s4
    ftosizs   s1,   s5
    ftosizs   s2,   s6
    ftosizs   s3,   s7
    fmrrs r0, r1,  {s0,s1}
    fmrrs r2, r3,  {s2,s3}
    uxtb      r0,   r0
    uxtb      r1,   r1
    uxtb      r2,   r2
    orr       r0,   r0, r1, lsl #8
    orr       r0,   r0, r2, lsl #16
    orr       r0,   r0, r3, lsl #24
    bx        lr

I didn't really put any effort into tuning this, because you wouldn't want to do conversions this way if they were performance-critical; you'd rather either operate on large arrays of values, and pipeline this code so that several conversions were in flight simultaneously, or interleave it with other operations that are doing useful work as well.

You may also like to insert ssats before the uxtbs to make any out-of-range values saturate instead of wrapping.

Also, be aware that this code will have poor performance on ARMv7 cores; you'll definitely want to use the NEON vector operations on that platform.

This video can help you solving your question :)
By: admin