An Introduction to GCC Compiler Intrinsics in Vector Processing

We're including integer math in the discussion. While integer math is not unique to vector processing, it's useful to have in case your vector hardware is integer-only or if integer math is much faster than floating-point math. Integer math will be an approximation of floating-point math, but you might be able to get a faster answer that is acceptably close.

The first option in integer math is rearranging operations. If your formula is simple enough, you might be able to rearrange operations to preserve precision. For instance, you could rearrange:

F = (9/5)C + 32

F = (9*C)/5 + 32

So long as 9 * C doesn't overflow the type you're using, precision is preserved. It's lost when you divide by 5; so do that after the multiplication. Rearrangement may not work for more complex formulas.

The seond choice is scaled math. In the scaled math option, you decide how much precision you want, then multiply both sides of your equation by a constant, round or truncate your coefficients to integers, and work with that. The final step to get your answer then is to divide by that constant. For instance, if you wanted to convert Celsius to Fahrenheit:

F = (9/5)C + 32
  = 1.8C + 32            -- but we can't have 1.8, so multiply by 10

sum = 10F = 18C + 320    -- 1.8 is now 18: now all integer operations

F = sum/10

If you multiply by a power of 2 instead of 10, you change that final division into a shift, which is almost certainly faster, though harder to understand. (So don't do this gratuitously.)

The third choice for integer math is shift-and-add. Shift-and-add is another method based on the idea that a floating-point multiplication can be implemented with a number of shifts and adds. So our troublesome 1.8C can be approximated as:

1.0C + 0.5C + 0.25C + ...   OR  C + (C >> 1) + (C >> 2) + ...

Again, it's almost certainly faster, but harder to understand.

There are examples of integer math in samples/simple/temperatures*.c, and shift-and-add in samples/colorconv2/scalar.c.

Vector Types, the Compiler and the Debugger

To use your processor's vector hardware, tell the compiler to use intrinsics to generate SIMD code, include the file that defines the vector types, and use a vector type to put your data into vector form.

The compiler's SIMD command-line arguments are listed in Table 1. (This article covers only these, but GCC offers much more.)

Table 1. GCC Command-Line Options to Generate SIMD Code

Here are the include files you need:

• arm_neon.h - ARM Neon types & intrinsics
• altivec.h - Freescale Altivec types & intrinsics
• mmintrin.h - X86 MMX
• xmmintrin.h - X86 SSE1
• emmintrin.h - X86 SSE2

X86: MMX, SSE, SSE2 Types and Debugging

The X86 compatibles with MMX, SSE1 and SSE2 have the following types:

• MMX: __m64 64 bits of integers broken down as eight 8-bit integers, four 16-bit shorts or two 32-bit integers.
• SSE1: __m128 128 bits: four single precision floats.
• SSE2: __m128i 128 bits of any size packed integers, __m128d 128 bits: two doubles.

Because the debugger doesn't know how you're using these types, printing X86 vector variables in gdb/ddd shows you the packed form of the vector instead of the collection of elements. To get to the individual elements, tell the debugger how to decode the packed form as "print (type[]) x" For instance if you have:

__m64 avariable; /* storing 4 shorts */

You can tell ddd to list individual elements as shorts saying:

print (short[]) avariable

If you're working with char vectors and want gdb to print the vector's elements as numbers instead of characters, you can tell it to using the "/" option. For instance:

print/d acharvector

will print the contents of acharvector as a series of decimal values.