Word Select
The word select instructions allow some limited non-SIMD operations to be performed at the word level between the x word of the destination register, and an arbitrary word from a source register.
Sometimes it may be necessary to perform simple operations only on a single word without having to sanitise the entire source vector, or copying to a temporary register and then copying the result back.
An example might be that you want to add a specific word from the constants registers to the program counter ri.x, you could do something like copy the entire ri word to a temporary register and perform arithmetic there with the constant register before doing a 1-word move back to ri or similar, or alternatively you can use the Word Select instructions to perform the arithmetic more directly.
In such a case as:
.memory ; This is c0.z, we want to add it to ri.x
; ↓↓↓↓
dead beef 0010 f00d
.code
mov c0, r0 ; Make a copy of the constant we can manipulate
swizzle r0.zwxy ; Swizzle the constant so that the value we want is at `r0.x`
add.w r0, r0, ri ; Add the result to `ri`
; Assuming ri was `$0052 $0001 $2000 $0000`
; `ri` now contains: `$0062 $f00e $fead $beef`
; `ri.yzw` have been clobbered with the other three
; potentially unrelated words
; Alternatively:
wadd ri, c0.z ; Adds `c0.z` to `ri.x` without touching `ri.yzw`
; `ri` now contains: `$0062 $0001 $2000 $0000`
; We avoided clobbering the other three words using
; the word select instruction `wadd`.
Word Select Modes
The Word Select instruction has the following modes with the specified mnemonics available:
| Mnemonics | Operation | Description |
|---|---|---|
wmovwmove | Word Move | Similar to the move instruction, this copiesthe data from the destination's selected word into the destination's x word. |
wswap | Word Swap | Swap exchanges the contents of the specified source word and the destination's x word. |
wadd | Word Add | Add performs addition between the specified source word and the destination's x word,storing the result in the destination's x word. |
wsub | Word Subtract | Subtract performs subtraction between the specified source word and the destination's x word, storing the result in the latter. |
Destination
The destination operand takes a general purpose writable register and specifies the register of which the x word will be written to.
Source
The source operand takes any register of which the word specified by the swizzle selector will be read.
If the operation is swap then the source register must also be writable.
Swizzle
The source operand also requires a word to be selected using the swizzle syntax.
A period . followed by a single x y z or w is used to specify the word to be selected.
Examples
wmove ri, c0.z
wadd r0, c1.y
wsub r4, r0.w
wswap r3, r1.z